STRUCTURAL DETERMINANTS OF GAS-PHASE BASICITIES OF PEPTIDES

Gas phase basicities of more than two dozen peptides have been measure d in studies of structural determinants. These were determined using t he kinetic method of dissociation of proton-bound dimers with a tandem four-sector mass spectrometer. Basicities of peptides were found to b e higher than those of amino acids, and values for polyalanines with r esidues ranging from 1 to 6 were found to increase with the length of the polymer. Basicities of systematically varied peptides are rank ord ered with increments analogous to the proton affinities of the most ba sic amino acids present in each. The position of the most basic residu e 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 > int ernal > carboxyl terminus. The kinetic method can provide good thermoc hemical values when it is used with caution, as is discussed in this p aper.