Enthalpies of solution and solvation of amides in N,N-dimethylformamide: Application of the random contact point approach

Calorimetrically determined molar enthalpies of solution at infinite dilution in N,N-dimethylformamide and densities of several amides at 25°C are reported. Some of the enthalpies are combined with literature data for enthalpies of vaporization to obtain molar enthalpies of solvation. Relations are found between the enthalpies of solution and the size, and between these enthalpies and the enthalpic pair interaction coefficients of the solute molecules. These relations are quantified by an extension of the random contact point approach. This additivity scheme is also applied to enthalpies of solvation, vaporization and cavity formation. With this approach thermodynamic quantities of solution, solvation, vaporization, and pair interaction of different solutes and solvents are correlated with a single consistent set of group interaction parameters. In addition, the random contact point model provides a simple method to calculate thermodynamics of cavity formation which appear to be as reliable as those of the much more complicated scaled particle theory.     

Standard molar enthalpies of solvation of aliphatic esters in 1,2-dichloroethane andn-hexane

The molar enthalpies of solution at infinite dilution of methyl-, ethyl-, n-butyl-and n-pentylacetate, and ethylpropionate, ethylbutanoate and ethylhexanoate were obtained by means of calorimetric determinations in 1,2-dichloroethane and n-hexane. From these values and the standard molar enthalpy of vaporization, the standard molar enthalpy of solvation of these esters in both solvents were calculated. These calculated values are shown to correlate with the number of methylene groups in the esters.     

Solvation of Na+ in N,N-dimethylformamide + methanol solutions. A nuclear magnetic relaxation study using dynamic solvent isotope effects

Inter- and intramolecular nuclear magnetic quadrupole relaxation measurements have been used to study the system methanol (CH₃OH)+ N,N-dimethylformamide (DMF)+NaI at 25°C. The dynamic behavior of the solvent molecules was investigated, throughout the composition range of the binary mixtures, by means of ¹⁴ N relaxation of DMF and ² H of methanol-d ₁ (CH ₃ OD). The intermolecular relaxation of ²³ Na⁺ in pure DMF was used to obtain information about the symmetry of the solvent electric dipole arrangement in the solvation sphere of the ion. The investigation of preferential solvation around Na⁺ in the binary mixtures was carried out by means of ²³ Na⁺ relaxation measurements using, for the first time, both the CH ₃ OH/CD ₃ OD and the DMF/DMF-d ₇ dynamic isotope effect. The results show that, throughout the composition range, there is preferential solvation by DMF. Furthermore, the use of the isotope effects of both components allowed for the first time a basic check of the reliability of the method since we obtained two independent sets of data for the composition of the Na⁺ solvation shell in the mixtures. The consistency of the two separate data sets demonstrates that the application of the dynamic isotope effect represents a powerful tool in preferential solvation studies.     

The solubility and solvation of salts in mixed nonaqueous solvents. 1. Potassium halides in mixed aprotic solvents

The solubilities of potassium fluoride, chloride, and bromide in acetonitrile, N,N-dimethylformamide, and dimethylsulfoxide and in binary mixtures of these solvents were determined at 25°C. The standard molar Gibbs free energies of solution, _solnG°, in the neat solvents were related to the polarizabilities and basicities of the anions and the dipole moments and acidities of the solvents. The values of _solnG° in the mixtures were fitted by expressions from the quasi-lattice quasi-chemical theory. The mean number of each kind of solvent in the nearest environment of the ions was obtained from these results.     

129Xe NMR法研究醇二元混合溶液

129Xe NMR可以作为研究液体结构状态的探针．甲醇是一个与水结构差别最小的有机溶剂．本文研究了129Xe在甲醇-丁醇,甲醇-戊醇,甲醇-辛醇,甲醇-癸醇混合溶剂中化学位移随组成的变化,并测定了混合溶剂的密度和折射率．129Xe化学位移随混合溶剂的摩尔分数的变化为四条曲率很近的曲线,而化学位移随混合溶剂的体积分数的变化为斜率随多碳醇碳原子数增加的直线．这表明甲醇-多碳醇二元混合溶剂中不存在疏溶剂作用,而构成分子均匀分布的体系．129Xe在甲醇-丁醇,甲醇-癸醇中的化学位移与混合溶剂折射率的一种函数的关系为两条斜率很接近的直线,表明Xe在溶液中主要受范德华力作用．     

Identification and energetic ranking of possible docking sites for pterin on dihydrofolate reductase

The reliability of new methodology for detecting sites for ligand binding on the surfaces of proteins has been tested using a range of dihydrofolate reductase (DHFR) crystal structures. Docking of the pterin molecule to ten such DHFR structures has been examined. Initial docking sites were selected using the VDW-FFT method we have developed recently. This procedure was followed by rigid geometry optimization and solvation energy calculations using our parametrized reaction field multipoles (PRFM) method and the finite difference solution of the Poisson equation (FDPB) method. Two different sets of MM parameters, from the OPLS and Amber94 force fields, have been used. In eight cases the energy of the complexes with pterin bound at the active site was the lowest with the recent Amber94 parameters. In one case the spurious first-ranked site was only 1.8 kcal/mol lower in energy compared with the active site. The other failure of the method may, in fact, represent a valid initial binding site. The calculations with the old OPLS parameters gave slightly worse results.     

取代基对螺噁嗪类化合物光致变色性能影响的探究

本文合成了4种螺噁嗪类化合物,并通过FTIR、1H NMR、ESI-MS对其结构进行了表征。研究了吲哚环上氮原子和噁嗪环9′位不同的取代基及溶剂化效应对其光致变色性能的影响。结果表明,吲哚环的氮原子引入体积较大的基团对螺噁嗪最大吸收波长无明显影响,然而有利于开环体的稳定性,且溴原子的引入会影响螺噁嗪的光响应性;噁嗪环9′位上的供电子基团会降低螺噁嗪的开环体稳定性;螺噁嗪开环体的褪色过程符合一级动力学方程;在非质子溶剂中,螺噁嗪开环体的最大吸收波长呈现正向溶剂化效应;有机酸的加入,可以增加开环体的热稳定性。     

Exploring Helical Folding in Oligomers of Cyclopentane-Based ϵ-Amino Acids: A Computational Study

The conformational preferences of oligopeptides of an ϵ-amino acid (2-((1R,3S)-3-(aminomethyl)cyclopentyl)acetic acid, Amc₅a) with a cyclopentane substituent in the C^β−C^γ−C^δ sequence of the backbone were investigated using DFT methods in chloroform and water. The most preferred conformation of Amc₅a oligomers (dimer to hexamer) was the H₁₆ helical structure both in chloroform and water. Four residues were found to be sufficient to induce a substantial H₁₆ helix population in solution. The Amc₅a hexamer adopted a stable left-handed (M)-2.3₁₆ helical conformation with a rise of 4.8 Å per turn. The hexamer of Ampa (an analogue of Amc₅a with replacing cyclopentane by pyrrolidine) adopted the right-handed mixed (P)-2.9_18/16 helical conformation in chloroform and the (M)-2.4₁₆ helical conformation in water. Therefore, hexamers of ϵ-amino acid residues exhibited different preferences of helical structures depending on the substituents in peptide backbone and the solvent polarity as well as the chain length.     

Singular Energy Distributions in Driven and Undriven Granular Media

We study the kinetic theory of driven and undriven granular gases, taking into account both translational and rotational degrees of freedom. We obtain the high-energy tail of the stationary bivariate energy distribution, depending on the total energy E and the ratio of rotational energy E _w to total energy. Extremely energetic particles have a unique and well-defined distribution f(x) which has several remarkable features: x is not uniformly distributed as in molecular gases; f(x) is not smooth but has multiple singularities. The latter behavior is sensitive to material properties such as the collision parameters, the moment of inertia and the collision rate. Interestingly, there are preferred ratios of rotational-to-total energy. In general, f(x) is strongly correlated with energy and the deviations from a uniform distribution grow with energy. We also solve for the energy distribution of freely cooling Maxwell Molecules and find qualitatively similar behavior.