Amino acids in aqueous solution. Effect of molecular structure and temperature on thermodynamics of dissolution

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The enthalpies of dissolution of glycine (Gly) and L-α-alanine (Ala) in water at 288.15–318.15 K were measured. The results were compared with the earlier obtained data for L-α-phenylalanine (Phe) and L-α-histidine (His). The standard enthalpies of dissolution (Δsoln H 0) and differences (ΔC p 0 ) between the limiting partial molar heat capacity of the amino acids in solution and the heat capacity of the amino acids in the crystalline state \((\Delta C_p ^0 = \bar C_{p,2}^\infty - C_p )\) were calculated in the temperature interval 273–373 K. Changes in the entropy of dissolution (ΔΔsoln S 0) and reduced Gibbs energy [Δ (Δsoln G 0/T)] in the temperature interval from 273 to 373 K were determined from the known thermodynamic relationships. The ΔC p 0 value is negative for hydrophilic glycine and positive for other amino acids. The ΔΔsoln S 0 values increase with an increase in the hydrophobicity of the amino acids. The Δ(Δsoln G 0/T) values become more negative in the order Ala, Phe, Gly, His.