Sequencing of ?-Peptides by Mass Spectrometry

Abstract

Seven ?-peptides containing up to 18 ?2-, ?3- and ?2,3-amino acids have been subjected to ESI-tandem mass spectrometry (low-energy fragmentation, positive ions). From the fragment ions formed from the free ?-peptides, as well as from the corresponding methyl esters (+14 U) and N-acetyl derivatives (+42 U), the known sequences of ?-amino acids could be confirmed unambiguously with the program Sherpa. Thus, the commonly used MS-sequencing procedure for ?-peptides can be adopted for ?-peptides without modification. However, there are pronounced differences in the fragmentation patterns of the two types of peptides: the ?-peptides disclose their relationship to Mannich bases in the mass-spectrometric experiment by the elimination of ammonia from the N-terminus (? RCH=CH-CO-NH-R') and the occurence of retro-Mannich cleavage (cf. formation of HN=CHR + CH3CO-NH-R' from ?-amino-acid residues).