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Article

Molecular Interaction of Water-Soluble Resorcinarenes for Potential Choline Detectors

by
Cielo Urquijo
,
Miguel Vela
,
Roger Sarmiento
and
Mauricio Maldonado
*
Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, Sede Bogotá, Carrera 30 No. 45-03, Bogotá 111321, Colombia
*
Author to whom correspondence should be addressed.
Processes 2025, 13(2), 553; https://doi.org/10.3390/pr13020553
Submission received: 14 January 2025 / Revised: 8 February 2025 / Accepted: 11 February 2025 / Published: 16 February 2025
(This article belongs to the Section Pharmaceutical Processes)
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Abstract

The molecular interactions of water-soluble crown resorcinarenes with choline were analyzed. To this end, four sulfonated resorcinarenes were synthesized and characterized by ATR-IR, 1H-NMR, and 13C-NMR spectroscopy. The molecular interaction studies with choline were carried out through FT-IR spectroscopy, 1H-NMR titrations, and conductimetric titrations, with which it was possible to determine that the complexes formed 1:1 stoichiometries with the host, in addition to showing good interaction in the electronic cavity of the macrocycle, demonstrating great potential for host–guest systems for choline detection in aqueous media. Finally, the incidence of the structural aspects of sulfonated resorcinarenes were analyzed.

1. Introduction

Choline is a quaternary ammonium cation that is synthesized in the body or consumed through the diet [1]. Choline is essential for the structure and function of the cell membrane and for the synthesis of the neurotransmitter acetylcholine [2]. Choline acts [3] on the central nervous system and helps with memory functions, the regulation of muscle movements, and learning in human beings. In addition, it plays an important role in sleep patterns [2,3]. Several methods for its detection are found in the literature, given its great importance in the human diet [3,4,5,6]; one clear example is represented by infant formulas that, according to regulations of the United States Food and Drug Administration (FDA), must contain up to 200 mg of choline per 100 g of sample to imitate breast milk, which is rich in choline [5]. The importance of choline in the body is summarized in Figure 1.
Due to the importance of choline, several methods are described in the literature for its detection: analysis with radioactive markers [5,6], chromatographic methods, electrochemical methods, colorimetric methods and fluorescent methods [7,8], and the development and use of selective organic cation receptors [9,10,11], among others. However, several of these methods have the problems of interference with the media used, low sensitivity, and poor stability [5]. Over the last few decades, advances have been reported in the detection of choline through supramolecular chemistry, which promises to overcome the problems presented so far in the methods used in a simple manner and without having to resort to robust techniques for tests. In this vein, systems such as calixarenes have been used as choline receptors with good results, finding very good interaction, with suitable characteristics for use as chemosensors of this neurotransmitter [10]. Within calixarenes, there is an interesting group of macrocycles known as calix[4]resorcinarenes, which have been used successfully in the host–guest interaction process. Calix[4]resorcinarenes are oligomeric macrocyclic polyphenols containing four resorcinol nuclei linked by four methyne bridges [11,12]. These macrocycles are generally synthesized by a direct reaction between resorcinol and an aliphatic or aromatic aldehyde [13], with this reaction being catalyzed by concentrated hydrochloric acid, and the synthesis process can generate conformers such as crown and chair, among others [14]. As shown in Figure 2, in the crown conformation, is possible to identify two active sites for functionalizing the calix[4]resorcinarenes: the lower rim, where the methylene bridges which can have an alkyl or aryl substituent are located, and the upper rim, in which hydroxyl groups or the ortho position can be functionalized.
These compounds are interesting because of their wide versatility, being modifiable on the upper rim, increasing the possibilities for applications. This is how these macrocycles have been used in the functionalization of monolithic columns [13,15,16,17], the manufacture of nanomaterials [18], as catalysts in organic chemistry [19,20,21], and also as cavitands [21,22,23,24], capsules [25,26,27,28], receptors [29], and dendrimers [14,30,31,32], in addition to being used as modifiers of polymeric surfaces [33]. As shown in Figure 2, there are several reactions that can be performed with these polyhydroxylated platforms. Among these possibilities, the sulfomethylation reaction is interesting because it significantly improves the solubility of these macrocycles in water [34], allowing their use in molecular interaction processes with biological analytes.
In the present investigation, we show the synthesis of four tetra(alkyl)calix[4]resorcinarenes and their modification with sodium sulfite and formaldehyde. The products were characterized via 1H- and 13C-NMR and ATR-IR spectroscopy. The interaction between calix[4]resorcinarenes and choline was investigated by FT-IR, 1H-NMR spectroscopy, and conductimetric titrations. In the results obtained, the important role played by the sulfonated group in the host–guest interaction in water is highlighted.

2. Materials and Methods

Resorcinol, ethanol, hydrochloric acid, acetone, acetaldehyde, hexanal, and decanal were obtained from Merck (Darmstadt, Germany) and were used without further purification. The IR-ATR (Infrared Attenuated Total Reflection) technique was used on a Thermo Fisher Scientific iD1 Nicolet iS5 IR spectrometer with a zinc selenide (ZnSe) ATR accessory, with frequencies in cm−1 (Thermo Scientific, Waltham, MA, USA). Nuclear magnetic resonance (1H-NMR, 13C-NMR) spectra were recorded on a BRUKER Avance 400 instrument (400.131 MHz for 1H and 100.263 MHz for 13C), and chemical shifts are given in δ units (ppm), using, in all cases, the residual signal of the respective deuterated solvent as a reference.

2.1. General Procedure for Synthesis of Resorcinarenes

Resorcinarenes were synthesized as per a reported method [33], which is described as follows: A resorcinol solution (5 mmol) in 10 mL of an ethanol/water (1:1) mixture was added dropwise to 0.4 mL of concentrated chlorine acid (37%), the mixture was stirred at 0 °C for 5 min, and then 5 mmol of aldehyde (acetaldehyde, hexanal, decanal, and lauric aldehyde) was added dropwise. After 10 min, the mixture was heated to 70–80 °C for varying times (6–12 h) depending on the aldehyde. After the heating period, the reaction was cooled and the respective resorcinarene was precipitated and filtered with water, producing four solids, which were characterized by means of IR, 1H-NMR, and 13C-NMR. Using this synthesis methodology, the following compounds were obtained: C-tetra(methyl)calix[4]resorcinarene (1a) yield 41.1%, C-tetra(pentyl)calix[4]resorcinarene (1b) yield 75.0%, C-tetra(nonyl) calix[4]resorcinarene (1c) yield 85%, and C-tetra(undecyl)calix[4]resorcinarene (1d) yield 64.6%.

2.2. General Procedure for Sulfomethylation of Resorcinarenes

First, 0.1 mol of calix[4]resorcinarene (1a, 1b, 1c or 1d) in 30 mL of ethanol was added to a mixture of 0.5 mol of 37% formaldehyde and 0.5 mol of sodium sulfite in H2O (10 mL), which was stirred and heated at 90–95 °C for 4 h. Dilute hydrochloric acid was added after cooling up to pH 7, and then acetone was added in order to precipitate products (2a, 2b, 2c or 2d). The solids were filtered, washed with acetone, and vacuum-dried. Using this synthesis methodology, the following compounds were obtained: tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(methyl)resorcinarene (2a) yield 10.6%, tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(pentyl)resorcinarene (2b) yield 26.5%, tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(nonyl)resorcinarene (2c) yield 50.0%, and tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(undecyl)resorcinarene (2d) yield 71.1%. The structure determination of the mentioned compounds was carried out by IR and NMR using the following deuterated solvents: 2a and 2b in D2O, 2c in DMSO-d6, and 2d in CDCl3-d1. The spectroscopic data are shown in Table 1.

2.3. ATR-IR Analysis

As mentioned above, IR spectra were obtained with an ATR accessory. Each mixture was prepared by mixing choline in an equimolar ratio with the respective sulfomethylated resorcinarene (2a–2d) in ethanol (5 mL). The resulting mixture was stirred for 1 h; then, the solvent was evaporated, and the IR spectrum of each inclusion complex was acquired.

2.4. Conductimetric Titrations

The experiment was carried out at a temperature of 19.2 °C, with the above-described conductometer. Initially, a solution of 40 mL (1∙10−3 M) of sulfomethylated resorcinarene was prepared, which was titrated with a solution of 20 mL (1∙10−2 M) choline chloride (10 times more concentrated); said solution was added in volumes of 0.2 mL with the help of a micropipette (100–1000 μL) until a 1:5 host–guest molar ratio was reached.

2.5. NMR Titrations

A total of twelve samples were prepared, of which two were reference samples, the guest (choline) and the host (sulfomethylated resorcinarenes). The remaining 10 samples consisted of a mixture of a fixed amount of host (20 mg in 600 μL), and there were increasing amounts of the guest solution (0.27 M) added until a 1:2 host–guest molar ratio was reached. All solutions were made up to a total volume of 700 μL.

3. Results

For the present study, the following resorcinarenes were chosen: tetra(methyl)calix[4]resorcinarene (1a), tetra(pentyl)calix[4]resorcinarene (1b), tetra(4-nonyl)calix[4]resorcinarene (1c), and tetra(4-undecyl)calix[4]resorcinarene (1d). As mentioned in the experimental section, synthesis was conducted by the acid-catalyzed cyclocondensation of aldehyde (ethanaldehyde, hexanaldehyde, decanaldehyde, or dodecanaldehyde) with resorcinol using ethanol as the solvent, and the reaction conditions were adapted according to previously described procedures [33] (Scheme 1). In all cases, the structures of the products were analyzed using spectral techniques, including ATR-IR, 1H-NMR, and 13C-NMR. Resorcinarenes 1a–d had been previously synthesized, and our spectroscopic data agreed with those reported [35,36]. The crown conformation of the four products was verified by the 1H-NMR spectra. For example, only one resonance signal was observed for the hydroxyl protons of 1a, 1b, 1c, and 1d at 8.53, 8.90, 9.60, and 9.56 ppm, respectively, as well as other signals in the spectrum; therefore, the 1H-NMR spectra of compounds 1a–d were consistent with the presence of the crown conformer.
Crown-type resorcinarenes have the ability to be functionalized through different reactions with functional groups to allow changes in their base structure and in order to fulfill special purposes. In this sense, the sulfonation of tetra(alkyl)calix[4]resorcinarenes was carried out by a direct reaction with a formaldehyde and sodium sulfite solution in water/ethyl alcohol at 90–95 °C, as previously described [34] (Scheme 2). After a work-up of the crude product and purification by recrystallization, the products obtained from each of the reactions were analyzed by spectroscopic methods to establish their structure. Initially, all products were analyzed by IR, specifically evaluating the bands corresponding to the functionalization; in this regard, as can be seen in Table 1, the four functionalized resorcinarenes present the characteristic S=O and S-O bands, confirming the presence of the sulfonate group. Secondly, the sulfomethylation reaction was also analyzed by NMR; e.g., the obtention of compound 2a was confirmed, with the presence of a new aliphatic proton signal at δ 4.29 ppm in 1H-NMR and a carbon signal of the methyne bridge between the sulfonate group and the resorcinol ring (Ar-CH2-SO3) at δ 48.1 ppm in the 13C-NMR spectrum. The confirmation of the structures of sulfonated tetra(pentyl)calix[4]resorcinarene (2b), tetra(nonyl)calix[4]resorcinarene (2c), and tetra(undecyl)calix[4]resorcinarene (2d) was carried out in a similar manner as shown in Table 1. In this context, the appearance of new aliphatic protons at 3.95 ppm for 2b, 3.82 ppm for 2c, and 3.85 for 2d in the 1H-NMR spectra and the appearance of carbon signals at 52.9 ppm (Ar-CH2-SO3), 48.3, and 48.6 ppm, respectively, in the 13C-NMR spectra confirmed the structure of compounds 2b, 2c, and 2d. Finally, for the four synthesized sulfonated calix[4]resorcinarenes 2a–d, in the 1H-NMR spectra, singlet signals are observed for some protons (Ha and Hb in Table 1), which confirms the presence of the crown conformer as the only product. These results indicate that during the sulfonation reaction, this conformation is retained in solution.
Once the sulfomethylated calix[4]resorcinarenes were synthesized, the next step consisted of studying the molecular interactions with choline. Initially, the complexometric properties of sulfomethylated resorcinarenes toward the choline ion were investigated through ATR-IR spectroscopy and molar conductance changes upon the addition of choline at room temperature. As can be seen in Figure 3, where the spectra are differentiated by color, indicating that of choline (blue), that of sulfomethylated resorcinarene (2d) (red), and that of the equimolar mixture (green), the green spectrum (mixture) presents some signals that shift with respect to the other two spectra. Clear evidence of this shift is the O-H stretching bands at 3393 cm−1 for the 2d spectrum (red) and 3382 cm−1 for the equimolar mixture (green), and, for its part, choline (blue) locates its signal at 3314 cm−1. Likewise, the differences in the intensity of the O-H stretching signals indicate a change in polarity in the equimolar mixture (green) with respect to the other two spectra, finding itself in an intermediate state. Other shifted signals are also observed, such as the following: C-C at 1183 cm−1, 1204 cm−1, and 1180 cm−1 for 2d, choline, and equimolar mixture, respectively; C-O at 1039 cm−1, 1053 cm−1, and 1038 cm−1 for 2d, choline, and equimolar mixture, respectively; and C-N at 1081 cm−1 and 1087 cm−1 for choline and equimolar mixture. In addition to the previously observed absorptions, the absorption at 952 cm−1, assigned to the NH2 vibration of choline, noticeably changes its intensity when it is located in the inner part of the resorcinarene cavity.
As mentioned in the experimental section, a solution of sulfomethylated resorcinarene was prepared, which was titrated with a solution of choline chloride (10 times more concentrated); this solution was added in volumes of 0.2 mL until a 1:5 host–guest molar ratio was reached. Conductometric titration curves were plotted as the dependence of conductance versus the molar ratio of resorcinarene to choline, as shown in Figure 4. For example, in the titration of 2a, since sulfomethylated resorcinarene is ionic, the addition of the choline ion solution resulted in a continuous increase in the conductance of the system studied [37]. The above-mentioned observation clearly demonstrated that in the system studied, there exists an interaction of the choline ion with that of 2a. With an increasing concentration of choline, a lower mobility of the complexed cation was recorded compared to the free solvated choline ion. As is evident from Figure 3, the slope of each molar conductance versus molar ratio titration curve changed sharply at the point where the resorcinarene 2a to choline ion molar ratio achieved a value equal to about 1. The obtained conductimetric data clearly demonstrated that the addition of choline to a 2a solution allowed the formation of a complex with a stoichiometric ratio of 1:1 with a high degree of stability. In agreement with this result, similar behavior was observed in the coordination process for the other resorcinarenes studied.
The next step of the study was the evaluation of complexation between sulfomethylated resorcinarenes and choline via 1H-NMR titration, a method that is commonly used to determine complexation constants via statistical analysis, with the data obtained by NMR. Titrations were carried out by the sequential addition of known amounts of the guest (choline) to a solution of the host (2a–2d), with the 1H-NMR spectrum observed after each addition. In Figure 5, the stacked 1H-NMR spectra of choline with resorcinarene 2c show great interaction due to changes in chemical shifts.
Following this procedure, in all four cases, it was observed that some signals were considerably affected. This interaction of choline with the bottom of the cavity of sulfonated resorcinarenes can be confirmed by various shifts in the signals in the spectra. The chemical shift variation for the proton of the aromatic ring (H meta to OH) as a function of its concentration is shown in Figure 6. This trend can be explained by the finding that the hydroxyl group is oriented toward the wider rim, according to what has been observed in previous studies for analogous systems [38]. The collected results for choline showed that the protons of the methylene group exhibited a significant downfield shift as well as the signals of the methyl groups of choline.
After the titration experiments were performed, the binding constants of the complexes were estimated by using the HypNMR2008 (v.4.071).20 computer program [39], and the results are shown in Scheme 3. The results indicate a moderate affinity of 2a and 2b with choline, with Kf values of 912 M−1 and 1148 M−1, respectively, while the long-chain resorcinarenes, 2c and 2d, showed a higher affinity with choline, with Kf values of 7000 M−1 and 9200 M−1, respectively. When compared to other analogous macrocycles [38], these observations together confirm a strong host–guest interaction in the electronic cavity of the sulfonated resorcinarenes.
An analysis of the inclusion complex between sulfonated resorcinarenes (2a–2d) and choline provided information on the molecular binding interactions. In this way, the differences between the magnitudes of the constants can be attributed to the differences in the chain sizes on the lower rim of the sulfonated resorcinarenes, so if an increase in the alkyl chain size results in an increase in the affinity of choline with the resorcinarene cavity, it indicates that hydrophobic interactions affect the size of the cavity, allowing better affinity in the case of choline complex 2c. Additionally, in all four cases, the interaction occurs through the trimethylammonium function; thus, the cation coordination is possibly dictated by hydrogen bonds and π interactions in the resorcinarene cavity. The presence of the sulfonate group eventually promotes better solubility in polar solvents such as water. Finally, the data obtained by conductimetric titrations and by NMR allow us to establish that the stoichiometry of the complexes is 1:1. In addition, we already observed this trend in previous studies of the interaction of choline with analogous macrocyclic systems [38].

4. Conclusions

Sulfonated calix[4]resorcinarenes were prepared using a direct reaction of the respective tetra(alkyl)calix[4]resorcinarene with a formaldehyde solution and sodium sulfite in water/ethyl alcohol. The formation of an inclusion complex was observed in solution between choline and sulfonated calix[4]resorcinarenes (2a–2d); in this regard, a detailed study of the interactions was performed in D2O via conductimetry titration, showing in all cases 1:1 stoichiometry. A detailed study of the interaction of 2a–2d with choline in D2O was performed with the help of 1H-NMR titrations. According to the 1H-NMR titration, in D2O, choline binds with 2c and 2d with a stability constant of K = 7000 and 920, respectively, while for complexes 2a–choline and 2b–choline, the constants were 912 and 1148, respectively. Finally, when the degree of interaction between compounds 2a–2d and choline was compared, it was found that the extension of the chain on the lower rim of sulfonated resorcinarenes improves their ability to interact with these organic cations.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr13020553/s1, Figure S1: IR spectrum for tetra(methyl)calix[4]resorcinarene (1a). Figure S2: 1H NMR spectrum for tetra(methyl)calix[4]resorcinarene (1a). Figure S3: 13C NMR spectrum for tetra(methyl)calix[4]resorcinarene (1a). Figure S4: IR spectrum for tetra(pentyl)calix[4]resorcinarene (1b). Figure S5: 1H NMR spectrum for tetra(pentyl)calix[4]resorcinarene (1b). Figure S6: 13C NMR spectrum for tetra(pentyl)calix[4]resorcinarene (1b). Figure S7: IR spectrum for tetra(nonyl)calix[4]resorcinarene (1c). Figure S8: 1H NMR spectrum for tetra(nonyl)calix[4]resorcinarene (1c). Figure S9: 13C NMR spectrum for tetra(nonyl)calix[4]resorcinarene (1c). Figure S10: 1H NMR spectrum for tetra(undecyl)calix[4]resorcinarene (1d). Figure S11: 1H NMR spectrum for tetra(undecyl)calix[4]resorcinarene (1d). Figure S12: 13C NMR spectrum for tetra(undecyl)calix[4]resorcinarene (1d). Figure S13: IR spectrum for tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(methyl)resorcinarene (2a). Figure S14: 1H NMR spectrum for tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(methyl)resorcinarene (2a). Figure S15: 13C NMR spectrum for tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(methyl)resorcinarene (2a). Figure S16: IR spectrum for tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(pentyl)resorcinarene (2b). Figure S17: 1H NMR spectrum for 5,11,17,23-tetrasodiotetrakis(methanesulfonate)-2,8,14,20-tetra(pentyl)calix[4]resorcinarene (2b). Figure S18: 13C NMR spectrum for tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(pentyl)resorcinarene (2b). Figure S19: IR spectrum for tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(nonyl)resorcinarene (2c). Figure S20: 1H NMR spectrum for tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(nonyl)resorcinarene (2c). Figure S21: ATP spectrum for tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(nonyl)resorcinarene (2c). Figure S22: IR spectrum for tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(undecyl)resorcinarene (2d). Figure S23: 1H NMR for tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(undecyl)resorcinarene (2d). Figure S24: 13C NMR for tetrasodium-5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(undecyl)resorcinarene (2d).

Author Contributions

Conceptualization, M.M.; methodology, C.U., M.V. and R.S.; software, M.V. and C.U.; validation, M.M. and R.S.; formal analysis, M.M.; investigation, C.U., M.V. and R.S.; resources, writing—original draft preparation, M.M.; writing—review and editing, M.M.; visualization, M.M.; supervision, M.M.; project administration, M.M.; funding acquisition, M.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author(s).

Acknowledgments

The authors would like to thank Universidad Nacional de Colombia.

Conflicts of Interest

The authors declare no conflicts of interest.

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Processes 13 00553 g001
Figure 1. Choline cation importance.
Figure 1. Choline cation importance.
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Figure 2. Crown conformation of resorcinarenes and reactive sites.
Figure 2. Crown conformation of resorcinarenes and reactive sites.
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Scheme 1. General synthesis of resorcinarenes.
Scheme 1. General synthesis of resorcinarenes.
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Scheme 2. General sulfonation of resorcinarenes.
Scheme 2. General sulfonation of resorcinarenes.
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Figure 3. ATR-IR of choline (blue, Processes 13 00553 i002), Sulfomethylated resorcinarene (2d) (red, Processes 13 00553 i003) and equimolar mixture (green, Processes 13 00553 i004).
Figure 3. ATR-IR of choline (blue, Processes 13 00553 i002), Sulfomethylated resorcinarene (2d) (red, Processes 13 00553 i003) and equimolar mixture (green, Processes 13 00553 i004).
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Figure 4. Graph of conductivity as a function of molar ratio 2a/choline.
Figure 4. Graph of conductivity as a function of molar ratio 2a/choline.
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Figure 5. Stacking of 1H-NMR spectra obtained from 2c titration with choline.
Figure 5. Stacking of 1H-NMR spectra obtained from 2c titration with choline.
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Figure 6. Graph of the chemical displacement of the hydrogen meta as a function of the molar ratio obtained from the titration of 2c with choline.
Figure 6. Graph of the chemical displacement of the hydrogen meta as a function of the molar ratio obtained from the titration of 2c with choline.
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Scheme 3. Binding constants (Ka) for hosts 2a–2d with choline.
Scheme 3. Binding constants (Ka) for hosts 2a–2d with choline.
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Table 1. Spectroscopic information of compounds 2a–2d.
Table 1. Spectroscopic information of compounds 2a–2d.
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2a2b2c2d
IR
(ATR/cm−1)		(O-H)3401341434183393
(ArC-H),3070305030503055
(aliphatic C-H)2932292729212851
(S=O)1075104610381039
(S-O)780757706770
13C and 1H NMR *(C-OHa)---
(C) 149.7		---
(C) 153.9		(s, 8H) 9.73
(C) 150.1		(s, 8H) 9.72
(C) 150.4
(C-Hb)(s, 4H) 6.59
(C) 109.1		(s, 4H) 7.17
(C) 110.7		(s, 4H) 7.13
(C) 109.1		(s, 4H) 7.23
(C) 109.6
(C-Hc)(s, 8H) 4.29
(C) 48.1		(s, 8H) 3.95
(C) 52.9		(s, 4H) 3.82
(C) 48.3		(s, 8H) 3.85
(C) 48.7
(C-Hd),(t, 4H) 4.51
(C) 28.4		(t, 4H) 4.29
(C) 35.8		(t, 4H) 4.18
(C) 34.1		(t, 4H) 4.20
(C) 32.3
(Alkyl chain), CH2---(32H)
1.23–2.71
(C)
23.7–35.8		(64H)
1.24–2.18
(C)
22.1–31.3		(80H)
1.25–2.18
(C)
22.6–32.3
(C-H), CH3(t, 12H) 1.40
(C) 20.2		(t, 12H) 0.85
(C) 14.0		(t, 12H) 0.84
(C) 13.9		(t, 12H) 0.83
(C) 14.2
* Solvents for NMR analysis: 2a and 2b in D2O, 2c in DMSO-d6, and 2d in CDCl3-d1.
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Urquijo, C.; Vela, M.; Sarmiento, R.; Maldonado, M. Molecular Interaction of Water-Soluble Resorcinarenes for Potential Choline Detectors. Processes 2025, 13, 553. https://doi.org/10.3390/pr13020553

AMA Style

Urquijo C, Vela M, Sarmiento R, Maldonado M. Molecular Interaction of Water-Soluble Resorcinarenes for Potential Choline Detectors. Processes. 2025; 13(2):553. https://doi.org/10.3390/pr13020553

Chicago/Turabian Style

Urquijo, Cielo, Miguel Vela, Roger Sarmiento, and Mauricio Maldonado. 2025. "Molecular Interaction of Water-Soluble Resorcinarenes for Potential Choline Detectors" Processes 13, no. 2: 553. https://doi.org/10.3390/pr13020553

APA Style

Urquijo, C., Vela, M., Sarmiento, R., & Maldonado, M. (2025). Molecular Interaction of Water-Soluble Resorcinarenes for Potential Choline Detectors. Processes, 13(2), 553. https://doi.org/10.3390/pr13020553

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