Boris N. Solomonov

Calorimetric and Fourier transform infrared spectroscopic study of solid proteins immersed in low water organic solvents

Calorimetric heat effects and structural rearrangements assessed by means of Fourier transform infrared (FTIR) amide I spectra were followed by immersing dry human serum albumin and bovine pancreatic alpha-chymotrypsin in low water organic solvents and in pure water at 298 K. Enthalpy changes upon immersion of the proteins in different media are in a good linear correlation with the corresponding IR absorbance changes. Based on calorimetric and FTIR data the solvents were divided into two groups. The first group includes carbon tetrachloride, benzene, nitromethane, acetonitrile, 1,4-dioxane, n-butanol, n-propanol and pyridine where no significant heat evolution and structural changes were found during protein immersion. Due to kinetic reasons no significant protein-solvent interactions are expected in such systems. The second group of solvents includes dimethyl sulfoxide, methanol, ethanol, and water. Immersion of proteins in these media results in protein swelling and involves significant exothermic heat evolution and structural changes in the protein. Dividing of different media in the two groups is in a qualitative correlation with the solvent hydrophilicity defined as partial excess molar Gibbs free energy of water at infinite dilution in a given solvent. The first group includes the solvents with hydrophilicity exceeding 2.7 kJ/mol. More hydrophilic second group solvents have this energy values less than 2.3 kJ/mol. The hydrogen bond donating ability of the solvents also assists in protein swelling. Hydrogen bonding between protein and solvent is assumed to be a main factor controlling the swelling of dry solid proteins in the studied solvents.

Pairwise substitution effects, inter- and intramolecular hydrogen bonds in methoxyphenols and dimethoxybenzenes. Thermochemistry, calorimetry, and first-principles calculations.

Methoxyphenols are the structural fragments of different antioxidants and biologically active molecules, which are able to form strong intermolecular and intramolecular hydrogen bonds in condensed matter. In the present work, thermochemical, Fourier transform infrared (FTIR)-spectroscopic and quantum-chemical studies of methoxyphenols and its H-bonded complexes in solution and gas phase have been carried out. Thermodynamic properties (standard molar enthalpies of formation, vapor pressure, vaporization enthalpies, sublimation enthalpies, and fusion enthalpies) of 2-methoxyphenol, 3-methoxyphenol, 4-methoxyphenol, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, and 1,4-dimethoxybenzene have been studied in this work. To verify the experimental data, ab initio calculations of all compounds have been performed using density functional theory (DFT), MP2, and G3 methods. The quantitative analysis of ortho, meta, and para pairwise-substituent effects in methoxyphenols has been performed. Solution enthalpies of methoxyphenols at infinite dilution in proton acceptor solvents have been measured. Calorimetric data shows that intermolecular hydrogen bond strength in complexes of 2-methoxyphenol with organic bases is less than that for 4-methoxyphenol. Two experimental approaches for determination of enthalpy of intramolecular hydrogen bonds in ortho-methoxyphenols were proposed. The new results help to resolve uncertainties in the available thermochemical data on methoxyphenols and dimethoxybenzenes and to realize relations among properties and structures for these compounds.

Thermochemistry of Dihalogen-Substituted Benzenes: Data Evaluation Using Experimental and Quantum Chemical Methods

Temperature dependence of vapor pressures for 12 dihalogen-substituted benzenes (halogen = F, Cl, Br, I) was studied by the transpiration method, and molar vaporization or sublimation enthalpies were derived. These data together with results available in the literature were collected and checked for internal consistency using structure-property correlations. Gas-phase enthalpies of formation of dihalogen-substituted benzenes were calculated by using quantum-chemical methods. Evaluated vaporization enthalpies in combination with gas-phase enthalpies of formation were used for estimation liquid-phase enthalpies of formation of dihalogen-substituted benzenes. Pairwise interactions of halogens on the benzene ring were derived and used for development of simple group additivity procedures for estimation of vaporization enthalpies, gas-phase, and liquid-phase enthalpies of formation of dihalogen-substituted benzenes.

FTIR study of H-bonds cooperativity in complexes of 1,2-dihydroxybenzene with proton acceptors in aprotic solvents: influence of the intramolecular hydrogen bond.

FTIR spectroscopic study of hydrogen bonding of 1,2-dihydroxybenzene (catechol) with proton acceptors has been carried out. The influence of intramolecular and intermolecular hydrogen bonds on the strengths of each other in complexes of 1,2-dihydroxybenzene with various proton acceptors has been analyzed. It was shown that intramolecular hydrogen bond is strengthened when 1,2-dihydroxybenzene interacts with bases (ethers, amines, nitriles, etc.) in inert solvents. The contribution of the cooperativity of intramolecular hydrogen bonds in the frequency of stretching vibrations of O-H groups linearly depends on the proton acceptor ability of the bases. The solvent effect on hydrogen bond cooperativity in 1,2-dihydroxybenzene-base complexes has been studied. The approach to determine the influence of cooperative effects on the formation of intermolecular complexes with 1,2-dihydroxybenzene is proposed. It was shown that the strength of intramolecular hydrogen bonds in the complexes of 1,2-dihydroxybenzene with bases due to cooperativity of interactions increases by 30-70%, and the strength of intermolecular hydrogen bond by 7-22%.

Calorimetric determination of the enthalpy of specific interaction of chloroform with a number of proton-acceptor compounds

Calorimetry was used to measure the enthalpies of solution of chloroform in various proton-acceptor solvents and, vice versa, proton-acceptors in chloroform. Based on a previously proposed equation, the enthalpies of specific interaction were calculated and compared with the published data on the enthalpy of hydrogen bonding of chloroform with various proton-acceptor solvents. The composition of the H-bonded complexes mainly formed during the dissolution of proton-acceptor solutes in chloroform was established. It was demonstrated that the dissolution of ethers in chloroform is predominantly accompanied by the formation of 1: 1 complexes, while the dissolution of acetone, dimethylformamide, and dimethyl sulfoxide in chloroform gives rise to more complex associates.

Hydrogen bonds formed by methyl groups of acetonitrile: Infrared and calorimetric study

Abstract Solutions of acetonitrile (I) in tetrachloromethane and deuteratred solvents (S) (benzene, acctonitrile, acetone and dimethylsulphoxide) have been studied by IR absorption spectra. The observed solvent effect on the IR spectrum of I was explained in terms of the existence of complexes with hydrogen bonding of the type NCCH3…solvent (S). The strength of the hydrogen bonding was characterized by enthalpies of specific interactions of I with solvents ΔHI/Sint(sp). The values ΔHI/Sint(sp) were determined both by IR spectroscopy and calorimetry and were found to be within the range 0.3–1.5 kcal mol−1.

Interactions of water with human serum albumin suspended in water-organic mixtures

Abstract Calorimetric enthalpy changes on suspending a partially hydrated preparation of human serum albumin (HSA) in various water-organic mixtures are discussed together with the water sorption isotherms. Experimental data indicate that suspending the HSA preparation is accompanied mainly by two processes. The first is water desorption-sorption which superficially obeys the Langmuir model. The influence of the medium on the thermodynamic parameters of water sorption can be described approximately by thermodynamic data on the solvation of water at infinite dilution. The second effect is a non-sorption process attributed tentatively to rupture of protein-protein contacts in the HSA preparation on suspending it. Depending on the nature of the solvent and its water content, such transformation of the HSA preparation can result in deviations from the Langmuir isotherm of water sorption by the suspended protein. This transformation is accompanied by the corresponding increase in the accessible surface area of the protein preparation and a significant enthalpy change. Experimental data cast doubt on the validity of the traditional opinion that the significant increase in water sorption by proteins at high water activities results from the various kinds of water-water interaction on the protein surface. It appears that the imposition of the transformation of the protein preparation on water sorption-desorption can determine both the calorimetric profile and thermodynamic data on suspending the protein preparation in various solvents.

Thermodynamics of water binding by human serum albumin suspended in acetonitrile

Abstract Heat effects resulting from the introduction of solid human serum albumin (HSA) into various water-acetonitrile mixtures were measured calorimetrically at 298 K. The amount of water bound to the suspended HSA as a function of the water content of the solvent was also determined. Introducing HSA into water-acetonitrile mixtures involves water binding according to the Langmuir isotherm with an adsorption constant K c = 1.0 ± 0.1 M −1 , enthalpy Δh = −9.0 ± 1.5 kJ mol −1 and entropy ΔS = −30 ± 6 J mol −1 K −1 . Placing HSA in the solvent has an additional heat effect of 46 ± 19 J g −1 , which is attributed to an unknown transformation of the protein preparation.

Hydrogen bonding in pure base media. Correlations between calorimetric and infrared spectroscopic data

The known correlations between calorimetric and IR spectroscopic data on hydrogen bonding were reinvestigated for hydrogen bond donors (AH) dissolved in pure bases (B). Ninety-five AH···B systems were considered for which the enthalpies of specific interaction due to hydrogen bonding AH···B (ΔHAH/Bsp.int., kJ mol−1), the weight centers of A-H stretching bands (v, cm−1) and their integral absorption coefficients (α, 104 cm mmol−1) had been measured. The relationships between ΔHAH/Bsp.int. and the parameters of the infrared absorption spectra [weight center shifts Δv and the changes in the square roots of α (Δα1/2)] were analyzed. It was found that the dependence of ΔHAH/Bsp.int. on Δα1/2 consists of two nearly parallel straight lines: the first (−ΔHAH/Bsp.int. = 12·0 Δα1/2 + 0·4) corresponds to water and the weak C(SINGLE BOND)H and O(SINGLE BOND)H H-bond donors (chloroethylenes, acetylenes acetonitrile, nitromethane, chloroform, o, o′-di-tert-butylphenol); the second (−ΔHAH/Bsp.int. = 12·1 Δα1/2 −4·2) corresponds to the stronger N(SINGLE BOND)H and O(SINGLE BOND)H H-bond donors (N-methylaniline, pyrrole, alcohols, phenol, carbon acids). Non-linear dependences of ΔHAH/Bsp.int. on Δv were obtained for all C(SINGLE BOND)H, N(SINGLE BOND)H and O(SINGLE BOND)H H-bond donors except water [−ΔHAH/Bsp.int. = 59·9 Δv/(Δv + 674)]. Enthalpies of specific interaction for water obey another dependence [−ΔHsp.int./2 = 28·5 Δv/(Δv + 269)]. The ΔHAH/Bsp.int. values can be estimated by the above correlations with an accuracy of ±3 to ±6 kJ mol−1. These relationships obtained for solutions of H-bond donors in pure bases differ from the well known dependences determined for the AH···B hydrogen bonding in carbon tetrachloride medium.

Interaction enthalpies of solid bovine pancreatic α-chymotrypsin with organic solvents: comparison with FTIR-spectroscopic data

Calorimetric heat effects and integral absorbance changes observed in the FTIR spectra were measured at immersing solid bovine pancreatic α-chymotrypsin in organic solvents and water at 298 K. Enthalpy changes upon the immersion of the enzyme in different media are in a good linear correlation with the corresponding IR-absorbance changes. Based on calorimetric and FTIR data, all the solvents were divided into two groups. The first group of solvents includes carbon tetrachloride, benzene, nitromethane, acetonitrile, 1,4-dioxane, n-butanol, n-propanol and pyridine in which no significant heat evolution and structural changes were found at the solid enzyme immersion. Second group of the solvents includes dimethyl sulfoxide, methanol, ethanol, and water. Immersion into these media, results in the solid protein swelling and involves significant exothermic heat evolution and structural changes in the protein. Dividing of different media in these two groups is in a qualitative correlation with the solvent hydrophilicity which is defined as partial excess molar Gibbs free energy of water at infinite dilution in a given solvent. The first group of solvents includes liquids with hydrophilicity exceeding 2.7 kJ/mol. The hydrophilicity of the second group solvents is <2.3 kJ/mol. Hydrogen bond donating ability of the solvents assists in the protein swelling. Hydrogen bonding between protein and solvent is assumed to be a main factor controlling the swelling of solid protein preparation in the solvents at room temperature.