Amplified histidine effect in electron-transfer dissociation of histidine-rich peptides from histatin 5

Abstract

We report electron-transfer dissociation (ETD) mass spectra of histidine-containing peptides DSHAK, FHEK, HHGYK, and HHSHR from trypsinolysis of histatin 5. ETD of both doubly and triply protonated peptides provided sequence ions of the c and z type. In addition, electron transfer to doubly protonated peptides produced abundant long-lived cation-radicals, (M+2H) +, whose relative intensities depended on the peptide sequence and number of histidine residues. CID-MS 3 spectra of (M+2H) + cation-radicals were entirely different from the ETD spectra of the doubly charged ions and involved radical-driven losses of C 4H 6N 2 neutral fragments from the histidine residues and charge-driven backbone cleavages forming b and y ions. Product ions from CID of (M+2H) + were further characterized by CID-MS 4 spectra to distinguish the histidine residues undergoing loss of C 4H 6N 2. The ETD-CID-MS n mass spectra are interpreted by considering radical-induced rearrangements of histidine side chains in the long-lived charge-reduced ions.