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

The enthalpies of dissolution of glycine (Gly) and L-alpha-alanine (Ala) in water at 288.15-318.15 K were measured. The results were compared with the earlier obtained data for L-alpha-phenylalanine (Phe) and L-alpha-histidine (His). The standard enthalpies of dissolution (Delta(soln) H-0) and differences (Delta C-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) = (C) over bar (infinity)(p,2) - C-p) were calculated in the temperature interval 273-373 K. Changes in the entropy of dissolution (Delta Delta S-soln(0)) and reduced Gibbs energy [Delta(Delta(soln)G(0)/T)] in the temperature interval from 273 to 373 K were determined from the known thermodynamic relationships. The Delta C-p(0) value is negative for hydrophilic glycine and positive for other amino acids. The Delta Delta S-soln(0) values increase with an increase in the hydrophobicity of the amino acids. The Delta(Delta(soln)G(0)/T) values become more negative in the order Ala, Phe, Gly, His.