Chirality effects on the intrinsic acidity of isomeric tripeptides containing a D/L-Cysteine on the N-terminus: CAA and dCAA

Chirality effects on the intrinsic gas-phase acidity of oligopeptides have been studied using a pair of stereoisomeric tripeptides consisting of a D/L-cysteine (C) and two residues of alanine (A): CAA and ^dCAA, where the C-terminus is amidated. Mass spectrometry measurements through bracketing via collision-induced dissociation clearly show that CAA is a stronger gas-phase acid than ^dCAA. Quantitative values of the acidity were determined using the extended Cooks kinetic method. The resulting deprotonation enthalpy (Δ_acidH) for CAA is 326.2 kcal/mol (1364.7 kJ/mol) and for ^dCAA it is 326.8 kcal/mol (1367.6 kJ/mol). The corresponding gas-phase acidity (Δ_acidG) for CAA is 321.3 kcal/mol (1344.2 kJ/mol) and for ^dCAA it is 322.0 kcal/mol (1347.3 kJ/mol). Changing the N-terminal cysteine from the L-form to the D-form reduces the gas-phase acidity by about 0.6 kcal/mol (2.5 kJ/mol). Extensive conformational searches followed by quantum chemical calculations at the ωB97X-D/6-311+G(d,p) level of theory yielded a set of lowest energy conformations for each peptide species. Theoretical gas-phase acidities calculated using the Boltzmann averaged conformational contributions are in good agreement with the experimental data. The shift in the acidity is likely due to the conformational effect induced by D-cysteine, which increases the stability of the neutral ^dCAA, and hence reduces its acidity. A chirality change on a single amino acid can have a noticeable effect on the biochemical properties of peptides and proteins.