Near-edge X-ray absorption mass spectrometry of a gas-phase peptide.

We have studied the dissociation of the gas-phase protonated peptide leucine enkephalin [YGGFL+H](+) upon X-ray absorption in the region of the C K-edge. The yield of photodissociation products was recorded as a function of photon energy. The total photoabsorption yield is qualitatively similar to near-edge X-ray absorption fine structure (NEXAFS) spectra recorded from condensed phase peptides and proteins. Fragment specificity reveals distinct quantitative differences between spectra obtained for different masses. Fragmentation channels can be assigned to specific electronic transitions some of which are site specific. For instance, C 1s → π(★) excitations in the leucine enkephalin aromatic side chains lead to relatively little fragmentation, whereas such excitations along the peptide backbone induce strong fragmentation.