ELECTROSPRAY MASS-SPECTROMETRIC STUDY OF PROTEIN-KETONE EQUILIBRIA INSOLUTION

The electrospray mass spectra of basic protein solutions containing ke tones exhibit ions that are attributable to protein-ketone adducts who se relative abundances are in accordance with the relative concentrati ons of these adducts in solution. In a basic solution, the deprotonate d (neutral) amino groups in a protein may be alkylated by ketone molec ules to form Schiff bases. The extent of alkylation may be approximate d by statistics if the reactivity of each amino group is identical. Th e relative ion abundances of these adducts have been found to follow t heir statistical distributions expected in solution. The reaction to f orm Schiff bases may be applied to the measurement of the number of pr imary amino sites (N-terminus plus lysine residues) in a protein. Exce llent agreement with known numbers was observed for all 18 proteins me asured.