Gas-phase conformation-specific photofragmentation of proline-containing peptide ions

Singly-protonated proline-containing peptides with N-terminal arginine are photodissociated with vacuum ultraviolet (VUV) light in an ESI linear ion trap/orthogonal-TOF (LIT/o-TOF). When proline is the n^th residue from the N-terminus, unusual b _n + 2 and a _n + 2 ions are observed. Their formation is explained by homolytic cleavage of the C_α− C bond in conjunction with a rearrangement of electrons and an amide hydrogen. The latter is facilitated by a proline-stabilized gas-phase peptide conformation.