APPLICATION OF MASS SPECTROMETERY TO THE SEQUENCING OF PROTEINS

Mass spectrometry is now established as a viable method for determing the sequence of amino acids in proteins. The approach involves chemical and enzymic digestion of the protein (as in classical methods) to small peptides (ten residues) which are then chemically modified to increase their volatility. The chemical modifications are (i) acetylation of basic nitrogen sites and (ii) Permethylation. The derivatives so obtained may be sequenced in the mass spectrometer as mixtures of several peptides. This is achieved by a temperature gradient in the mass spectrometer source, which effects partial, or total separation of the derivatized peptides according to their volatilities. In this way, much tedious separation work (sometimes the rate-determining factor in classical approaches) may be avoided. The sequencing process is carried out from the N-terminal residue, utilizing the preferential cleavage of the ionized petide at the amide bonds; each amino acid is associated with a characteristic mass increment and thus the sequence can be determined. Mass spectrometry is especially useful in the detection and characterization of novel amino acids which might otherwise be difficult to identify. Although almost all work to date has used electron impact mass spectrometery, in the future it is possible that newer methods of ionization, such as field desorption and chemical ionization, might be profitably used.