C-terminal sequence determination of modified peptides by MALDI MS

Peptides, cleaved by a mixture of carboxypeptidases CPP and CPY, can be detected by MALDI MS and the amino acid sequence thereby determined by calculation of the differences between consecutive peaks. In the present study we have used derivatizations of Lys and Cys to facilitate identification of these residues. Since the mass values do not readily distinguish Lys from Gln, we have converted Lys to homoarginine by guanidination, allowing simple detection of Lys. To identify the Cys positions in peptides that contain cystine, cysteic acid, or carboxymethylcysteine is not possible using CPY and CPP because of the lack of proteolytic cleavage. Instead we find that identification of Cys residues within the sequence can be achieved after conversion to a basic derivative, 4-thialaminine (Thi), by trimethylaminoethylation.