N-Cα bond cleavage of the peptide backbone via hydrogen abstraction

The specific cleavage of N-C-alpha bonds on the peptide backbone to form the so-called 'c ' and 'z + 2 ' products, which can be used for the rapid determination of protein amino-acid sequences, has been examined to clarify the mechanism(s) that occur during hydrogen abstraction induced by bombardment with 337-nm laser photons in matrix-assisted laser desorption/ionization (MALDI) method. Intramolecular hydrogen abstraction, which results from the hydrogen(s) on the C-alpha or C-beta carbon, did not occur with a deuterium-labeled dodecapeptide. To confirm a proposition that intermolecular hydrogen abstraction occurs between the peptide and the MALDI matrix, a deuterium dodecapeptide embedded in a deuterium 2,5-dihydroxybenzoic acid matrix at a molar ratio of 1:7000 was analyzed. The resulting deuterium c product ions suggested that c ions form via intermolecular hydrogen abstraction, although the results obtained did not deny any other possibilities such as intramolecular transfer of labile hydrogen. A mechanism for the N-C-alpha bond cleavage has been proposed that the formation of hypervalent radical species and subsequent prompt bond cleavages occur. The proposed mechanism successfully rationalizes the formation of both the z + 2 and the a product ions. (C) 2001 American Society for Mass Spectrometry.