Modelling of Cathinone–Carbon Nanotube Complexes’ Stability: Theory with a Cancer Treatment Perspective

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This work could provide preliminary data for experimental studies of the stability of cathinone–COOH/OH functionalized carbon nanotube complexes in environments with different pH. These data may have potential use in the development of new targeted anticancer therapies.

Abstract

Today, cathinone and its synthetic derivatives are among the most popular narcotics in the world. Their different cytotoxic activities on humans are increasingly and rapidly being published in forensic reports and the scientific literature. New studies demonstrate that these compounds target the apoptosis of some human cancer cell lines. These results could potentially open a new perspective for cathinones’ use as potential therapeutic agents. Even so, the psychostimulant effects of these compounds require testing their activity in the form of drug delivery systems. In our work, we report on the first theoretical studies concerning the potential use of functionalised carbon nanotubes (CNTs) as carriers in the targeted transport of cathinones. Using density functional theory (DFT), we predicted cathinone–CNT interaction energies in environments with different polarity, as well as the stability of complexes in simplified models of healthy and cancer tissue. The results of the current work provide first-step insights into cathinone–CNT non-covalent complex formation in neutral and acidified environments. This study may serve as the theoretical basis for further experimental works on the possibility of using cathinone–CNT complexes in targeted anticancer therapy.

1. Introduction

Changing urban environment conditions lead to the physiological and psycho-emotional modification of the human organism. Consequently, its carcinogenic factor resistance is reduced. Morbidity and mortality caused by various forms of cancer are increasing every year [1]. According to official world statistics, in 2022, about 20 million new cases of the disease and more than 9 million deaths caused by various neoplasms were registered [2]. Therefore, today, new anticancer therapies are one of the leading topics in medicinal chemistry [3]. An efficient treatment without side cytotoxic effects has not been developed yet [3].

Many studies present the use of known cytostatics against new forms of neoplasms, as well as in synergistic therapies to increase their anticancer activity [4,5,6,7]. Scientific works increasingly describe the anticancer potential of new and known natural [8,9,10,11,12] and synthetic compounds [11,13,14]. Particular attention is also being paid to drugs targeting various forms of cancer cell death—reactive oxygen species (ROS) formation [15], immunogenic cell death [16], ferroptosis [17,18], cuproptosis [19], and others [20,21,22]. The diverse localization and degree of neoplasm development, the emergence of new drug-resistant tumours, and the different interactions of cytostatics with patients’ healthy organs still do not allow the preparation of universal anticancer agents. This leaves a question regarding the anticancer properties open for different classes of substances. Attention is being drawn to new derivatives of known anticancer drugs (for example, transition metal complexes [13] or coumarin-based derivatives [11]), as well as to new organic and inorganic compounds.

One of the once “new” groups with unexpected therapeutic potential is cannabinoids. Naturally occurring in Cannabis sativa L. and Cannabis indica plants, these substances have long been known as psychostimulants [23]. The discovery of their anticancer properties in the past century [24] led to active works on the development of new synthetic derivatives with an improved therapeutic profile. The effectiveness of cannabinoids in the treatment of multiple myeloma [25], osteosarcoma [26], glioblastoma multiforme [27], and triple-negative breast cancer [28] was described. In addition, their anti-inflammatory properties [29,30,31], potential for treating Alzheimer’s [32] and Parkinson’s [33] disease, possible uses in treating neurocognitive deficiencies associated with HIV-1 infection [34], and potential for promoting bone [35] and skin [36] regeneration have been presented.

Another well-known group of psychostimulants is cathinone and its synthetic derivatives. Cathinone is a metabolite of the Catha edulis plant, which grows in North Africa and Western Asia. This compound is an amphetamine β-keto derivative and is popular as a cheap stimulant in countries where C. edulis naturally grows [37]. In most countries, the use of this narcotic substance is restricted.

Due to the ease and low cost of structure modification, many synthetic derivatives of cathinone have been produced over the years [38,39,40]. These substances are used as cheap and less psychoactive amphetamine and cocaine substitutes [39,41,42]. The production of cathinone and its derivatives is so massive that it is difficult to describe the current size of this drug group [43]. Their psychostimulant activities and addiction properties significantly limit research on their pharmacological properties. Despite this, the potential of some cathinones for the treatment of depression has been described. Currently, bupropion is a registered antidepressant and smoking cessation medicine [44,45]. Pyrovalerone has shown effectiveness in treating chronic fatigue [46], and amphepramone has demonstrated strong appetite-suppression effects without psychostimulation [47]. It is worth noting that some synthetic cathinone derivatives have demonstrated the targeting of apoptosis in some human cancer cell lines [48,49,50,51,52,53,54,55,56,57,58,59].

Legal restrictions significantly limit research on cathinones in the pharmaceutical field and do not allow extensive studies to determine the structural–psychostimulation correlation. Even so, some reports on decreasing the psychotropic properties of cathinones while increasing their cytotoxicity can be found in the literature [60,61,62,63]. A tendency towards a selective cytotoxic effect on cancer tissues has also been noted. Moreover, the increase in mephedrone cytostatic properties with the development of carcinogenesis in cancer tissue is also described [64,65,66,67]. For this reason, similar to cannabinoids, research into the anticancer potential of cathinone and its derivatives should not be ignored. On the other hand, possible psychostimulant and addictive properties require introduction of cathinones into the body in a form that avoids these side effects.

Anticancer agents are commonly administered via classical chemotherapy. This method has a number of limitations. The main disadvantages of conventional chemotherapy are toxic side effects on patients’ healthy organs, the limited solubility of cytostatics in physiological fluids, and the repeated administration of large doses of drugs, which carries the risk of overdose. A potential solution to all the above-mentioned limitations is targeted transport of cytostatics in drug delivery systems (DDSs). This form of anticancer therapy involves a preparation of reversible drug complexes with a biocompatible carrier and subsequent controlled drug transport to the cancerous tissue [3]. The increase in cytostatic activity of DDS administrated medicines compared to classical injection is described [68,69]. The targeted delivery also helps overcome the multidrug resistance of neoplasms [69,70,71] and increases general drugs’ permeability into tumours [72,73,74]. Growing anticancer activity has also been observed for alternative therapies in DDS forms—nanocytostatics [75,76], immunotherapy [77], ferroptosis targeting [78], and potential natural-origin drugs [79,80]. Nevertheless, the effectiveness of the DDS is often limited by the difficulties in choice of a good carrier. The key criteria are controlled drug transport and release near the molecular target, as well as the carrier removal from the body without excessively harming the patient’s health [3]. For this reason, intensive research on new carriers’ preparation and the improvement of materials with good effectivity in DDS therapies is being conducted. Attention is paid to synthetic and natural polymers [73,74,77], vesicular systems [70,72,79], antibodies [20], and others. In addition, the development of nanotechnology in recent years introduces new possibilities for the use of nanostructures as effective DDS carriers [69,74,76]. The high efficiency of metal nanoparticles [80,81], as well as ordered carbon nanostructures in the targeted transport of anticancer agents [82,83,84,85,86], is described. The introduction of the nanocomplexes enhances the absorption of the drug by the cancer cell [87], increases the therapy selectivity [88], and improves the transport of the drug into drug-resistant neoplasms [82]. Some studies demonstrate that carbon nanocarriers with certain particle sizes do not provide toxic effects on the human and animal body in certain concentrations [89,90]. It has also been noted that the additional functionalization of these structures allows for preventing possible negative effects on the human body [88,91]. For this reason, this work is intended to present introductory molecular modelling of the stability of the complexes, with cathinones as potential drugs in cancer treatment and carbon nanotubes as potential carriers.

The stability of DDS complexes and their controlled release is often based on the environment pH gradient. It is reported that the extracellular pH value of healthy tissues is close to neutral (about 7), and in the case of neoplasms it acquires an acidic character [92,93]. Therefore, in this work, the geometry and stability of the potential DDS complexes at different pH values are modelled. The influence of the cathinone and its derivatives structure on the number and strength of the interactions formed with different structural fragments of the nanotube—functional groups and an aromatic carbon skeleton—is considered. The influence of the environment polarity on the complexes’ geometrical parameters and energy prediction is also considered. The main goal is the analysis of the complexes’ stability in the simplified model of healthy tissue environment, their degradability in an acidic simplified model of neoplasm surface, and the preliminary determination of the efficiency of the carbon nanotubes’ classical functional groups (hydroxyl and carboxyl) in the targeted transport of cathinones. These results could provide a theoretical foundation for future experimental studies on the stability and anticancer efficacy of cathinone–nanotube complexes under varying physiological pH conditions.

2. Methods

All calculations were performed using Gaussian 16 C.01 program package [94]. Density functional theory was used to predict the structural, spectroscopic, and energy parameters for cathinone and its derivatives, carboxylated and hydroxylated carbon nanotubes, and the drug-nanotube complexes. All structures of CNTs and studied cathinones were created in GaussView [95]. The B3LYP hybrid density functional with Grimme’s D3BJ empirical correction for dispersion were used [96,97,98,99]. This method demonstrated good prediction of studied parameters for models of carbon nanostructures, as well as other small- and middle-size organic molecules [100,101,102,103,104,105,106,107]. Due to the relatively large size of interacting complexes, Pople’s type triple-zeta quality 6-311++G** basis set was used for all calculations [108]. All studied compounds and their complexes were optimised without any geometrical constrains and with tight convergence criteria. The lack of imaginary harmonic frequencies was used as a criterion for the obtained equilibrium structures. The environment polarity effect on CNT–cathinone complexes and model dimers (protonated/deprotonated formic acid—protonated methylamine), geometrical parameters, and interaction energies was studied in the gas phase and using a polarisable-continuum model (PCM) of chloroform (only for model dimers) and water [109]. To improve the intermolecular non-covalent interactions prediction, the first hydration sphere was further enhanced by adding one or two discrete water molecules near the sides of polar intermolecular non-covalent interactions. The interaction energy ${∆E}_{CNT-Cath}$ of complexes (kcal/mol) was determined using the following Formula (1):

$${∆\mathrm{E}}_{\mathrm{C}\mathrm{N}\mathrm{T}-\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{h}}={(\mathrm{E}}_{\mathrm{C}\mathrm{N}\mathrm{T}-\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{h}}-{\mathrm{E}}_{\mathrm{C}\mathrm{N}\mathrm{T}}-{\mathrm{E}}_{\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{h}})\ast 627.509$$

(1)

where E are the electronic energies of the studied complex or its fragments. The obtained values were corrected for the basis set superposition error (BSSE) using the counterpoise method [110].

3. Results and Discussion

The systematic theoretical studies revealed several cathinone complexes with single-walled carbon nanotubes (SWCNTs) end-functionalised by a single carboxyl or hydroxyl group. The non-substituted ends of all SWCNT models were capped with hydrogen atoms. To avoid overcrowding the work with figures and tables, the obtained complexes’ structures are shown in Figures S1–S3 in the Supporting Information. To simplify the analysis of the obtained results, each structure is named by the kind of “Nanotube substituent _ cathinone or its derivative code _ specific intermolecular interactions”. In order to reduce the computational power and to avoid the influence of intramolecular interactions between CNT substituents occurring in real multi-walled nanotubes (MWCNTs), a simplified model of a single-walled zigzag nanotube (5,0) SWCNT with one substituent was used in this work. The nanotube with a carboxyl substituent is represented by the abbreviation “COO” for the deprotonated and “COOH” for the protonated functional group. Similarly, a hydroxylated model is labelled using the abbreviation “OH”. The structures of cathinone and its derivatives with their chemical names and the attached letter code A–D are shown in Figure 1.

Cathinone was chosen as the primary structure of the studied drug groups. The other three cathinones were chosen to compare the influence of their structural modifications on the type and energy of intermolecular interactions with the carbon nanotube. Since the pKa of the cathinones’ amine end is in the range of 8–10 [111], the cationic form of these substances will dominate in both neutral and acidic environments. It is reported that only S-cathinone is synthesised by C. edulis and exhibits psychotropic and cytotoxic properties [37,48]. For this reason, its R-configuration was not considered in this work. In the case of synthetic cathinones, there are few data on the R/S-configuration influence on their activity [51,53,112,113]. Despite this, the main goal of the work was to determine the stability of potential DDS carbon nanotube–cathinone complexes in an environment resembling healthy and cancer cells and the contribution of CNT substituents and different cathinone modifications to the interaction energy. In this case, R-configurations of cathinone derivatives were considered. Further experimental determination of the cytostatic properties of cathinones in a free form and in the DDS complex will help to determine the most efficient derivative and its configuration. For a more accurate analysis of intermolecular interactions in nanotube–cathinone complexes, the total interaction energies with correction for the basis set superposition error (BSSE) were considered. E_BSSE values are shown in red in the figure of each structure (see Figures S1–S3).

3.1. The Impact of Environment Polarity and SWCNT-Model Length on the Structure and Intermolecular Interactions Energies in the Complex

The first factor analysed in this work is the effect of the environment polarity on the structure and the total interaction energy values in nanotube–cathinone complexes. For this purpose, two media with significantly different dielectric constants ε were selected—a non-polar gas phase (ε = 1) and water (ε = 78.4) using a PCM. Hydrogen bonds in the gas phase and solution have different dynamics depending on the structural features of the proton donor and acceptor molecules. Therefore, it was difficult to determine which position of hydrogen in the CNT–cathinone complexes will correspond to the equilibrium structure. To solve this problem, scan calculations were performed for model complexes—protonated/deprotonated formic acid and methylamine cation. The reaction path of the bridged hydrogen was studied in the gas phase, chloroform, and water using PCMs to track the effect of a gradual increase in the medium polarity on the proton movement. The proton transfer energy profile analysis has demonstrated reliability in predicting hydrogen bond geometric parameters and energies in other molecular systems, supported by experimental data [114,115,116,117]. The proton transfer from the -NH₃⁺ in three possible directions is considered as follows: to the carbonyl oxygen of the protonated carboxyl group, to the hydroxyl oxygen of the protonated carboxyl group, and to the oxygen of the deprotonated carboxyl group. Since the hydroxyl groups of CNTs are significantly more resistant to deprotonation than the carboxyl groups [118,119], the proton transfer in this case was not analysed. Figure 2 shows the dependence of the dimer energy on the N-H interatomic distance in all analysed models in vacuum and solution. For better analyses, each energy profile is presented separately in Figures S4–S6 in SI. As can be seen, the proton transfer from nitrogen to oxygen in both the carbonyl and hydroxyl fragments of the protonated carboxyl group is not energetically favoured (the energy barrier is about 25 kcal/mol in all environments for N-H··O=C-OH models and about 30–40 kcal/mol for N-H··O(H)-C=O models). In complexes with a deprotonated carboxyl group (Figure 2G–I), the proton transfer barrier decreases with an increase in the polarity of the medium from about 10 to about 1 kcal/mol. Because the energy barrier is very small, it is expected that in a real polar physiological environment, a rapid proton transfer occurs between the two minimum energy positions (Figure 2I). Consequently, it is expected that in the equilibrium structure the N-H distance will be the average of the two energy minima.

In the case of cathinone-nanotube complexes, unconstrained geometry optimisation was performed. In Figure 3, the structures of COO_A_NH nanotube–cathinone complexes optimised in vacuum (Figure 3A) and water using a PCM (Figure 3B) are shown. As can be seen from Figure 3A, in the gas phase, a transfer of a hydrogen atom from cationic to anionic form is observed, rather than the expected hydrogen bond formation (according to the previous energy profiles in Figure 2G–I). It can be concluded that the lack of solvent significantly worsens the prediction of the structure of complexes with ionised components (models in simplified neutral environment of healthy tissue). Similar shortcomings of the hydrogen bond DFT predictions in vacuum have also been noted for small clusters of ionic liquids [120,121,122]. A similar artefact is also observed for the cathinone–CNT complex in water using PCM (Figure 3B). It is worth noting that the medium polarisation only slightly improves the prediction of hydrogen bonds (the O··H interatomic distance in vacuum was 1.0004 Å, and in water—1.0018 Å, see Figure 3).

In the case of interactions between the cationic form of cathinones and the neutral form of SWCNT-COOH, as a model in a simplified acidic environment of neoplasms, the situation was similar. As can be seen from Figure 4A, the transfer of a hydrogen atom from S-cathinone to O=C of the CNT carboxyl group occurred in the gas phase. In the case of this complex in water using a PCM (Figure 4B), the formation of two hydrogen bonds with partial delocalization of the hydrogen atom between the amine group of the cathinone and the carbonyl group of SWCNT-COOH was observed. The detachment of the proton to CNT hydroxyl oxygen was not observed in the complexes with SWCNT-OH, both in vacuum and in water (Figure 5).

The above-mentioned problems in a prediction of the complexes structure are caused by a significant simplification of the solvent effect using the PCM, which is unable to predict specific interactions. On the other hand, using the discrete solvent molecules in the first hydration sphere is very computationally demanding. To improve the prediction with minimal calculation efforts, we used only one or two water molecules placed close to the sites of unwanted proton transfer. This simplified approach to providing the first hydration sphere was presented as effective in the modelling of specific interactions—the breaking of the C-P bond in an acidic environment [123]. As can be seen from Figure 6, predicting complexes of SWCNT-COO⁻ or SWCNT-COOH with the discrete water molecule addition in the PCM leads to the formation of the non-covalent complexes of realistic geometry (according to energy profiles in Figure 2). It is worth noting that the addition of a water molecule to complexes with SWCNT-OH does not introduce significant changes in these complexes’ geometric parameters (see Figure 7).

As can be seen, the partial inclusion of the first hydration sphere plays a key role in the prediction of intermolecular H-bonds in the SWCNT-COO–cathinone and SWCNT-COOH–cathinone complexes. In the SWCNT-OH–cathinone complexes, such a tendency was not observed. Such differences in the discrete water molecules effects on interactions with -OH and -COO/COOH could be explained by the presence of the cathinones amine group in the cationic form. Its higher reactivity compared to the neutral one could lead to a tendency to form hydrogen bonds with the additional donation of a hydrogen atom to a more electronegative oxygen. The oxygen atom in -C=O of the -COOH group and the oxygen atoms of the -COO⁻ group are more electronegative compared to the hydroxyl group. Therefore, in the SWCNT-OH models, a significantly lower ability of the oxygen atom to form H-bonds is observed, and the transfer of the hydrogen atom did not occur during modelling. Additionally, when the -OH substituent is an H-bond donor, proton abstraction does not occur due to the greater chemical stability of this group compared to the carboxyl substituent. Addition of the discrete water molecules near a -COOH/COO⁻ oxygen atom with the formation of the additional H-bond leads to the partial shift in electron density toward the water. As a consequence, the water molecules block the hydrogen transfer in CNT–cathinone interactions. It can also be assumed that in real systems, solvent molecules significantly affect the type and energy of the formed intermolecular interactions and, as a consequence, the stability of the considered complexes in a medium with different polarity and pH values.

Since this work is the first step in determining the stability of potential DDS complexes under different conditions, the simplified effect of the environment polarity was considered. In the future, both the effect of discrete solvent molecules on the type and strength of the bonds formed, and the dynamics of complex formation and degradation in a medium with different polarity and pH should be studied in more detail.

The inclusion of one (or two) discrete water molecules significantly influenced hydrogen bonds, while non-polar interactions remained largely unaffected. In the case of H-π and π-π interactions, the introduction of both the PCM and its addition by discrete solvent molecules did not significantly affect their interatomic distances (see Figure 5, Figure 6 and Figure 7). This can be explained by the non-polar nature of these interactions, and, as consequence, the small impact of environment polarity. It is worth noting that these non-covalent bonds additionally stabilise the complex structure.

Another important aspect in the first step determining cathinone–CNT complexes’ stability is the choice of relatively small CNT model. To determine the optimal size of SWCNTs, the interactions of S-cathinone with (5, 0) zigzag SWCNT-OH built with 1, 2, or 3 “bamboo” units [100,101,102,103,104,105] were analysed. Full optimisation of the complexes was performed in water using the PCM. The effect of one discrete water molecule addition to the first hydration sphere on the complexes geometries and interaction energies prediction was analysed. As can be seen from Figure 8 and Figure 9, reducing the model to 1 “bamboo” unit will not allow us to predict all possible interactions of cathinone with the SWCNT wall. Increasing the model to 3 “bamboo” units only slightly changes the complex structure. In addition, it can be seen from Figure 8 and Figure 9 that the structural parameters of the complexes varied insignificantly upon an addition of the discrete water molecule. It can be seen from

Table 1. BSSE-corrected interaction energies (kcal/mol) in complexes of the S-cathinone with/without a discrete water molecule addition with zigzag (5,0) SWCNT-OH with one, two, and three “bamboo” units.

Without Water		With Water
Complex	EBSSE	Complex	EBSSE
1 unit	−10.9464	1 unit	−10.8627
2 units	−11.0556	2 units	−11.3299
3 units	−11.4193	3 units	−11.4700

that the interaction energies in the complexes of S-cathinone with 1–3 “bamboo” units SWCNT-OH, with and without one water molecule, varied insignificantly. This difference could be explained by the accidental compensation of the calculation errors and some small deviations between the non-covalent interaction distances. Therefore, using the model with 2 “bamboo” unit allows us to obtain results with acceptable accuracy and calculation time.

3.2. Effect of the CNT Substituent Type and Cathinone Structural Modifications on the Intermolecular Interaction Energy Values

Carbon nanotubes are massive and chemically stable hydrophobic ordered structures. However, under the influence of concentrated and oxidising acids, they are functionalized with oxygen-containing substituents, mainly with hydroxyl and carboxyl groups. Functionalization occurs mainly at the ends, and partially on the outer surface of the nanotube. CNTs’ potential applications are promising [82,118,119,124]. Since this work is intended to be a first-step study of pH and CNT functionalization effects on the type of intermolecular interactions and the complexes’ stability, two models with single -COOH or -OH group were considered. A nanotube with a narrow diameter was chosen to avoid the synergistic effect of the substituent and the penetration of cathinone into the CNT. In future works, the effect of the substituent should be supplemented by the influence of several functional groups of the same and different type, as well as the possibility of drug penetration into the CNT.

In

Table 2. BSSE-corrected interaction energies (kcal/mol) and non-covalent intermolecular interaction lengths (Å) in complexes of S-cathinone and its derivatives (A–D) with and without discrete water molecule(s) and (5,0) zigzag SWCNT with two “bamboo” units. Label “W” stands for discrete water molecule(s) in the system.

Complex	EBSSE	Kind of Interaction	H··A or π··π Length
SWCNT-OH
OH_A_CO	−11.0556	CNT-O-H··O=C	1.7279
		CNT-C(π)··H-N	2.2097
		CNT-C(π)··H-C	2.8326
OH_A_NH	−5.6333	CNT-O··H-N	2.4271
		CNT-C(π)··H-N	2.1972
		CNT-C(π)··H-C	2.6250
		CNT-C(π)··H-C	2.9627
OH_A_NH_π	−9.9913	CNT-O··H-N	1.9932
		CNT-C-H··O=C	3.1801
		CNT-C(π)··C(π)	3.2626
OH_B_CO	−11.0684	CNT-O-H··O=C	1.7519
		CNT-O··H-C	2.8541
		CNT-C(π)··H-C	3.3073
		CNT-C(π)··C(π)	3.2787
OH_C_CO	−10.5607	CNT-O-H··O=C	1.7193
		CNT-C(π)··H-N	2.1034
		CNT-C(π)··H-C	3.4077
OH_C_F	−3.2792	CNT-O-H··F	1.8392
OH_C_NH	−5.6249	CNT-O··H-N	1.9322
		CNT-C(π)··H-C	2.8021
OH_C_NH_π	−6.0032	CNT-O··H-N	2.0279
		CNT-C-H··O=C	2.4568
		CNT-C-H··C(π)	2.6999
OH_C_NH_π2	−10.0124	CNT-O··H-C	3.2526
		CNT-C(π)··H-N	2.1870
		CNT-C(π)··H-C	2.9081
		CNT-C(π)··H-C	2.5219
		CNT-C(π)··H-C	2.8047
OH_C_π	−9.0711	CNT-C(π)··H-C	2.6274
		CNT-C(π)··H-C	2.7309
		CNT-C(π)··C(π)	3.2898
SWCNT-COO−
COO_A_CO-CH	−7.4879	CNT-C-H··O=C	2.6517
		CNT-C-H··C(π)	2.6270
		CNT-C-H··C(π)	2.7392
		CNT-C=O··H-C	2.3810
		CNT-C=O··H-C	2.5079
COO_A_NH_π_W	−22.8341	CNT-C=O··H-N	1.3952
		CNT-C-H··C(π)	2.4972
COO_A_NH_W	−23.2044	CNT-C=O··H-N	1.5029
		CNT-C(π)··H-N	2.9131
COO_B_CO-CH	−6.2739	CNT-C=O··H-C	2.7538
		CNT-C=O··H-C	2.1771
		CNT-C-H··O=C	2.3519
		CNT-C-H··O=C	2.4012
COO_B_NH_W	−18.6932	CNT-C=O··H-N	1.4511
COO_B_π	−8.8260	CNT-C-H··CI	3.1526
		CNT-C-H··CI	2.8095
		CNT-C=O··H-C	2.3634
		CNT-C=O··H-C	2.5071
		CNT-C-H··C(π)	2.8230
COO_C_NH_W	−20.2424	CNT-C=O··H-N	1.4768
COO_C_π	−7.4756	CNT-C=O··H-C	2.4253
		CNT-C=O··H-C	2.4082
		CNT-C-H··C(π)	2.7564
COO_C_π2_W2	−11.8132	CNT-C=O··H-C	2.2884
		CNT-C=O··H-C	2.0081
		CNT-C=O··H-C	2.4404
		CNT-C-H··O=C	2.5782
		CNT-C-H··C(π)	2.6376
COO_D_COC_W	−8.8299	CNT-C=O··H-C	2.3822
		CNT-C-H··O-C	2.6209
		CNT-C-H··O=C	2.3299
COO_D_NH_W2	−16.5568	CNT-C=O··H-N	1.6187
		CNT-C=O··H-C	2.4678
		CNT-C=O··H-C	2.3770
SWCNT-COOH
COOH_A_Dimer_W	−15.3782	CNT-O-H··O=C	1.8247
		CNT-C=O··H-N	1.5116
COOH_A_NH_W	−16.4735	CNT-C=O··H-N	1.6398
		CNT-C(π)··H-C	3.0042
COOH_A_π_W	−7.1474	CNT-C=O··H-C	2.4025
		CNT-C-H··C(π)	2.7412
COOH_B_CI	−7.6597	CNT-O-H··CI	3.4914
		CNT-C(π)··H-C	2.3745
		CNT-C(π)··H-C	2.7376
		CNT-C(π)··H-N	2.6783
COOH_B_Dimer_W	−14.5553	CNT-O-H··O=C	1.8315
		CNT-C=O··H-N	1.5659
COOH_B_NH_W	−16.2372	CNT-C=O··H-N	1.5541
		CNT-C(π)··H-N	2.4967
		CNT-C(π)··H-C	3.0067
COOH_C_Dimer_W	−16.9588	CNT-O-H··O=C	1.8993
		CNT-C=O··H-N	1.6445
COOH_C_F_CO	−5.5671	CNT-O-H··F	2.0677
		CNT-C-H··O=C	2.3553
COOH_C_NH_W	−13.1462	CNT-C=O··H-N	1.5767
		CNT-C(π)··H-C	3.0016
		CNT-C(π)··H-C	3.0241
COOH_C_π2	−9.4891	CNT-C(π)··H-C	2.8867
		CNT-C(π)··H-C	2.8332
		CNT-C(π)··H-C	2.8735
		CNT-C(π)··H-C	2.7112
		CNT-C-H··C(π)	3.3450
COOH_D_COC_W	−11.5632	CNT-O-H··O-C	2.1689
		CNT-C(π)··H-C	2.8143
		CNT-C(π)··H-C	2.8537
		CNT-C(π)··H-C	2.5988
		CNT-C(π)··H-C	3.4458
		CNT-C-H··C(π)	2.6291
COOH-D_Dimer_W	−16.2812	CNT-O-H··O=C	1.8966
		CNT-C=O··H-N	1.6131
		CNT-C(π)··H-C	2.9518
		CNT-C(π)··H-C	2.9298
COOH_D_π	−10.2033	CNT-O··H-C	2.2241
		CNT-O··H-C	2.5506
		CNT-C(π)··H-C	2.5417
		CNT-C(π)··H-C	2.7299
		CNT-C-H··C(π)	2.5632

, the BSSE-corrected interaction energies and selected intermolecular distances of SWCNT-OH–cathinones, SWCNT-COO–cathinones, and SWCNT-COOH–cathinones are gathered.

It is worth noting that for both protonated and deprotonated carboxyl groups, the energy of CNT–cathinone complexes was higher than for hydroxylated SWCNTs for the majority of complexes. As can be seen from Table 2, the SWCNT-OH complexes’ E_BSSE values were about 3–11 kcal/mol. The most stable were the “CO”-type complexes. In each of them, one hydrogen bond was formed, where the donor was the hydroxyl substituent of the CNT, and the acceptor was the carbonyl group of cathinones. In “CO”-type complexes, a number of interactions of the π-type were also observed. It is known from the literature that these interactions with low-polarity groups and benzene rings are characterised by lower energy than typical H-bonds [107,125]. However, as can be seen from the OH_C_π complex, where only three interactions of the H-π and π-π types were observed, the complex total interaction energy is about 9 kcal/mol (see Table 2 and Figure S1(C_6)). Therefore, the H-bonds and π-type interactions play an equal role in stabilising the SWCNT-OH–cathinone complexes. A significant contribution of low-polarity π interactions has also been observed in a number of biological macromolecular systems [126,127,128]. It is worth noting that the hydrogen bonds with the hydroxyl substituent as a donor were stronger than H-bonds with the -OH oxygen as an acceptor. The interaction energies of the OH_A_NH and OH_C_NH complexes (E_BSSE approx. 6 kcal/mol for both) with one hydrogen bond of the CNT-O··H-N type and a number of the π-type interactions were significantly lower than for the complexes with the CNT-O-H··O=C type hydrogen bond (E_BSSE approx. 11 kcal/mol, see Table 2). This can be explained by the lower electronegativity of the oxygen atom in the hydroxyl group in contrast to the carbonyl group. As a consequence, the lower ability of -OH to accept a proton is observed, which is shown in the elongation of the hydrogen bond and its weakening (see Table 2 and Figure S1(A_2,C_3)). In addition to hydrogen bonds with the -OH substituent, H-bonds between cathinones carbonyl oxygen and hydrogen atoms at the ends of the nanotube were also observed. It is worth noting that, partly due to the steric blockade by the side methyl group in cathinones, the corresponding H-bonds’ lengths were 3.1801 Å in OH_A_NH_π and 2.4568 Å in OH_C_NH_π. In both complexes with the energetically less favourable CNT-O··H-N hydrogen bond, these interactions played an additional stabilising role (see Table 2 and Figure S1(A_3,C_4)). Analysing the effect of cathinones benzene ring halogenation on the ability to form non-covalent interactions with SWCNT-OH, its small contribution to the complex stabilisation should be noticed. As can be seen for the p-Cl cathinone B, the chlorine atom did not form non-covalent interactions with either the -OH substituent or the terminal nanotube hydrogens. In the case of fluorine, the ability to form hydrogen bonds with the -OH substituent was noted. As can be seen from the OH_C_F complex (see Figure S1), the CNT-O-H··F bond was characterised by a longer length (1.8392 Å) and lower energy (approx. 3 kcal/mol) compared to the H-bonds of the CNT-O-H··O=C type. An explanation for such a low energy may be due to low ability of the fluoride substituent in a specific chemical environment to form intermolecular hydrogen bonds. This effect was reported in theoretical studies of 5-fluorouracil hydration [107,129].

Analysing the interactions of cathinones with SWCNT-COO⁻ (a model of a nanotube in neutral pH of healthy tissue), it is worth paying attention to Table 2 and Figure S2. It seems that the key role in stabilising these complexes is played by charge-assisted hydrogen bonds -COO⁻ with hydrogen at the N⁺-end of cathinones. As can be seen for the COO_B_NH_W and COO_C_NH_W complexes (Figure S2(B_2,C_1)), where one H-bond of the -C=O⁻··H-N⁺ type was observed, the stabilisation energies were about 19 and 20 kcal/mol, respectively. The second most significant were the π-type interactions. In the case of COO_A_NH_π_W and COO_A_NH_W complexes (Figure S2(A_2,A_3)), the appearance of one additional bond of the -CNT-C(π)··H-N or -CNT-C-H··π type increased the complexes stabilisation energy by about 3 kcal/mol, compared to COO_B_NH_W and COO_C_NH_W. In addition to the canonical H-bonds with the -COO⁻ substituent, bifurcated hydrogen bonds were also observed. As can be seen from Figure S2(D_2), bifurcated H-bonds were formed in the COO_D_NH_W2 complex. Their appearance could be explained by the structural modification of cathinone D, where a propyl side group is present opposite the pyrrolidine ring. As a result, two hydrogen atoms of these substituents were close to each other and formed bifurcated H-bonds with one oxygen of -COO⁻. At the same time, one canonical H-bond of considerable length (2.4678 Å) was formed by a hydrogen atom from the pyrrolidine ring with the second oxygen of -COO⁻. As a result, the complex was stabilised by three hydrogen bonds, but the canonical H-bond was long due to the steric effect of pyrrolidine, and the remaining two were bifurcated. This led to a decrease in the complex total interaction energy to about 17 kcal/mol. It is also worth noting that hydrogen atoms of the cathinones’ low-polarity benzene ring, as well as aliphatic substituents, also formed H-bonds with -COO⁻. In addition, similar H-bonds were formed between the cathinones carbonyl oxygen and the nanotubes’ aromatic hydrogen atoms. As can be seen from Figure S2(A_1,C_2,C_3), these interactions weakly stabilised the SWCNT-COO–cathinone complexes. In the COO_A_CO-CH complex, two H-bonds were formed between two hydrogen atoms of the cathinone A benzene ring and two -COO⁻ oxygens, one -CNT-C-H··O=C- bond, and two -CNT-C-H··π bonds. All non-covalent bonds were fairly long, and their total interaction energy was low (approx. 7 kcal/mol). Similar interaction energy and non-covalent bond lengths could be seen in case of the COO_C_π complex. In the COO_C_π2_W2 complex, -COO⁻ interactions with hydrogens of cathinone C aliphatic substituents were also formed. As can be seen from Table 2 and Figure S2(C_3), the complex total interaction energy was about 4 kcal/mol higher compared to COO_A_CO_CH and COO_C_π complexes. This could be due to the overall contribution of five non-covalent interactions in COO_C_π2_W2 compared to three interactions in COO_C_π, as well as by the higher polarity of some functional groups contributing to non-covalent interactions in COO_C_π2_W2, compared to COO_A_CO_CH. The synergistic effect of interaction energy weakening by bifurcated H-bonds with low-polar functional groups could be seen in the COO_B_CO-CH complex. As can be seen from Figure S2(B_1), four bifurcated H-bonds were formed in the complex, which led to a low value of interaction energy—about 6 kcal/mol. Another type of hydrogen bonds with low-polarity groups was observed for the complex COO_D_COC_W. It was formed between a nanotube terminal hydrogen atom with the cathinone D dioxolane oxygen. As can be seen from Figure S2(D_1), a polarised but longer (2.3822 Å) H-bond between the -COO⁻ and the dioxolane hydrogen and an interaction of the -CNT-C-H··O=C type were also formed in the system. Consequently, the complex also had a fairly low interaction energy (about 9 kcal/mol, see Table 2). Analysing the effect of cathinone halogenation on the type and energy of interactions in the SWCNT-COO–cathinone complexes, it is worth noting that the fluorine did not form hydrogen bonds with the nanotube terminal hydrogen atoms. On the other hand, the chlorine atom in the COO_B_π complex was an acceptor of two bifurcated hydrogen bonds with the nanotube terminal hydrogen atoms (see Figure S2(B_3) and Table 2). Even so, these H-bonds were long and appeared together with two canonical hydrogen bonds with -COO⁻ and one interaction of the -CNT-C-H··π type, and they weakly stabilised the complex (E_BSSE about 9 kcal/mol).

Finally, SWCNT-COOH complexes with cathinones were studied as models in neoplasms acidic pH conditions. It is apparent form Table 2 and Figure S3 that the highest contribution to the stabilisation of the complexes was made by canonical strong hydrogen bonds of the -CNT-C=O··H-N and -CNT-O-H··O=C types. In the complexes COOH_A_Dimer_W, COOH_B_Dimer_W, and COOH_C_Dimer_W (Figure S3(A_1,B_2,C_1)), one interaction of both types is visible, so these complexes are characterised by the highest interaction energy values among the complexes with SWCNT-COOH—15–17 kcal/mol. The next most important non-covalent interactions were the π-type ones. As can be seen from Figure S3(D_2), in the COOH_D_Dimer_W complex, two canonical H-bonds with -COOH were formed. However, due to the larger and more complex structure of cathinone D, its N-end was sterically less accessible to interactions with -COOH than cathinones A–C. Therefore, an elongation and weakening of H-bonds was observed. On the other hand, the cathinone propyl side chain formed two H-π interactions with the CNT sidewall. As a result, the weakened H-bridges were compensated by π-type interactions, and the complex had a high interaction energy of about 16 kcal/mol. A similar effect of stabilisation by the π-type interactions was also observed for the complexes with a single canonical hydrogen bond. As can be seen from Figure S3(A_2,B_3,C_3), in COOH_A_NH_W, COOH_B_NH_W and COOH_C_NH_W complexes, a single canonical H-bond of the -CNT-C=O··H-N type was formed. The complexes were additionally stabilised by one or two H-π-type interactions. Consequently, the complexes energies were close to the E_BSSE of the complexes with two conventional H-bonds—about 13–16 kcal/mol. It can also be noted that the -COOH substituent, in addition to classical polar H-bonds, participated in a number of the H-bridges with low-polar functional groups. Thus, in the COOH_A_π_W complex (Figure S3(A_3), Table 2), the carbonyl oxygen was an acceptor of hydrogen atom from the cathinone A benzene ring. In the COOH_C_π2 complex (Figure S3(C_4), Table 2), the same carbonyl oxygen atom was an acceptor of a hydrogen atom from the cathinone C aliphatic side chain. The hydroxyl fragment of -COOH also participated in the H-bonds with low-polar groups. In the COOH_D_COC_W complex (Figure S3(D_1), Table 2), the hydroxyl group was an H-bond donor to the dioxolane oxygen atom of the cathinone D. On the other hand, the hydroxyl oxygen atom was an acceptor of the H-bond with the cathinone D benzene hydrogen in the COOH_D_π complex (Figure S3(D_3)). All the mentioned complexes with H-bridges formed by the low-polar functional groups were additionally stabilised by the π-type interactions. Due to the lower energy of all non-covalent interactions, compared to canonical polar H-bonds, the complexes’ total interaction energy was not very high (about 7–12 kcal/mol, see Table 2). Analysing the effect of cathinone halogenation on the intermolecular interaction types and energies in the SWCNT-COOH–cathinone complexes, it can be noted that the fluorine and chlorine atoms were acceptors of H-bonds with the -COOH substituent. As can be seen for the COOH_B_CI and COOH_C_F_CO complexes (Figure S3(B_1,C_2)), the interaction lengths were longer than 2 Å, and their contribution to the complexes’ stabilisation was small. As can be seen in the COOH_C_F_CO complex, a single H-bond of the -CNT-C-H··O=C type was additionally formed. The same type of H-bond and several π-type interactions were formed in the COOH_B_CI complex. The total E_BSSE of both complexes was quite low (about 6 kcal/mol for the COOH_C_F_CO and about 8 kcal/mol for the COOH_B_CI).

It can be summarised that both SWCNT functionalization and cathinone structure modification significantly contribute to the complexes’ stability. It can be noted that the SWCNT-OH complexes were characterised by a lower E_BSSE values compared to most of the SWCNT-COO⁻ and SWCNT-COOH complexes. In addition, in the SWCNT-OH complexes, the hydroxyl substituent was capable of forming a smaller number of H-interactions compared to -COO⁻/-COOH. In the case of cathinones, the key role was played by the modifications of the N-end. The spatial accessibility of H-N played a key role in the formation of the both H-bonds and π-type interactions. In addition, the modification of the cathinones side aliphatic chain played a significant role in its location relative to the side wall of the CNT and, as a consequence, in the number and strength of the π-type interactions in the complex. The halogenation of the cathinones played a role in stabilising the complexes with SWCNTs, although the formed H-bonds involving fluorine and chlorine atoms were weak and occurred only in the case of some favourable positions of cathinone over the CNT surface.

3.3. The Influence of pH on the Complex Stability

The process of protonation–deprotonation in the environment of different pH is dynamic and occurs chaotically and quickly. Therefore, in each environment, one should expect the presence of the protonated and deprotonated forms of each pH-sensitive functional group. One should also expect the dominance of the negative ion of the acidic group at a pH higher than the pKa value. According to experimental data, the pKa of carboxyl groups in CNT is about 7, and that of hydroxyl groups is about 10 [130]. Therefore, in the neutral environment of healthy tissue, one should expect the dominance of deprotonated -COO⁻ and protonated -OH. In the acidic pH of cancer cells, one should expect the protonation of both functional groups.

Since the deprotonation of -OH occurs to a small extent both at neutral and acidic pH, the interaction energy of this functional group of CNT with cathinones will remain virtually the same under the analysed conditions. As mentioned earlier, the interaction energies in the CNT-OH–cathinone complexes were lower than those of both CNT-COOH–cathinone and CNT-COO⁻–cathinone. Deprotonated hydroxyl groups are expected to contribute minimally to the overall complex stability. As a result, the decisive contribution to the complex stability will be made by interactions with the carboxyl group, as it is more sensitive to changes in the physiological pH in cancer cells. Analysing the total E_BSSE of -COO⁻/COOH complexes, it should be noted that it significantly differs, depending on the pH value. It is worth paying attention to

Table 3. The BSSE-corrected interaction energies (kcal/mol) in complexes of the S-cathinone and its derivatives B-C with SWCNT-COOH and SWCNT-COO⁻ and discrete water molecules.

Complex	EBSSE (SWCNT-COO−)	EBSSE (SWCNT-COOH)
A_NH_W	−23.2044	−16.4735
B_NH_W	−18.6932	−16.2372
C_NH_W	−20.2424	−13.1462

. It presents the E_BSSE values for “NH_W”-type complexes. Each of them is stabilised by one strong H-bond, which in the case of -COO⁻ is additionally strengthened by the negative charge of the carboxylate substituent. In the case of -COOH and COO_A_NH_W complexes, a stabilising effect of π-type interactions occurs too. The structures of “A_NH_W”, “B_NH_W”, and “C_NH_W” with SWCNT-COO⁻ and SWCNT-COOH are also presented in Figure 10. The energies of all obtained complexes at different pH are presented in Table S1 in SI.

As can be seen from Table 3, all complexes with -COO⁻ (model in the healthy tissues) show higher interaction energy and are therefore more stable in comparison to complexes with -COOH. Therefore, in a real system, one should expect the intermolecular hydrogen bond to break as result of the protonation of the oxygen atom participating in the H-bond and the complex dissociation. It corresponds to the pH value decrease in the neoplasms’ medium. If protonation occurs on the oxygen atom not participating in the H-bond, one should also expect a significant weakening of the complex’s stability (see Table 3) and its gradual degradation. Analysing other complexes with the -COO⁻/COOH motif, it is worth noting that a high dependence of the interaction energy on the medium pH value was obtained only for classical H-bonds (see Table 2). The E_BSSE of complexes with H-bridges with low-polar groups, H-bonds with halogens, and the π-type interaction dominance varied only slightly in different pHs. Therefore, in the case of the experimental design of CNT–cathinone DDS complexes, nanotubes functionalized with both -OH and -COOH or only -COOH substituents could be recommended. Due to the high pKa value and, therefore, significant resistance to deprotonation in healthy and cancerous tissues, hydroxyl substituents will additionally stabilise the complex during transport to the molecular target and slow down the rate of the complex degradation to an optimal value near the neoplasm. Carboxyl substituents will significantly stabilise the complex during transport to the molecular target, and they will serve as a trigger for drug release near the neoplasm surface. It could be supposed that standard functionalization with polar -COOH/OH substituents will be sufficient to ensure the stability and controlled release of the drug from the carrier. This assumption is based on the relatively high E_BSSE values in complexes with conventional H-bonds and moderate E_BSSE values in complexes with the bifurcated and low-polarity H-bonds for CNT with one substituent. Even so, this explanation requires further in-depth theoretical and experimental confirmation.

4. Conclusions

As a result of systematic theoretical studies, the first data on the interaction of cathinone and its three derivatives with simplified models of hydroxylated and carboxylated nanotubes in the context of new potential anticancer DDS preparation were obtained. The effect of the CNT substituent, as well as the structural modification of cathinones on the type and energies of intermolecular interactions in the complex was analysed. It was noted that both structural features of CNT and cathinones are equally important in the complexes’ formation and stabilisation. It was observed that in the SWCNT-OH complexes, a smaller number of non-covalent intermolecular interactions are formed compared to the SWCNT-COO⁻/COOH complexes. The stabilisation energies of the SWCNT-OH complexes were lower compared to most of the SWCNT-COO/COOH complexes. Therefore, the hydroxyl substituents in real systems will stabilise the complex less strongly during transport to molecular targets. On the other hand, due to the high -OH pKa value, its very low ability to deprotonate should be expected both at neutral pH healthy tissue and at acidic pH neoplasms. Therefore, the functionalization of CNT with -OH substituents is recommended due to the possibility of the additional stabilisation of the complex during transport and the slowing down of the drug release near the molecular target. Carboxyl substituents will stabilise the complex significantly more during transport. Their ability to protonate during a pH decrease in the neoplasms will serve as the start of the complex’s degradation and controlled drug release. Therefore, it is recommended to consider the possibility of using CNT functionalized with either carboxyl or carboxyl and hydroxyl substituents, in the future, for more detailed theoretical and experimental studies. The obtained theoretical data show that the H-bonds with functional groups of different polarity and π-type interactions will dominate in the cathinone–CNT complexes. In the case of cathinone halogenation, H-bonds with halogen atoms can also be expected, but they will be significantly weaker than classical H-bridges. Based on the obtained data, it could be assumed that the classically functionalized CNT will provide more effective transport of cathinones compared to traditional drug injection. It is suspected that the release of the drug near the molecular target will allow for the more effective use of its cytostatic potential. It is also worth mentioning that the introduction of cathinones in complexes with the CNT will potentially decrease side cytotoxic effects on healthy tissues, as well as the drugs’ psychotropic activity.