Search Results (104)

A brief summary of the effect of nonspecific interactions upon chemical equilibria in solutions containing a high total concentration of macromolecular solutes comparable to that found in biological fluid media is presented. Analyses of experimental measurements permitting relatively direct quantitation of the free energy of nonspecific intermolecular interaction in solutions of one or two macrosolutes are described, and a table listing published experimental studies of both homo- and hetero-interactions is provided. Methods for calculating the free energy of nonspecific interaction via theory and computer simulation are described. Recommendations for further progress in both measurement and calculation of interaction free energies are presented. Full article

An experimental and computational methodology was developed for ionic speciation of Pb²⁺ and Cd²⁺ with dissolved organic matter (DOM) in surface seawater (SSW) from the Bay of Bengal (BoB) in eastern Bangladesh. Differential pulse anodic stripping voltammetry (DPASV) with a thin mercury film glassy carbon electrode (TMFGC) was used to measure free and DOM-bound Pb²⁺ and Cd²⁺. A continuous binding model was used to calculate the binding constants for metal ions with experimentally found complex ligands like DOM in the BoB. The ionic speciation and distribution of all major naturally occurring ions and toxic Pb²⁺, Cd²⁺, and DOM were calculated using a computational chemical equilibrium model, MINTEQA. We found that the change in pH with increasing dissolved carbon dioxide due to global warming will cause drastic changes in the bioavailability of Pb²⁺ by the year 2050. Full article

In the quest of the pivotal origin of the very strong gas-phase proton basicity for some iminopyrrole derivatives, proposed in the literature on the basis of quantum chemical calculations, the full tautomeric and acid/base equilibria were investigated in vacuo for 2-aminopyrrole exhibiting enamino–imino tautomerism. Thermochemistry of these processes investigated at the Density Functional Theory (DFT) level indicates a lower stability for the imino than for the enamino tautomers. However, the imino N atom in the imino forms displays an exceptionally high basicity, particularly in the minor and rare tautomers containing at least one tautomeric proton at the pyrrole C atom. This explains why derivatives of CH tautomers (being free of prototropy) display exceptionally high gas-phase proton basicity. As predicted by the Maksić group using quantum chemical methods, these derivatives can be considered as good organic imino N-superbase candidates. Unfortunately, some other structures of iminopyrrole derivatives (proposed by the same group) possess labile protons, and, thus, exhibit prototropy, resulting in the transformation into the more stable but less basic aminopyrrole derivatives under synthesis conditions or acid/base equilibria measurements. Full article

All modern process simulators rely on thermodynamic methods to estimate physical properties and calculate phase equilibria. The critical properties of individual components or pseudo-components, which represent undefined mixtures, play a crucial role in these calculations. However, the chemical compositions and characteristics of whole crude oils, petroleum fractions, and fuels, which are very complex mixtures of individual hydrocarbons, can vary significantly depending on the specific crude oil and the processing involved. For instance, straight-run petroleum fractions differ from those obtained through cracking processes due to differences in unsaturated hydrocarbon content. Consequently, effective methods for predicting critical temperature and pressure must account for a wide range of compositional scenarios. To address this challenge, we utilized a database of 176 individual hydrocarbons to evaluate the existing correlations for critical temperature and pressure calculations. Intercriteria analysis was performed to evaluate the relations between the different variables to be used for critical temperature and pressure predictions. Additionally, we proposed new correlations and ANN models for these properties and assessed their performance. Our study aims to provide robust predictive models that can accurately estimate critical properties across diverse petroleum fractions and compositions. Full article

Molecular network conformations in the over-stoichiometric arsenoselenides of canonical As_xSe_100−x system (40 ≤ x ≤ 100) covering a full row of thioarsenide-type As₄Se_n entities (0 ≤ n ≤ 6) are analyzed with ab initio quantum-chemical modeling employing cluster-simulation code CINCA. Native (melt-quenching-derived) and nanostructurization-driven (activated by nanomilling) polymorphic and polyamorphic transitions initiated by decomposition of the thioarsenide-type As₄Se_n cage molecules and incorporation of their remnants into a newly polymerized arsenoselenide network are identified on the developed map of molecular network clustering in a binary As-Se system. Within this map, compositional counter lines corresponding to preferential molecular or network-forming tendencies in the examined arsenoselenides are determined, explaining that network-crystalline conformations prevail in the boundary compositions corresponding to n = 6 and n = 0, while molecular-crystalline ones dominate inside the rows corresponding to n = 4 and n = 3. A set of primary and secondary equilibrium lines is introduced in the developed clustering map to account for inter-phase equilibria between the most favorable (regular) and competitive (irregular) thioarsenide phases. Straightforward interpretation of decomposition reactions accompanying induced crystallization and amorphization (reamorphization) in the arsenoselenides is achieved, employing disproportionality analysis of thioarsenide-type molecular network conformations within the reconstructed clustering map. The preference of network clustering at the boundaries of the As₄Se_n row (at n = 6 and n = 0) disturbs inter-phase equilibria inside this row, leading to unexpected anomalies, such as absence of stable tetra-arsenic triselenide As₄Se₅ molecular-crystalline species; polyamorphism in mechanoactivated As₄Se_n alloys (2 ≤ n ≤ 6); breakdown in the glass-forming ability of melt-quenching-derived arsenoselenides in the vicinity of tetra-arsenic biselenide As₄Se₂ composition; plastically and normally crystalline polymorphism in tetra-arsenic triselenide As₄Se₃-based thioarsenides, and so on. Full article

Fourteen non-colored flavonoids, including flavanols (catequine and epicatequine) and flavonols (myricetin, quercetin, and isorhamnetin and their glucoside/glucuronide derivatives) were investigated in over two hundred monovarietal red wines from the Canary Islands, as a continuation of a previous study available in this journal. This is the first comprehensive study on non-colored flavonoids in monovarietal Canary wines, highlighting their potential as chemical fingerprints for wine characterization. Flavanal and flavanol concentrations were similar to values reported in other regions. Concentrations of these flavonoids significantly varied by grape cultivar, denomination of origin (DO), island, and aging. International grape cultivars generally showed higher non-colored flavonoid concentrations than autochthonous cultivars. Strong correlations were observed among flavonols of the same chemical groups, as well as between flavonols and anthocyanins, indicating a shared biosynthesis pathway in grapes and equilibria in wine. Principal component analysis revealed relationships among the individual flavonoids. Lineal discriminant analysis (LDA) resulted in high percentages of correct classification by cultivar, DO, precedence island, and aging. The classification accuracy achieved through LDA, except for aging, notably improved compared to the previous study that examined only visible flavonoids, underscoring the importance and effectiveness of non-colored flavonoid profiling. Full article

The Bozhushan in southeastern Yunnan is a composite granite body that was formed by multi-phase magmatic intrusion. The genesis of the tourmaline-supergroup minerals occurring therein remains uncertain, as it has been the subject of only a limited number of studies. This investigation employs an integrated analytical approach combining EPMA, LA-ICP-MS, and boron isotope geochemistry, supplemented by detailed field geological investigations and petrographic observations of tourmaline textural characteristics. This study aims to elucidate the genetic relationships between distinct tourmaline varieties, establish temporal correlations between mineral crystallization stages and magmatic–hydrothermal evolution processes, and evaluate the petrogenetic significance of tourmaline geochemical signatures for regional mineralization events. This study analyzes tourmaline-supergroup minerals in granitic pegmatites and aplites, which occur as nodular, radial, and columnar aggregates. Most tourmaline crystals exhibit well-defined rhythmic zoning patterns, which are clearly observable under cross-polarized light microscopy. Chemical composition analysis has identified two tourmaline species: schorl and dravite. The formation of tourmaline is primarily of magmatic origin and is characterized by a magmatic–hydrothermal transition. It predominantly belongs to the alkali subgroup and is formed in Li-poor granitoids and associated pegmatites and aplites, Ca-poor metapelites, metapsammites, and quartz-tourmaline rocks. The inter-ionic substitution mechanism in this system is predominantly governed by Fe²⁺Mg₋₁ and (X_vacAl)(NaR²⁺)₋₁ exchange equilibria. Additionally, geochemical evidence indicates that the primary ore-forming fluids originate from granitic magmas, which are likely sourced from the partial melting of metasedimentary rocks. During the late Yanshan period, the upwelling of granitic magma in the Bozhushan area introduced a substantial heat source and mineralizing fluids, which interacted with the Cambrian units to form tungsten–tin mineralization. The geochemical data on tourmaline indicate that the Bozhushan granite body has considerable potential for ore mineralization. Full article

Prototropic conversion (prototropy) for heterocyclic nucleobases was already signaled by Watson and Crick about seventy years ago as one of the reasons for nucleic acids mutations. This isomeric phenomenon has been investigated for neutral derivatives by means of both experimental and theoretical procedures, and their favored tautomers discussed in numerous articles published in the last fifty years. Protonation/deprotonation reactions in the gas phase have also been studied using both quantum-chemical calculations and experimental techniques. Some thermochemical parameters of these processes have been documented. However, prototropy has not always been taken into account in protonation/deprotonation reactions. Most frequently, tautomeric heterocycles have been treated as simple polyfunctional compounds without possible intramolecular protontransfers. Taking into account the lack of data for the complete tautomeric mixtures, quantum-chemical investigations have been undertaken by us about twenty-five years ago for prototropic heterosystems. In this work, the pyrimidine base uracil (U) was chosen. It possesses two identical exo groups (=O/OH) at the 2- and 4-positions, two labile (tautomeric) protons, and five conjugated sites (N1, N3, C5, O7, and O8). Different types of isomerism, prototropy and OH-rotation, were considered for the neutral, protonated, and deprotonated forms. Using quantum-chemical methods, thermochemical stabilities of all possible tautomers-rotamers were examined in vacuo and the potential isomers selected. The selected isomeric mixtures for the neutral and ionic forms were applied for the determination of the thermochemical parameters in the four-step acid/base equilibria: B²⁻ BH⁻ BH₂ BH₃⁺ BH₄²⁺, where BH₂ indicates U. For each step, the microscopic (kinetic) and macroscopic (thermodynamic) acid/base parameters were estimated, and sites of the proton gain and proton loss examined. The similarities and differences between the acid/base equilibria for uracil and other pyrimidine nucleobases were discussed. Full article

Push-pull imines with strong electron donor group(s) display exceptional basicity in the gas phase. Most of them do not exhibit prototropic tautomerism, and gas-phase acid-base equilibria have been already well described and reviewed. Some questions remain for tautomeric systems, particularly for their uncommon forms. As shown by quantum-chemical calculations, some often-neglected tautomers display higher basicity than the thermodynamically favored forms. However, their participation in tautomeric mixtures being in equilibrium is negligible, and their basicity can be impossible to measure in the gas phase by the equilibrium method. During this work, we examined the gas-phase proton basicity for some acyclic and cyclic push-pull organic bases containing the tautomeric amidine or guanidine group. By quantum-chemical calculations, we confirmed the existence of very low amounts of rare tautomeric forms, in particular, those bearing a methylidene (=CH₂) group. We also demonstrated that the alkyl derivatives of rare tautomers, being free of prototropy, can be good candidates as very strong push-pull C bases, i.e., bases protonated on the =CH₂ group. Full article

Using the framework of an investigation of the stimuli-responsive behavior of peptide assembly on a solid surface, this study on the behavior of a chemisorbed peptide on a gold surface was performed. The studied peptide is a dimeric form of the antimicrobial peptide Trichogin GAIV, which was also modified by substituting the glycine with lysine residues, while the N-terminus octanoyl group was replaced by a lipoic one that was able to bind to the gold surface. In this way, a chemically linked peptide assembly that is pH-responsive was obtained because of the protonation/deprotonation of the sidechains of the Lys residues. Information about the effect of protonation/deprotonation equilibria switching the pH from acid (pH = 3) to basic (pH = 11) conditions was obtained macroscopically by performing Quartz crystal microbalance with dissipation monitoring (QCM-D), Surface Plasmon Resonance (SPR), Nanoplasmonic Sensing (NPS), and FTIR techniques. Using molecular dynamics (MD) simulations, it is possible to explain, at the molecular level, our main experimental results: (1) pH changes induce a squeezing behavior in the system, consisting in thickness and mass variations in the peptide layer, which are mainly due to the pH-driven hydrophilic/hydrophobic character of the lysine residues, and (2) the observed hysteresis is due to small conformational rearrangements from helix to beta sheets occurring mainly on the first half of the peptide, closer to the surface, while the second half remains almost unaffected. The latter result, together with the evidence that the layer thickness is not simply double the assembly of the monomeric analog, indicates that the dimeric peptide does not behave as a double monomer, but assumes very peculiar features. Full article

In the present paper, a modification of the standard mean-field model is considered, allowing for the description of the formation of a dynamic equilibrium between infected and recovered persons in a population of constant size. The key point of this model is that it highlights two-infection transfer mechanisms depending on the physical nature of the contact between people. We separate the transfer mechanism related directly to the movement of people (the so-called transport processes) from the one occurring at zero relative speed of persons (the so-called social contacts). Under the framework of a physical chemical analogy, the dependencies for the infection transfer rate constants are proposed for both purely transport and social mechanisms of spread. These dependencies are used in discussing the formation of quasi-stationary states in the model, which can be interpreted as endemic equilibrium states. The stability of such endemic equilibria is studied by the method of Lyapunov function. Full article

Double hydrophilic, random, hyperbranched copolymers were synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization of oligo(ethylene glycol) methyl ether methacrylate (OEGMA) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) utilizing ethylene glycol dimethacrylate (EGDMA) as the branching agent. The resulting copolymers were characterized in terms of their molecular weight and dispersity using size exclusion chromatography (SEC), and their chemical structure was confirmed using FT-IR and ¹H-NMR spectroscopy techniques. The choice of the two hydrophilic blocks and the design of the macromolecular structure allowed the formation of self-assembled nanoparticles, partially due to the pH-responsive character of the DMAEMA segments and their interaction with -COOH end groups remaining from the chain transfer agent. The copolymers showed pH-responsive properties, mainly due to the protonation–deprotonation equilibria of the DMAEMA segments. Subsequently, a nanoscopic polymer–lipid (lipomer) mixed system was formulated by complexing the synthesized copolymers with cosmetic amphiphilic emulsifiers, specifically glyceryl stearate (GS) and glyceryl stearate citrate (GSC). This study aims to show that developing lipid–polymer hybrid nanoparticles can effectively address the limitations of both liposomes and polymeric nanoparticles. The effects of varying the ionic strength and pH on stimuli-sensitive polymeric and mixed polymer–lipid nanostructures were thoroughly investigated. To achieve this, the structural properties of the hybrid nanoparticles were comprehensively characterized using physicochemical techniques providing insights into their size distribution and stability. Full article

Hydrogen and carbon monoxide over-equilibria have been found computationally in kinetic dependencies of methane-reforming catalytic reactions (steam and dry reforming) using the conditions of the conservatively perturbed equilibrium (CPE) phenomenon, i.e., at the initial equilibrium concentration of hydrogen or carbon monoxide. The influence of the pressure, temperature, flow rate and composition of the initial mixture on the position of the CPE point (the extremum point) was investigated over a wide domain of parameters. The CPE phenomenon significantly increases the product concentration (H₂ and CO) at the reactor length, which is significantly less than the reactor length required to reach equilibrium. The CPE point is interpreted as the “turning point” in kinetic behaviour. Recommendations on temperature and pressure regimes are different from the traditional ones related to Le Chatelier’s law. The obtained results provide valuable information on optimal reaction conditions for complex reversible chemical transformations, offering potential applications in chemical engineering processes. Full article

Formic acid is a promising candidate fuel that can be produced by reacting renewable hydrogen with carbon dioxide. However, the burning safety characteristics of formic acid–air mixtures have not been fully studied. This paper presents an extensive theoretical study of the adiabatic explosion pressure of formic acid–air premixed laminar flames at various initial conditions (composition of formic acid: 17–38% volume; initial pressure: 0.1–1.5 bar; initial temperature: 333–500 K), using the GASEQ software package. GASEQ software calculates chemical equilibria based on ideal gas behavior and is based on the hypothesis of adiabatic expansion inside a closed containment that allows ideal expansion. The influence of the initial conditions (pressure, temperature, and concentration) of formic acid–air mixtures on the adiabatic explosion pressures, maximum flame temperature, and peak concentrations of the main reaction intermediates is investigated and discussed. It is found that the adiabatic peak explosion pressure (calculated equilibrium pressure) of the studied concentrations decreases with increasing initial temperature and increases linearly with increasing initial pressure. Full article

Nicotinamide mononucleotide (NMN), the direct precursor of nicotinamide adenine dinucleotide (NAD⁺), is involved in the regulation of many physiological and metabolic reactions in the body. NMN can indirectly affect cellular metabolic pathways, DNA repair, and senescence, while also being essential for maintaining tissues and dynamic metabolic equilibria, promoting healthy aging. Therefore, NMN has found many applications in the food, pharmaceutical, and cosmetics industries. At present, NMN synthesis strategies mainly include chemical synthesis and biosynthesis. Despite its potential benefits, the commercial production of NMN by organic chemistry approaches faces environmental and safety problems. With the rapid development of synthetic biology, it has become possible to construct microbial cell factories to produce NMN in a cost-effective way. In this review, we summarize the chemical and biosynthetic strategies of NMN, offering an overview of the recent research progress on host selection, chassis cell optimization, mining of key enzymes, metabolic engineering, and adaptive fermentation strategies. In addition, we also review the advances in the role of NMN in aging, metabolic diseases, and neural function. This review provides comprehensive technical guidance for the efficient biosynthesis of NMN as well as a theoretical basis for its application in the fields of food, medicine, and cosmetics. Full article