Kinetic Method Analysis of the Effect of cis- and trans-Hydroxylation on the Proton Affinity of Proline

Abstract

Proton affinities of proline and hydroxyproline were measured using the Cooks’ kinetic method. The measurements show that hydroxylation increases the proton affinity, which is consistent with X3LYP computation results. This work supports findings from a previous study (S. Mezzache et al., Rapid Commun. Mass Spectrom. 2005, 19, 2279) that modification of proline increases its proton affinity, but it does not provide compelling evidence for the prediction in the same study that proton affinity of the molecule is substantially influenced by intramolecular interactions involving the proton. PBE1PBE calculations suggest that isotropic polarizability rather than intramolecular hydrogen-bonding interactions provides a more suitable diagnosis for trends in proton affinity changes associated with modifications.