Electrospray Mass Spectrometric Study of Protein-Ketone Equilibria in Solution

Abstract

The electrospray mass spectra of basic protein solutions containing ketones exhibit ions that are attributable to protein-ketone adducts whose relative abundances are in accordance with the relative concentrations of these adducts in solution. In a basic solution, the deprotonated (neutral) amino groups in a protein may be alkylated by ketone molecules to form Schiff bases. The extent of alkylation may be approximated by statistics if the reactivity of each amino group is identical. The relative ion abundances of these adducts have been found to follow their statistical distributions expected in solution. The reaction to form Schiff bases may be applied to the measurement of the number of primary amino sites (N-terminus plus lysine residues) in a protein. Excellent agreement with known numbers was observed for all 18 proteins measured. 25 refs., 10 figs., 1 tab.