Photocatalytic redox reactions for in-source peptide fragmentation

In-source photocatalytic redox reactions based on a photosensitive target plate have been developed to realize peptide fragmentation during laser desorption ionization. Sample peptides and glucose are simply deposited on a spot of sintered TiO2 nanoparticles. With the irradiation of UV laser on TiO2, electrons are excited from the valence to the conduction band, leaving oxidative holes and reductive electrons to drive various in-source redox reactions. Glucose, working here as a hole scavenger and conductor, can favor both on-surface reduction and long distance in-plume oxidation, therefore inducing peptide fragmentation. C(alpha)-C backbone cleavage was observed to generate a,x fragment decay, while the N-C(alpha) bond cleavage was also sometimes obtained to induce c,z fragmentation, but was rather weaker. The former dissociation is believed to originate from oxidative routes induced by the valence band holes, based on the oxidation of nitrogen atom at the peptide backbone, including hydrogen-radical abstraction and electron transfer. In contrast, the latter dissociation is supposed to be the result of reductive processes by the conduction band electrons, which are then rather similar to electron capture dissociation in tandem mass spectrometry.