Estimation of peptide N-C-alpha bond cleavage efficiency during MALDI-ISD using a cyclic peptide

摘要

Matrix-assisted laser desorption/ionization in-source decay (MALDI-ISD) induces N-C-alpha bond cleavage via hydrogen transfer from the matrix to the peptide backbone, which produces a c'/z(center dot) fragment pair. Subsequently, the z(center dot) generates z' and [z + matrix] fragments via further radical reactions because of the low stability of the z(center dot). In the present study, we investigated MALDI-ISD of a cyclic peptide. The N-C-alpha bond cleavage in the cyclic peptide by MALDI-ISD produced the hydrogen-abundant peptide radical [M+2H](+center dot) with a radical site on the alpha-carbon atom, which then reacted with the matrix to give [M+3H](+) and [M+H+matrix](+). For 1,5-diaminonaphthalene (1,5-DAN) adducts with z fragments, post-source decay of [M+H+1,5-DAN](+) generated from the cyclic peptide showed predominant loss of an amino acidwith 1,5-DAN. Additionally, MALDI-ISD with Fourier transform-ion cyclotron resonance mass spectrometry allowed for the detection of both [M+3H](+) and [M+H](+) with two C-13 atoms. These results strongly suggested that [M+3H](+) and [M+H+1,5-DAN](+) were formed by N-C-alpha bond cleavage with further radical reactions. As a consequence, the cleavage efficiency of the N-C-alpha bond during MALDI-ISD could be estimated by the ratio of the intensity of [M+H](+) and [M+3H](+) in the Fourier transform-ion cyclotron resonance spectrum. Because the reduction efficiency of a matrix for the cyclic peptide cyclo(Arg-Gly-Asp-D-Phe-Val) was correlated to its tendency to cleave the N-C-alpha bond in linear peptides, the present method could allow the evaluation of the efficiency of N-C-alpha bond cleavage for MALDI matrix development. Copyright (C) 2016 John Wiley & Sons, Ltd.