Estimation of peptide N–C α bond cleavage efficiency during MALDI‐ISD using a cyclic peptide

Matrix‐assisted laser desorption/ionization in‐source decay (MALDI‐ISD) induces N–C α bond cleavage via hydrogen transfer from the matrix to the peptide backbone, which produces a c ′/ z • fragment pair. Subsequently, the z • generates z ′ and [ z  + matrix] fragments via further radical reactions because of the low stability of the z •. In the present study, we investigated MALDI‐ISD of a cyclic peptide. The N–C α bond cleavage in the cyclic peptide by MALDI‐ISD produced the hydrogen‐abundant peptide radical [M + 2H] + • with a radical site on the α‐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 acid with 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 13 C atoms. These results strongly suggested that [M + 3H] + and [M + H + 1,5‐DAN] + were formed by N–C α bond cleavage with further radical reactions. As a consequence, the cleavage efficiency of the N–C α 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 α bond in linear peptides, the present method could allow the evaluation of the efficiency of N–C α bond cleavage for MALDI matrix development. Copyright © 2016 John Wiley & Sons, Ltd.