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

Matrix-assisted laser desorption/ionization in-source decay (MALDI-ISD) induces N-Cα 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-Cα 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-Cα bond cleavage with further radical reactions. As a consequence, the cleavage efficiency of the N-Cα 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-Cα bond in linear peptides, the present method could allow the evaluation of the efficiency of N-Cα bond cleavage for MALDI matrix development. Copyright © 2016 John Wiley & Sons, Ltd.