Optical studies of the Soret effect. Part III. Entropies and heat capacities of transfer of norvaline, ε-aminocaproic acid, L-proline, DL-hydroxyproline, and glycylglycine in aqueous solution

Abstract

Soret coefficients for aqueous solutions of isoelectric norvaline, ε-aminocaproic acid, L-proline, DL-hydroxyproline, and glycylglycine have been measured over concentration ranges from about 0.1 molal upwards, at mean temperatures of 25 and 35°, using the optical beam-displacement method described in Part I. Entropies and heat capacities of transfer have been calculated from the results. On the basis of the qualitative model discussed in Part II, we conclude that (i) a terminal NH₃⁺ group is more efficient in raising the “structural temperature” of the solvent round the solvated solute than a similar group placed elsewhere in an amino-acid molecule, and (ii) a significant increase in this effect probably occurs when a hydroxyl group is introduced into the molecule. The dipeptide glycylglycine has a higher heat capacity of transfer than has so far been observed for amino-acids themselves.