Gas-Phase Acidity of Proline from Density Functional Computations

Density functional computations using the B3LYP hybrid potential coupled with the extended 6-311++G∗∗ basis set were performed to evaluate gas-phase acidity of proline α-amino acid. The computed value of gas-phase acidity is 1403.8 kJ/mol in good agreement with the experimental counterpart (1395.0 ± 13.8 kJ/mol) and is referred to the deprotonated species deriving from the first relative minimum of neutral proline characterized by the exo ring conformation with the carboxyl group in cis position. The most stable neutral proline isomer gives rise to a species for which the gas-phase acidity value (1384.5 kJ/mol) is also close to the experimental data.