Calculation Of Equilibrium Constants Research Articles

Investigation of ion pairs in electrochemical hexacyanoferrate(II)– hexacyanoferrate(III) system in presence of supporting electrolyte

The study of ion pair formation involving redox couples and the cation or anion of supporting electrolytes in solutions is a fundamental question in the field of fluid mechanics and electrochemistry. In this research, density functional theory (DFT) calculations were performed to investigate the ion pair reactions of hexacyanoferrate(III) and hexacyanoferrate(II) with varying numbers of potassium ions from the supporting electrolyte KCl. The binding energy and Gibbs free energy for the ion pair reactions were evaluated. The DFT results revealed a non-monotonic behavior in the binding energy and Gibbs free energy for the ion pair reaction of hexacyanoferrate(III) with different numbers of K ions.Further exploration of the chemical structures of ion pair compounds of hexacyanoferrate(III) and hexacyanoferrate(II) with varying numbers of K ions was conducted using DFT methods. Notably, in the first step of the ion pair reaction with K ions, the iron-potassium chemical bond length in hexacyanoferrate(III) was observed to be smaller than in subsequent steps.The DFT results also provided insights into the capacity of hexacyanoferrate(III) and hexacyanoferrate(II) to accept potassium ions for forming ion pair structures, showing that the former has a lower acceptance capacity than the latter. Furthermore, DFT calculations of equilibrium constants confirmed experimental results based on reduction electrical conductivity data obtained from ion pair formation in the solution.

Using hydrogen bond to promote the ionization of benzene sulfonamide for extracting Li+

Ensuring a continuous and sustainable supply of lithium requires the development of green and efficient separation processes, such as liquid-liquid extraction for primary sources. In this work, a green method of extracting Li+ from Qinghai old brine by using benzene sulfonamides is proposed. The method forms a green extraction system by hydrogen bonding between long-chain alcohol and benzene sulfonamide substance N221. It is proved that the formation of hydrogen bond promotes the ionization of N221 by means of conductivity measurement, equilibrium constant calculation, molecular dynamics simulation. This system has an obvious attraction to Li+, and the highest extraction efficiency of Li+ can reach 95 %, and amine substances are added to maintain the pH of the system. The extraction effect is comparable to TBP (tributyl phosphate) and functional ionic liquid. After stripping, Li+ enters the aqueous phase, and the separation effect is desirable. Therefore, a new green extractant is proposed in this study, which provides a new idea for green and environmentally friendly lithium extraction.

Machine Learning Derived Collective Variables for the Study of Protein Homodimerization in Membrane.

The accurate calculation of equilibrium constants for protein-protein association is of fundamental importance to quantitative biology and remains an outstanding challenge for computational biophysics. Traditionally, equilibrium constants have been computed from one-dimensional free energy surfaces derived from sampling along a single collective variable. Importantly, recent advances in enhanced sampling methodology have facilitated the characterization of multidimensional free energy landscapes, often exposing multiple thermodynamically important minima missed by more restrictive sampling methods. A key to the effectiveness of this multidimensional sampling approach is the identification of collective variables that effectively define the configurational space of dissociated and associated states. Here we present the application of two machine learning methods for the unbiased determination of collective variables for enhanced sampling and analysis of protein-protein association. Our results both validate prior work, based on intuition derived collective variables, and demonstrate the effectiveness of the machine learning methods for the identification of collective variables for association reactions in complex biomolecular systems.

ISOLATION OF FULVIC ACID FROM COAL AND STUDY OF PHYSICAL AND CHEMICAL PROPERTIES

Due to its high solubility and low molecular weight, fulvic acid is a very active participant in natural chemical processes. Due to its smaller molecule size than humic acid, it can act both inside and outside the cell. The work presents the method of isolation and purification of fulvic acid obtained from oxidized brown coal and the analysis of its physical and chemical properties by the methods of IR, NMR spectroscopy, and the amount of carboxyl and hydroxyl groups studied. Conducted work on calculation of equilibrium constant (pK) and concentration (C). Stages of purification: adsorption, ion purification and dialysis, where adsorbents obtained from brown ("Shoptykol") and hard ("Shubarkol") coal and coconut activated carbon were used as adsorbents.

Consistency Checks for Pressure-Volume-Temperature Experiment of Formation Oil and Gas at High Temperature

The oil and gas phase behavior of high temperature is complex and changeable, which is usually obtained by PVT experiments. The accuracy of the experiment data plays a crucial role in the reserve evaluation and development plan of oil and gas reservoirs. However, the current PVT experiment consistency checks are not suitable for high-temperature reservoir conditions. This paper proposes a systematic check method for the PVT experiment data consistency at high temperature. These checks revise the material balance method, Hoffman method, and equilibrium constant method by using the equilibrium constant calculation method at high temperature. The consistency check of component data and constant volume depletion experiment data is carried out by combining the three improved methods with the component check method, so as to judge the experiment data accurately. In this paper, two high-temperature reservoir fluids—gas condensate sample fluid X and volatile oil sample fluid Y—are selected to carry out consistency checks with component data and constant volume depletion data. This check method is of great significance to study the phase behavior of formation oil and gas at high temperature, especially for volatile oil and gas condensate fluid.

Different metal coordination in sub- and super-critical fluids: Do molybdenum(IV) chloride complexes contribute to mass transfer in magmatic systems?

Molybdenum (Mo) is extracted predominantly from magmatic-hydrothermal ore deposits, and knowledge of the thermodynamic properties of aqueous Mo complexes over a wide range in pressures (P) and temperatures (T) is required to predict the transport and deposition of Mo in ore-forming environments. These properties are usually derived from experimental studies conducted over a limited P-T range, with uncertainties increasing as temperature and pressure increase, because of inherent technical difficulties and decreasing number of studies. There is a discrepancy between numerous experimental studies at T < 400 °C and a handful of studies at magmatic-hydrothermal conditions. The low T (<400 °C) experiments suggested that Mo(VI) chloride complexes are unlikely to play a significant role in nature, whereas some studies in magmatic-hydrothermal conditions showed the predominance of Mo(VI) chloride complexes. In an attempt to resolve these inconsistencies, we investigated Mo(VI)-Cl complexations from 350 to 750 °C (500 to 2000 bar) using ab initio molecular dynamics (MD) simulations. The simulations reveal that in high-temperature magmatic-hydrothermal fluids (750 °C, 2000 bar), a tetrahedral Mo(VI) chloride complex, MoO2(OH)Cl(aq), becomes stable; this coordination differs from the octahedral Mo(VI)-chloride complexes identified in highly acidic sub-critical fluids. The ab initio thermodynamic integration method was employed to calculate the equilibrium constants of the reaction.MoO2(OH)Cl(aq) + H2O = MoO2(OH)2(aq) + HCl(aq)Thermodynamic modelling of the speciation using the calculated equilibrium constants shows that at lower temperatures (350 °C, 500 bar), Mo(VI)-chloro complexes are only important at highly acidic conditions (pH < 1), in accordance with previous studies. However, the MD results show that at higher temperatures, Mo-chloro-complexes may be important in transporting Mo in mildly acidic conditions up to the quartz-muscovite-K-feldspar buffered pH. Hence, Mo(VI) chloro-complexes could play an important role in the formation of magmatic-hydrothermal Mo deposits (e.g., Climax-type Mo deposits). We conclude that the mode of metal transport can change dramatically at high temperatures as a result of coordination changes caused by changes in the solvation properties of water; and that quantitative MD is an important tool to support the interpretation of scant experimental results available for magmatic-hydrothermal systems.

Random and block architectures of N-arylitaconimide monomers with methyl methacrylate

Abstract “Itaconimide” is the members of imide (–CO–NH–CO–) family with reactive exocyclic double bond and it is easily obtained from the renewable resource i.e. D-glucose. The polymerization of various N-arylitaconimide (NAI) monomers with methyl methacrylate (MMA) have been reported to improve the glass transition temperature (T g) and thermal stability of poly(methyl methacrylate) (PMMA). In literature, these studies have been done mostly using conventional free radical polymerization methods, which restricts the architecture of copolymers to “random” only. The block copolymers of NAI and MMA are an important due to the combination of glassy PMMA and thermally stable poly(NAI), which offers its applications for higher temperature service. The architectural control of polymers in provisions of its topology, composition, and various functionalities is possibly obtained using reversible-deactivation radical polymerizations (RDRPs). In RDRPs, the concentration of free radical is controlled in such a way that the termination reactions are minimized (normally in range of 1–10 mol%), and not allowed to obstruct with the desired architecture. However, this is possible by achieving (or by establishing) a rapid dynamic equilibrium between propagating radical and dormant species (i.e. R–X). Among all RDRPs, the atom transfer radical polymerization (ATRP) is very popular and adaptable method for the synthesis of polymers with specifically controlled architecture. Two different architectures of NAI and MMA copolymers are reported using ATRP process. The effect of various pedant groups on the rate constants of propagation (k p) and thermal properties NAI and MMA copolymers is studied. The poly(NAI-ran-MMA)-b-poly(MMA) are stable up to 200 °C and degraded in three steps. Whereas, the poly(NAI-ran-MMA)-b-poly(NAI) are stable up to 330 °C and degraded in two steps. The density functional theory methods are used for calculation of equilibrium constants (K ATRP) for the ATRP process for the series of laboratory synthesized alkyl halides. A good agreement was observed between the experimentally determined and theoretically calculated K ATRP values. The mechanistic studies are carried for poly(NAI-ran-MMA) copolymer system using statistical model discrimination method along with 1H decoupled 13C NMR spectroscopy. For studying the mechanism of copolymerization of NAI and MMA via ATRP methods, “trimer model or penultimate model” will be more accurate than “dimer model or terminal model”.

Charge Regulation of Poly(acrylic acid) in Solutions of Non-Charged Polymer and Colloids.

Weak polyelectrolytes (WPEs) are responsive materials used as active charge regulators in a variety of applications, including controlled release and drug delivery in crowded bio-related and synthetic environments. In these environments, high concentrations of solvated molecules, nanostructures, and molecular assemblies are ubiquitous. Here, we investigated the effect of high concentrations of non-adsorbing, short chains of poly(vinyl alcohol), PVA, and colloids dispersed by the very same polymers on charge regulation (CR) of poly(acrylic acid), PAA. PVA does not interact with PAA (throughout the full pH range) and thus can be used to examine the role of non-specific (entropic) interactions in polymer-rich environments. Titration experiments of PAA (mainly 100 kDa in dilute solutions, no added salt) were carried out in high concentrations of PVA (13-23 kDa, 5-15 wt%) and dispersions of carbon black (CB) decorated by the same PVA (CB-PVA, 0.2-1 wt%). The calculated equilibrium constant (and pKa) was up-shifted in PVA solutions by up to ~0.9 units and down-shifted in CB-PVA dispersions by ~0.4 units. Thus, while solvated PVA chains increase the charging of the PAA chains, as compared to PAA in water, CB-PVA particles reduce PAA charging. To investigate the origins of the effect, we analyzed the mixtures using small-angle X-ray scattering (SAXS) and cryo-TEM imaging. The scattering experiments revealed re-organization of the PAA chains in the presence of the solvated PVA but not in the CB-PVA dispersions. These observations clearly indicate that the acid-base equilibrium and the degree of ionization of PAA in crowded liquid environments is affected by the concentration, size, and geometry of seemingly non-interacting additives, probably due to depletion and excluded volume interactions. Thus, entropic effects that do not depend on specific interactions should be taken into consideration when designing functional materials in complex fluid environments.

Optical Behavior of Nile Red in Organic and Aqueous Media Environments

A simple model is proposed to calculate the dimerization constant of a dye in non-aqueous solvents. Alkan-1ols of the formula H–(CH2)n–OH are used to study the spectroscopic behavior of Nile Red dye. The number n varied from 1 to 8 to modulate the medium hydrophobicity. Generally, Nile red is used to localize lipid droplets within cells. This molecule is non-fluorescent in water and other polar solvents but undergoes fluorescence enhancement and large absorption and emission blue shifts in non-polar environments. The calculated equilibrium constants suggest that the aggregation process is solvent-assisted. The absorption and fluorescence emission spectra reveal a marked red shift, which is studied by breaking the wavelength of the maximum band into two terms, showing the contribution of the solvent and the effect of the dye concentration. Both contributions were investigated as a function of the number n, and it was found that alkan-1ols with large n tend to aggregate and produce a smaller red shift. Conversely, it was also noticed that short-chain alkan-1ols stabilize the excited state of the dye via H-bond and the red shift increases. The hydrophilicity of the medium was found to be modulated by adding pure water, in a controlled way, to the binary systems dye–H–(CH2)n–OH (n = 1–8). The quantification of solvent hydrophilicity is described with the ratio R = water moles/alcohol moles. From this investigation, we realized that the absorption spectra values are strictly connected with the R parameter. In this context, we realized that fluorescence emission spectra allow us to determine the adjustable parameters.

Influence of the model used for the calculation of equilibrium constants at moderate temperature for electrolytes

The formation of a solid mineral deposit can cause major problems in the conduct of industrial processes such as high pressure acid leaching. Knowledge of the chemical speciation and solubility product of the mineral can help control or inhibit its formation. This speciation is based on knowledge of the thermodynamic equilibrium constants of the dissociation/complexation reactions involved in the system and their temperature dependence. It also depends on the concentration of the different species in the system, the pressure and the activity coefficients of the chemical species present in their molecular or electrolytic form. From these thermodynamic quantities and the state of the system, it is possible to predict the direction of evolution of the reaction, i.e. whether the reactants or the products of the chemical reactions are favoured. This work presents and compares different models for the calculation of activity coefficients for ionic strengths between 0.001 – 6 molal and the calculation of thermodynamic equilibrium constants at temperatures up to 300°C (at zero ionic strength). It confirms the need for reliable thermodynamic data for modeling aqueous systems.

Temperature and Pressure-Dependent Rate Constants for the Reaction of the Propargyl Radical with Molecular Oxygen.

Ab initio CCSD(T)/CBS(T,Q,5)//B3LYP/6-311++G(3df,2p) calculations have been conducted to map the C3H3O2 potential energy surface. The temperature- and pressure-dependent reaction rate constants have been calculated using the Rice–Ramsperger–Kassel–Marcus Master Equation model. The calculated results indicate that the prevailing reaction channels lead to CH3CO + CO and CH2CO + HCO products. The branching ratios of CH3CO + CO and CH2CO + HCO increase both from 18 to 29% with reducing temperatures in the range of 300–2000 K, whereas CCCHO + H2O (0–10%) and CHCCO + H2O (0–17%) are significant minor products. The desirable products OH and H2O have been found for the first time. The individual rate constant of the C3H3 + O2 → CH2CO + HCO channel, 4.8 × 10–14 exp[(−2.92 kcal·mol–1)/(RT)], is pressure independent; however, the total rate constant, 2.05 × 10–14 T0.33 exp[(−2.8 ± 0.03 kcal·mol–1)/(RT)], of the C3H3 + O2 reaction leading to the bimolecular products strongly depends on pressure. At P = 0.7–5.56 Torr, the calculated rate constants of the reaction agree closely with the laboratory values measured by Slagle and Gutman [Symp. (Int.) Combust.1988, 21, 875−883] with the uncertainty being less than 7.8%. At T < 500 K, the C3H3 + O2 reaction proceeds by simple addition, making an equilibrium of C3H3 + O2 ⇌ C3H3O2. The calculated equilibrium constants, 2.60 × 10–16–8.52 × 10–16 cm3·molecule–1, were found to be in good agreement with the experimental data, being 2.48 × 10–16–8.36 × 10–16 cm3·molecule–1. The title reaction is concluded to play a substantial role in the oxidation of the five-member radicals and the present results corroborate the assertion that molecular oxygen is an efficient oxidizer of the propargyl radical.

Simple Approximation for the Ideal Reference State of Gases Adsorbed on Solid-State Surfaces

Reference states are useful as models for facilitating calculations of equilibrium constants, and they may also serve as standard states that are convenient for organizing and tabulating thermodynamic data; however, standard state conventions and appropriate reference states for adsorbed species have received less attention than those for pure substances and solutes. Here, we compare seven choices of reference states for calculations of equilibrium constants and transition state theory rate constants for flat surfaces, in particular (1) an ideal 2D harmonic oscillator, (2) an ideal rigid-molecule harmonic oscillator, (3) an ideal 2D harmonic oscillator with separable surface modes, (4) a 2D ideal gas, (5) an ideal 2D hindered translator, (6) an ideal 2D hindered translator with lowest-order barriers, and (7) a simple ideal 2D hindered translator proposed in this work. The advantage of models 5-7 is that they can treat both mobile and localized adsorbates in a consistent way, whereas models 1-3 are only appropriate for localized adsorbates, and model 4 is only appropriate for a freely translating adsorbate. Furthermore, models 6 and 7 reduce the computational cost without the user having to calculate barrier heights for diffusion. An advantage of the simple ideal 2D hindered translator is that it has a physical high-temperature limit. We also propose a reference state for nonflat surfaces. The user is encouraged to choose a reference state based on the appropriateness of the model and the practicality of the calculations.

Comments on the calculation of the standard equilibrium constant using the Langmuir model in Journal of Hazardous Materials 422 (2022) 126863

The calculation of equilibrium constant from the Langmuir model is widely used in the literature. However, the dimensional problem of the equilibrium constant has often been ignored. For example, a recent publication on the strong properties of the new adsorbent for tetracycline. Nevertheless, this used an improper calculation of the standard equilibrium constant, confusing solute and solution. This creates dimension for K0 obtained from Eq. (13). Unfortunately, K0 should be dimensionless. In this comment, the origin and background of this kind of error is analyzed and the method for correcting the error is presented. Specifically, we establish a clear relationship between the Langmuir constant (KL) and the standard equilibrium constant (K0). We hope this comment clarifies the essence of calculating the standard equilibrium constant by using the Langmuir model, so others avoid the propagation of this kind of error.

Modulating the thermodynamics, kinetics and photochemistry of 7-diethylamino-4'-dimethylaminoflavylium in water/ethanol, SDS and CTAB micelles.

The thermodynamics and kinetics of compound 7-diethylamino-4'-dimethylaminoflavylium were studied in water : ethanol (1 : 1) and water in the presence of SDS and CTAB micelles. The blue flavylium cation is in equilibrium with the pink protonated flavylium cation defined by pKAH2+/AH+ and the yellow trans-chalcone, defined by pKAH+/Ct. The difference between these two pKs gives the pH domain of the flavylium cation, ΔpK = 1.95 in CTAB, ΔpK = 5.6 in water/ethanol (1 : 1) and ΔpK = 8.5 in SDS micelles. On the other hand, the pH domain of the trans-chalcone is limited by pKAH+/Ct and pKCt/Ct-. It is lower in SDS micelles ΔpK = 2.7, increases in ethanol/water (1 : 1) ΔpK = 5.1 and is maximum in CTAB micelles, ΔpK = 6.8. All these effects can be explained by the electric charge present at the micellar surface. Relative energy level diagrams that allow for the explanation of the driving forces for any pH stimuli or light absorption were constructed from the calculated equilibrium constants. Irradiation of the trans-chalcone at 466 nm leads to the formation of the flavylium cation. In water : ethanol (1 : 1), the photochemistry is residual with Φ < 0.00002, while in SDS micelles at pH = 7 light increases the rate of the spontaneous conversion of trans-chalcone to the flavylium cation, with quantum yield Φ = 0.002; photochromism from trans-chalcone to give the flavylium cation with the same quantum yield is also observed in CTAB micelles.

Thermodynamic study of canned mince meat and fish in metal cans during 30 months storage

Long storage period for the meat and fish cans of 30 months was presented. The provide investigation aims to found thermodynamic stability in mackerel in tomato sauce and finely ground pork minced meat. During the storage period a decrease in pH in two types of cans were seen. pH values for mackerel in tomato sauce and for finely ground pork minced meat cans were measured between 6.2±0.19 to 4.8±0.22 and 5.8 ±0.24 to 3.9±0.17 in the starting to the end of storage, respectively. Determined absorption is used to calculated equilibrium constant, Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS). For mackerel in tomato sauce ΔG exhibit value -8.33±0.07 kJ.mol−1 and for finely ground pork -6.79±0.09 kJ.mol−1. Microscopic examinations were performed and the size of the particles was determined. In measurements in mackerel in tomato sauce the sizes are between 25±0.79 to 62±1.28 μm and for finely ground pork minced meat the particle sizes are between 32±1.12 to 89±2.48 μm in the investigation period. Based on the experiment, both types of canned food are defined as thermodynamically stable after negative Gibbs energies observations, but canned fish is defined as more stable and may have a longer stored.

Platinum(II)‐mediated disulfide/thiolate interconversion in organic disulfides: Density functional theory thermodynamic study

AbstractWe present a complete Density Functional Theory (DFT) study of the thermodynamics of the reactions of disulfides (l‐cystine, cystamine, dl‐homocystine, and 3,3′‐dithiodipropionic acid) with Pt(II) ions in aqueous media. SMD solvation model was fitted to take into account the solvation effects of the medium. DFT calculations predict the oxidative addition reaction with the formation of thiolate complexes of Pt(IV)—[PtCl4(S‐R)2] which is consistent with experimental data. We propose a theoretical model that rationalizes the experimental data on Pt(II) interactions with homological disulfides. The role of explicit solvation in reactions is also investigated. We propose that the stability of S,S′‐binuclear disulfide complexes correlates with the redistribution of charges on sulfur atoms. We compared MN15 and PBE0 functionals for the calculation of equilibrium constants.

Synthesis, identification, density functional and Hirshfeld surface studies of 2,2'-disulfanediylbis(tetrahydro-4H-cyclopenta[d][1,3,2]dioxaphosphole-2-sulfide).

The new compound 2,2'-disulfanediylbis (tetrahydro-4H-cyclo penta[d][1,3,2]dioxaphosphole 2-sulfide), the dimeric form of 2-mercaptotetrahydro-4H-cyclopenta[d] [1,3,2] dioxaphosphole 2-sulfide, has been synthesized and characterized by elemental analysis, molecular weight determination and spectral data (1 H-NMR, 13 C-NMR, 31 P-NMR, FTIR). The molecular geometry was confirmed by single X-Ray crystallography. The ground state property was examined by PBE0 and B3LYP density functionals using aug-cc-pV(Q+d)Z basis set in the gas phase and in DMSO solution. The preference of PBE0 functional was statistically established. Thermodynamic parameters and standard heat of dissociation reaction ( ) have been established. The calculated equilibrium constants at different temperatures reflect the stability of the dimer over the monomers at low temperatures and vice versa. Valency and Fukui indices calculations showed that the monomer is more reactive than the dimer. 2D-fingerprint revealed that, while the H…X; [X=H, O and S] nonbonding intermolecular interactions and reciprocals play a crucial role in strengthening of molecules packing in the crystal unit cell while the S…S ones contribute negatively on it.

Molecular and Mechanistic Characterization of PddB, the First PLP-Independent 2,4-Diaminobutyric Acid Racemase Discovered in an Actinobacterial D-Amino Acid Homopolymer Biosynthesis.

We recently disclosed that the biosynthesis of antiviral γ-poly-D-2,4-diaminobutyric acid (poly-D-Dab) in Streptoalloteichus hindustanus involves an unprecedented cofactor independent stereoinversion of Dab catalyzed by PddB, which shows weak homology to diaminopimelate epimerase (DapF). Enzymological properties and mechanistic details of this enzyme, however, had remained to be elucidated. Here, through a series of biochemical characterizations, structural modeling, and site-directed mutageneses, we fully illustrate the first Dab-specific PLP-independent racemase PddB and further provide an insight into its evolution. The activity of the recombinant PddB was shown to be optimal around pH 8.5, and its other fundamental properties resembled those of typical PLP-independent racemases/epimerases. The enzyme catalyzed Dab specific stereoinversion with a calculated equilibrium constant of nearly unity, demonstrating that the reaction catalyzed by PddB is indeed racemization. Its activity was inhibited upon incubation with sulfhydryl reagents, and the site-directed substitution of two putative catalytic Cys residues led to the abolishment of the activity. These observations provided critical evidence that PddB employs the thiolate-thiol pair to catalyze interconversion of Dab isomers. Despite the low levels of sequence similarity, a phylogenetic analysis of PddB indicated its particular relevance to DapF among PLP-independent racemases/epimerases. Secondary structure prediction and 3D structural modeling of PddB revealed its remarkable conformational analogy to DapF, which in turn allowed us to predict amino acid residues potentially responsible for the discrimination of structural difference between diaminopimelate and its specific substrate, Dab. Further, PddB homologs which seemed to be narrowly distributed only in actinobacterial kingdom were constantly encoded adjacent to the putative poly-D-Dab synthetase gene. These observations strongly suggested that PddB could have evolved from the primary metabolic DapF in order to organize the biosynthesis pathway for the particular secondary metabolite, poly-D-Dab. The present study is on the first molecular characterization of PLP-independent Dab racemase and provides insights that could contribute to further discovery of unprecedented PLP-independent racemases.

Supramolecular Zn(II)-Dipicolylamine-Azobenzene-Aminocyclodextrin-ATP Complex: Design and ATP Recognition in Water.

Cyclodextrins (CyDs) are water-soluble host molecules possessing a nanosized hydrophobic cavity. In the realm of molecular recognition, this cavity is used not only as a recognition site but also as a reaction medium, where a hydrophobic sensor recognizes a guest molecule. Based on the latter concept, we have designed a novel supramolecular sensing system composed of Zn(II)-dipicolylamine metal complex-based azobenzene (1-Zn) and 3A-amino-3A-deoxy-(2AS,3AS)-γ-cyclodextrin (3-NH2-γ-CyD) for sensing adenosine-5′-triphosphate (ATP). 1-Zn showed redshifts in the UV-Vis spectra and induced circular dichroism (ICD) only when both ATP and 3-NH2-γ-CyD were present. Calculations of equilibrium constants indicated that the amino group of 3-NH2-γ-CyD was involved in the formation of supramolecular 1-Zn/3-NH2-γ-CyD/ATP. The Job plot of the ICD spectral response revealed that the stoichiometry of 1-Zn/3-NH2-γ-CyD/ATP was 2:1:1. The pH effect was examined and 1-Zn/3-NH2-γ-CyD/ATP was most stable in the neutral condition. The NOESY spectrum suggested the localization of 1-Zn in the 3-NH2-γ-CyD cavity. Based on the obtained results, the metal coordination interaction of 1-Zn and the electrostatic interaction of 3-NH2-γ-CyD were found to take place for ATP recognition. The “reaction medium approach” enabled us to develop a supramolecular sensing system that undergoes multi-point interactions in water. This study is the first step in the design of a selective sensing system based on a good understanding of supramolecular structures.

Isotopic equilibrium constants for very low-density and low-temperature nuclear matter

Yields of equatorially emitted light isotopes, $1\le Z\le 14$, observed in ternary fission in the reaction $^{241}$Pu($n_{\rm th}$,f) are employed to determine apparent chemical equilibrium constants for low-temperature and low-density nuclear matter. The degree of liberation and role of medium modifications are probed through a comparison of experimentally derived reaction quotients with equilibrium constants calculated using a relativistic mean-field model employing a universal medium modification correction for the attractive $\sigma$ meson coupling. The results of these comparisons indicate that equilibrium is achieved for the lighter ternary fission isotopes. For the heavier isotopes experimental reaction quotients are well below calculated equilibrium constants. This is attributed to a dynamical limitation reflecting insufficient time for full equilibrium to develop. The role of medium effects leading to yield reductions is discussed as is the apparent enhancement of yields for $^8$He and other very neutron rich exotic nuclei.