C_(alpha)-C Bond Cleavage of the Peptide Backbone in MALDI In-Source Decay Using Salicylic Acid Derivative Matrices

摘要

MALDI-ISD of peptides has been studied using several salicylic acid derivatives as matrices. The abundances of a/x- and c'/z'-series ions were dependent on the hydrogen-donating and hydrogen-accepting ability of matrix, respectively. In the MALDI-ISD with 5-FSA and 5-NSA, the nitrogen-centered radical can form by intermolecular hydrogen transfer from the peptide backbone amide hydrogen to the matrix with subsequent radical induced cleavage at the C_(alpha)-C bond. This C_(alpha)-C bond cleavage first leads to a(centre dot)/x pairs followed by hydrogen extractions from a~(centre dot) radicals to yield a-series ions. However, for Xxx-Pro and Xxx-Sar bonds, cleavage of the CO-N bond occurs via a hydrogen abstraction from the Pro and Sar C_(alpha)-H bonds. This CO-N bond cleavage leads to the formation of a b(centre dot)/y fragment pair, and the b(centre dot)-series ion undergoes further degradation to b- and a-series ions. The present matrix provides a useful complementary method to the conventional MALDI-ISD for the analysis of amino acid sequencing and site localization of PTMs in peptides.