Thermochemistry of the Dissolution of L-Threonine in Acetonitrile, 1,4-Dioxane, Acetone, and Dimethylsulfoxide Aqueous S
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ICAL CHEMISTRY OF SOLUTIONS
Thermochemistry of the Dissolution of L-Threonine in Acetonitrile, 1,4-Dioxane, Acetone, and Dimethylsulfoxide Aqueous Solutions at Т = 298.15 K V. I. Smirnova,* a Krestov
Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, 153045 Russia *e-mail: [email protected] Received December 5, 2019; revised February 28, 2020; accepted March 17, 2020
Abstract—The enthalpies of dissolution of L-threonine in acetonitrile, 1,4-dioxane, acetone, and dimethylsulfoxide aqueous solutions are measured via calorimetry at mole fraction of the organic solvent x2 = 0–0.25 and T = 298.15 K. Standard enthalpies of dissolution Δsol H ° and transfer Δ tr H ° of L-threonine from water to a mixed solvent are calculated along with the enthalpy coefficients of pairwise interactions hxy between Lthreonine and the solvent molecules. Relationships between the enthalpies of dissolution of L-threonine and the composition of a mixture, the energy of interaction between the components of the mixture, and the electron-donating ability of organic solvents are found. The enthalpy coefficients of the pairwise interaction between L-threonine and other amino acids (glycine, L-alanine, L-valine, and L-methionine) in the mixed solvents are analyzed comparatively. The enthalpy contributions from the side chains of L-amino acids to the energy of interaction with molecules of these organic solvents in an aqueous solution are assessed quantitatively. Keywords: amino acids, enthalpies of dissolution and transfer, water-organic mixtures, enthalpy coefficients of pairwise interactions DOI: 10.1134/S0036024420100271
INTRODUCTION This work is a continuation of our studies of the energy of interaction between amino acids, peptides, and alcohols, amides, and other organic solvents in aqueous solutions [1–7]. L-Threonine, an essential amino acid found in many proteins, is contained in human blood plasma and participates in the metabolism of proteins and fats and in the synthesis of collagen. L-Threonine is an optically active form of threonine. This form of threonine plays an important role in the metabolism of humans, plants, and animals. A deficiency or excess of it in a living organism can have serious consequences. The main structural difference between L-threonine and other amino acids is that its side chain contains both hydrophobic CH3 and hydrophilic OH groups. H
H
O
N C C H
CH OH HO CH3
Organic solvents with different physicochemical properties were used as co-solvents. Acetone (AC) and dimethylsulfoxide (DMSO) are polar aprotic organic
solvents with high dipole moments and high dielectric constants. They are widely used in industry, biochemistry, and pharmacology. 1,4-Dioxane (DO) is a nonpolar cyclic ether used in technical fluids and cleaning products, and acetonitrile (AN) is an extractant with a high dielectric constant and a dipole moment. The main aims of this work were (a) to find the dependences of the enthalpies of dissolution and the transfer of L-threonine on the composition of aqueous organic mixtu
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