Evaluation of a novel approach for peptide sequencing: Laser-induced acoustic desorption combined with P(OCH3)2+ chemical ionization and collision-activated dissociation in a fourier transform ion cyclotron resonance mass spectrometer
A novel mass spectrometric method has been developed for obtaining sequence information on small peptides. The peptides are desorbed as intact neutral molecules into a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR) by means of laser-induced acoustic desorption (LIAD). Reactions of the neutral peptides with the dimethoxyphosphenium ion, P(OCH3)2+, occur predominantly by addition of the peptide to P(OCH3)2+ followed by the loss of two methanol molecules, thus yielding product ions with the composition (peptide+P−2H)+. Upon sustained off-resonance irradiation for collision-activated dissociation (SORI-CAD), the (peptide+P−2H)+ ions undergo successive losses of CO and NH=CHR or H2O, CO, and NH=CHR to yield sequence-related fragment ions in addition to the regular an- and bn-type ions. Under the same conditions, SORI-CAD of the analogous protonated peptides predominantly yields the regular an- and bn-type ions. The mechanisms of the reactions of peptides with P(OCH3)2+ and the dissociation of the (peptide+P−2H)+ ions were examined by using model peptides and molecular orbital calculations.