Abstract
Gas phase basicities of more than two dozen peptides have been measured in studies of structural determinants. These were determined using the kinetic method of dissociation of proton-bound dimers with a tandem four-sector mass spectrometer. Basicities of peptides were found to be higher than those of amino acids, and values for polyalanines with residues ranging from 1 to 6 were found to increase with the length of the polymer. Basicities of systematically varied peptides are rank ordered with increments analogous to the proton affinities of the most basic amino acids present in each. The position of the most basic residue was found in a series of tripeptides to have only a small influence (less than 2 kcal/mol) on basicity, in the order: amino terminus > internal > carboxyl terminus. The kinetic method can provide good thermochemical values when it is used with caution, as is discussed in this paper.
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