Proton Affinity of β-Oxalylaminoalanine (BOAA): Incorporation of Direct Entropy Correction into the Single-Reference Kinetic Method

A new version of the single-reference-extended kinetic method is presented in which direct entropy correction is incorporated. Results of calibration experiments with the monodentate base pyridine and the bidentate base ethylenediamine are presented for which the method provides proton affinities in excellent agreement with published values and reasonable predictions for the protonation entropies. The method is then used to determine the proton affinity and protonation entropy of the non-protein amino acid β-oxalylaminoalanine (BOAA). The PA of BOAA is found to be 933.1 ± 7.8 kJ/mol and a prediction for the protonation entropy of -39 J mol−1 K−1 is also obtained, indicating a significant degree of intramolecular hydrogen bonding in the protonated form. These results are supported by hybrid density functional theory calculations at the B3LYP/6–311++G**//B3LYP/6–31+G* level. They indicate that the preferred site of protonation is the α-nitrogen atom (PA = 935.0 kJ/mol) and that protonated BOAA has a strong hydrogen bond between the hydrogen on the α-amino group and one of the carbonyl oxygen atoms on the side chain