Aspartic acid side chain effect-experimental and theoretical insight.

Gas-phase H/D exchange and density functional theory study of the Asp and Glu side-chain carboxylic group intrinsic reactivity is reported. H/D exchange site specific treatment and some additional theoretical calculations showed that a side-chain carboxylic group may initiate proton transfer along with bond formation to one of its oxygens, i.e., possibility to initiate selective of cleavage peptide bond ("aspartic acid effect"). That finding is used to select aspartic acid cleavage mechanisms (side-chain proton transfer either to backbone carbonyl or to amide nitrogen) for further computational study. B3LYP/6-31G(d) and G3(MP2)//B3LYP potential energy profiles of both mechanisms on a model system CH3CO-Asp-NHCH3 were constructed. Although energy employed in low-energy collision induced dissociation suffices for both mechanisms thresholds, energy transferred to specific modes suggests a complex one-step mechanism of proton transfer (from the side-chain carboxylic group to the backbone amide group), bond formation (between deprotonated carboxylic group and carbon atom of the backbone carbonyl), and peptide bond cleavage as favorable.