GAS-PHASE BASICITIES AND PROTON AFFINITIES OF LYSINE AND HISTIDINE MEASURED FROM THE DISSOCIATION OF PROTON-BOUND DIMERS

Gas-phase basicities and proton affinities of lysine and histidine wer e measured by the kinetic method. Using low-energy collisional activat ion to change the effective temperature in the dissociation of proton- bound dimer, proton affinities of lysine and histidine were determined to be 235.3 and 234.0 kcal/mol respectively. A large entropy effect w as found for lysine, and thus at temperatures above 300 K, histidine i s slightly more basic than lysine. Peptides that contain lysine or his tidine are more basic than lysine or histidine. The basicities of thes e peptides depend on their structures and the internal hydrogen bondin g.