Bibliography: p. 114-123A few pages are in colour.Includes copies of copyright permission. Original copies with original Partial Copyright Licence.This work reports a protein labeling method using non-selective carbene reactions of sufficiently high efficiency to permit detection by mass spectrometric methods at the protein, peptide and residue level. The approach uses a diazirine-modified amino acid (L-2-amino-4,4' -azipentanoic acid, "photoleucine") as a label source, which is converted to a highly reactive carbene by pulsed laser photolysis at 355 nm. Labeling of model proteins and peptides was achieved with sensitivity to changes in protein topography brought about by conformational change and ligand binding. Labeling yield is independent of protein concentration over approximately two orders of magnitude, but is weakly dependent on the presence of other chromophores in a photon-limited apparatus. The current configuration required 2 minutes of irradiation for full reagent conversion, however it is shown that comparable yields can be achieved with a single high-energy laser pulse (>100 mJ/pulse, <10 nsec), offering a labeling method with high temporal resolution
