ASSESSMENT OF GAS-PHASE BASICITIES OF PROTONATED PEPTIDES BY THE KINETIC METHOD

Gas phase basicities of protonated peptides can be obtained, based on values of apparent basicities determined by a modified kinetic method, and the reverse activation energy barrier due to Coulombic repulsion can be determined by ion kinetic energy release spectrometry. The gas phase basicity for protonated bradykinin has been determined to be 217 .8 +/- 1.7 kcal/mol. The value of the reverse activation energy barrie r due to Coulombic repulsion has been determined by MIKES experiments to be 13.6 +/- 1.4 kcal/mol. The influence of the distance between bas ic residues in the peptide chain on Coulombic repulsion and gas phase basicity was also investigated in a series of protonated isomeric pept ides.