Protonation Thermochemistry of α-Amino Acids Bearing a Basic Residue

Abstract

The proton affinity ( PA) and protonation entropy, ΔpS°, of glycine (Gly), 1, aspartic acid (Asp), 2, asparagine (Asn), 3, histidine (His), 4, lysine (Lys), 5, glutamic acid (Glu), 6, and glutamine (Gln), 7, have been reinvestigated by the extended kinetic method, using the “isothermal point” method and the orthogonal distance regression technique. The proton affinity values of α-amino acids bearing a basic residue ( PA = 926.8, 965.2, 996.0, 993.9, 981.8 and 988.1 kJ mol−1 for 2–7, respectively) show significant deviation from the tabulated values. As expected from the effect of a strong intramolecular hydrogen bond in the protonated forms of these peculiar amino acids, negative protonation entropies are detected (ΔpS° = −36, −43, −37, −29, −95 and −55 J mol−1 K−1 for 2–7, respectively).