Abstract
The mechanism of in-source decay (ISD) under matrix-assisted laser desorption/ionization (MALDI) conditions has been described. The ISD characteristic is the formation of c- and (z+2)-ions originated from the N-Cα bond cleavage on the peptide backbone. The ISD processes occurring with 337 nm laser photon irradiation for peptide or protein proceed to form hyper-valent radical species via intermolecular hydrogen transfer between matrix and analyte molecules following the non-ergodic N-Cα bond cleavage. The non-ergodic N-Cα bond cleavage occurs in the MALDI ion source within nanosecond order, as an α-cleavage initiated with unpaired electron at the hydroxyl methyl radical. The ISD is named as hydrogen-attachment dissociation (HAD). Furthermore, the ISD data of three different myoglobins showed a common intense product of c35 ion originated from the cleavage of N-Cα bond at Gly35-His36 or Ser35-His36 residues. It was suggested that the relatively abundant formation of c35 ion was not due to the N-side of His36 residue, but due to a turn region between helices in secondary structure of myoglobins.
