Studies on the dissociation constants and solubility of amino acids in water + urea mixtures at 298 K, interaction of urea with amino acids and the role of urea in the denaturation of proteins in terms of structural aspects of water

Abstract

The first and second dissociation constants of amino acids (glycine, L -proline, L -valine, α-alanine, β-alanine, L-asparagine, L -methionine, L -leucine, L -threonine, L -glutamine, L -serine and L -histidine) were determined pH-metrically in water + urea (0-8 M urea) mixtures at 298 K. Solubilities of amino acids in water + urea mixtures and the interaction constants of amino acids with urea were also determined pH-metrically. The Gibbs energies of transfer ΔG 0 t (1) and ΔG 0 t (2) for the reactions, RH 2 + + H 2 O ⇄ RH ± + H 3 O + and RH ± + H 2 O ⇄ R - + H 3 O + (2) were coupled with Gibbs energies of transfer for neutral amino acids [ΔG 0 t (RH ± )] and H + -ions [ΔG 0 t (H + )] (determined previously) to get the Gibbs energies of transfer of RH 2 + + [ΔG 0 t (RH 2 + )] and R- [ΔG 0 t (R-)] from water to water+urea mixtures. ΔG 0 t (H + ) were also calculated directly and utilized. ΔG 0 t (RH 2 + ) and ΔG 0 t (R-) are negative and positive respectively. These give the quantitative measure of ion-solvent interactions of the cations and anions of the amino acids. The interaction constants of amino acids with urea were calculated. The results were discussed in terms of structural changes of water in presence of urea in water + urea mixtures, changed basicities of the solvent mixtures and solute-solvent interactions. However interaction of urea with amino acids appears not to be responsible for the denaturation of proteins but the destruction of the tetrahedral structure of H 2 O by urea is responsible for the alteration of protein folding and protein structure leading to denaturation. Direct mechanism involving microscopic properties suggests that no structure breaking of water by urea but a number of indirect evidences suggest urea to be the structure breaker of water.