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

Department of Chemistry, Vivekananda Mission Mahavidyalaya, Chaitanyapur (Haldia), Purba Medinipur-721 645, West Bengal, India E-mail : bisnupada.dey@gmail.com Research Advisor, Central Forensic Science Laboratory, MHA, Government of India, 30, Gorachand Road, Kolkata-700 014, India E-mail : sujitclahiri@yahoo.com Manuscript received 24 August 2009, accepted 25 August 2009 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-tbreonine, L-glutamine, L-serine and L-histidine) wen determined pH-metrically In water + urea (0-8 M urea) mixtures at 2911 K. Solubilities of amino acids in water + urea mixtures and the interaction constants of amino acids with urea wen also determined pH-metrically. The Gibbs energies of transfer ΔG0t (1) and ΔG0t (2) for the reactions, RH2+ + H2O ⇋ RH± + H3O+ (1) and RH± + H2O ⇋ R- + H3O+ (2) were coupled with Gibbs energies or transfer for neutral amino acids [ΔG0t (RH±)] and H+ -ions [ΔG0t (H+)] (determined previously) to get the Gibbs energies of transfer or RH2+ [ΔG0t (RH2+)] and R- [ΔG0t (R-)] from water to water+ urea mixtures. ΔG0t (H+) were also calculated directly and utilized. ΔG0t (RH2+) and ΔG0t (R-) are negative and positive respectively. These give the quantitative measure of ion-solvent Interactions or the cations and anions or 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 adds appears Dot to be responsible for the denaturation of proteins but the destruction of the tetrahedral structure or H2O 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 or water