Pairwise Gibbs Energy Interaction Parameters for R-Amino Acid-Amide Interactions in Aqueous Solution: A Kinetic Study

In order to establish the way in which the hydration characteristics of R-amino acids influence their noncovalent interactions with other solutes in aqueous solution, we measured medium effects for the water-catalyzed hydrolysis of an activated amide in aqueous solution in the presence of R-amino acids and some of their derivatives at 25 °C and pH 4. With the exception of phenylalanine and 3-phenylserine, all common R-amino acids induce significant rate enhancements. The kinetic results are analysed using the thermodynamic description of solutesolute interactions as formulated by Savage and Wood. This procedure allows the evaluation of pairwise Gibbs energy interaction parameters. These parameters neither reflect the hydrophobicity of the R-amino acid side chain nor the additivity of functional group interactions in the cosolute. The carboxylate group in the zwitterionic R-amino acid dominates the rate enhancing kinetic medium effect and reduces the availability of nearby methylene groups for interaction with the apolar initial state of the reaction. Some R-amino acid side chains clearly influence the interaction of the carboxylate group with the reactant, most probably due to destructive hydration shell overlap. Only for phenylalanine and 3-phenylserine are marked rate retardations observed, and presumably the hydrophobic effect of the side chains now dominates over the rate enhancing effect of the carboxylate moiety. The results provide quantitative insights into intermolecular noncovalent interactions between R-amino acids and the activated amide. It is shown that destructive hydration shell overlap effects play a significant role in noncovalent interactions in aqueous solution.