THERMOCHEMISTRY OF MULTIPLY-CHARGED MELITTIN IN THE GAS-PHASE DETERMINED BY THE MODIFIED KINETIC METHOD

The modified kinetic method was expanded to measuring gas-phase basici ties of biopolymers carrying two or more charges. Relative rates of th e unimolecular dissociation of metastable multiply charged proton-boun d dimers formed by melittin and reference bases provided the values of 'apparent' gas-phase basicities, These numbers are offset by the heig ht of the reverse activation energy barrier due to Coulombic repulsion in charge separation processes, which have been measured experimental ly using mass-analyzed ion kinetic energy spectrometry. The gas-phase basicity of doubly protonated melittin was determined to be 200.0+/-3. 4 kcal/mol, while the basicity of the same polypeptide carrying three protons was determined to be 179.1+/-3.6 kcal/mol.