DIELECTRIC AND STRUCTURAL BEHAVIOR OF L-C YSTEINE AND L-CYSTINE IN AQUEOUS-SOLUTION

Based on dielectric measurements of very dilute solutions of L-Cystein e and L-Cystine at 25-degrees-C the average dipole moment for each ami no-acid was calculated using the theoretical equations of Buckingham a nd Baron. We obtained the following results: mu(L-Cysteine)BAR = 11,43 D and mu(L-Cystine)BAR = 15,50 D. Owing to the discrepancies between our empiric dipole moment value and the theoretical one reported for L -Cysteine as an isolated zwitterion in gas phase, we studied models in which the aminoacid is linked to water molecules by hydrogen bonds. T he analysis of the different vectorial calculation alternatives using bond lengths and angles led us to postulate that L-Cysteine is attache d to two water molecules by hydrogen bonds: one to the NH3, group and one to the COO-group, which diminishes its dipole moment. For L-Cystin e it is found by the same systemic analysis that, only one water molec ule can hydrate this aminoacid, possibly associated to an oxygen atom of a COO. group. We suppose that an intramolecular bond could be estab lished between an NH3. and COO. of both extremes in a folded conformat ion, although some contribution of an extended form present in a dynam ic equilibrium was not excluded. The experimental result of V2BAR for L-Cysteine, remarkably lower than the theoretical value, confirms the solute-solvent association for this aminoacid.