MALDI in-source decay (MALDI-ISD) is a fragmentation
that occurs in the MALDI source. It has already been successfully used for the de novo sequencing of peptides and proteins, including the localization of
post-translational modifications. MALDI-ISD involves the hydrogen attachment to
peptides, leading the N–Cα bond cleavage. The source of the hydrogen radical
involved in MALDI-ISD was investigated by Takayama using deuterium-labeled
peptide and/or matrix. The result clearly shows that MALDI-ISD is initiated by
the hydrogen transfer from the matrix to the peptide. However, the mechanism of
fragmentation that follows this transfer is not fully understood.During our investigations, we observed that the
formation of ISD ions was suppressed by using ionic liquid and amorphous
structure of matrix whereas strong signal were observed on sharp crystals. This
seems to indicate that the initial step of MALDI-ISD forming hydrogen-abundant
peptide radical mainly occurs on the matrix crystal during the dissipation of
the laser energy. Resulting hydrogen-abundant peptide radical fragments into c’/z• fragments pair by radical-induced N–Cα
bond cleavage located on the right side of radical site. Subsequently, z•
radicals evolve by reacting with a matrix molecule or by losing their
side-chain. These two reactions competitively occur and are controlled by the
collision rate in MALDI plume.
This work led to new insights in the hydrogen-abundant
radical formation, N–Cα bond cleavage and further radical reaction processes
during MALDI-ISD that will be helpful to interpret
MALDI-ISD spectra of peptides and proteins.