Enthalples of solution and dilution of L-a-aminobutyric acid, L-a-isoleucine, L-aphenylalanine, L-proline and L-a-methionine in water at a temperature of 298.15 K have been measured. The values of dilution enth.aIpy were used to determine enthalpic homogeneous interaction coefficients which characterize the interactions between zwitterions of the examined L-a-amino acids in water. The effect of side substituents in a molecule of L-a-amino acids on the direct homogeneous interactions in water and the effect of these substituents on the surrounding water molecules have been discussed. Quantum chemical semiempirical cakuktti~us of the structure of L-a-aminobutyric acid and several of its hydrates Wa caked Out 0 2OC0 Elsevier Science B.V. All rights reserved.
The behaviour of L-a-amino acids, basic building materials of proteins, in water ha been studied in many laboratories [l-6]. Amino acids in aqueous solutions exist in ion&d forms, 'HfiCH(R)COz-. The amino acid molecules consist of a dipolar head and various side substituents -R.
The side substituents of La-amino acids are functional groups of poly-peptide chains which are responsible for the structure and properties of proteins. It was of interest to examine the effect of side substituents, both polar and non-polar in L-aamino acid molecules and also those substituents that have electric charges, on the direct interaction between amino acid molecules in water and the effect of these substituents on the surrounding water molecules. Spectroscopic measurem ents of aqueous sohltions of Lu-amino acids carried out recently by Ide et al. [7] have confirmed that non-polar substituents bring about structuring of water surrounding these groups, which is proven by the increase in the relative intensity of the collective band which reflects the probability of water-water hydrogen bonding in the solutions. This suggestion is confirmed by NMR spectrum examinations carried out by Bagno et al. [8], which show that the water-water bonding in the surrounding of hydrophobic side substituents are stronger than those within bulk water, According to Hechte et al. [9] the increase in the alkyl chain length in amino acid molecules brings about an increase in water clusters around these non-polar groups.
To understand better the interactions taking place in water between Lu-amino acids and consequently between their side substituents, we have determined the enthalpic interaction coefficients of the zwitterions of La-amino acids on the basis of dilution enthalpies. These coefficients, derived from M&Wan-Mayer's theory [IO] characterize the energy effects of mutual interaction between the examined molecules in water. In order to complete the thermochemical charac&istic of the investigated systems at 298.15 K the heat of solution of selected amino acids in water was measured additionally. In addition, in order to go more deeply into the salvation mechanism of a-amino acid zwitterions having l Author to whom correspondence should be addressed. 0167-732XJW -see front matter 0 2000 Elsevier Science B.V. AI1 rights resewed,