A general method for the determination of the carboxyl-terminal sequence of proteins.

A method is described for the selective isolation of peptides derived from the carboxyi-terminus of proteins. The method is of general applicability since it is independent of the primary structure of the protein in this region. The principle behind the isolation procedure is as follows: when an enzymic digestion is performed on a protein in which all free carboxyl groups have been modified by coupling with ethanolamine. the carboxyl-terminal peptide will be unique in that it does not have a carboxyl group. This difference is utilized to selectively purify carboxyl-terminal peptides. These peptides have the same electrophoretic mobility at pH 4.4 and at pH 2.1, whereas all other peptides will migrate more rapidly toward the cathode at the lower pH. Using a diagonal electrophoretic procedure based on this property, carboxyl-terming peptides may be isolated, and their sequence determined by standard procedures.