Is the first step in MALDI in-source decay electron transfer or hydrogen atom abstraction?

Matrix -assisted laser desorption/ionization in -source decay (MALDI-ISD) with reducing matrices enables rapid mass spectrometric characterization of intact proteins. Several novel matrices have been developed for improving the fragmentation efficiency of MALDI-ISD. In particular, matrix having aniline group stronger facilitate MALDI-ISD than that having phenol group. Because the aniline N-H bond is stronger than the phenol O-H bond, the efficient fragmentation induced using matrices containing aniline group cannot be explained by the previously proposed mechanism involving the matrix -peptide hydrogen atom transfer. To explain these phenomena, electron transfer from the matrix to the peptide was recently proposed as the initial step in the MALDI-ISD processes. In this study, to estimate whether a peptide acquires hydrogen atoms or electrons in the first step of MALDI-ISD, the transition -state barriers of the reaction between dipeptide and hydrogen atoms produced from the excited -state matrix were calculated, assuming the peptide in the ground state during the MALDI-ISD process. The barrier obtained for the corresponding hydrogen atom attachment reaction did not correlate with the yield of the fragment ions produced by MALDI-ISD. In contrast, a correlation has been reported between the yields of the fragment ions produced by MALDI-ISD and the ionization energy of the matrix solids, indicating that MALDI-ISD occurs efficiently using a matrix that easily emits electrons. These results strongly suggest that MALDI-ISD of peptides using reducing matrices is well explained by the transfer of electrons and subsequent protons from the matrix to the peptide.