Sequencing of β-Peptides by Mass Spectrometry

Abstract

Seven ?-peptides containing up to 18 β²-, β³- and β^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 β-peptides, as well as from the corresponding methyl esters (+14 U) and N-acetyl derivatives (+42 U), the known sequences of β-amino acids could be confirmed unambiguously with the program Sherpa. Thus, the commonly used MS-sequencing procedure for α-peptides can be adopted for β-peptides without modification. However, there are pronounced differences in the fragmentation patterns of the two types of peptides: the β-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 β-amino-acid residues).