Radical Anions of Oxidized vs. Reduced Oxytocin: Influence of Disulfide Bridges on CID and Vacuum UV Photo-Fragmentation.

The nonapeptide oxytocin (OT) is used as a model sulfur-containing peptide to study the damage induced by vacuum UV (VUV) radiations. In particular, the effect of the presence (or absence in reduced OT) of oxytocin's internal disulfide bridge is evaluated in terms of photo-fragmentation yield and nature of the photo-fragments. Intact, as well as reduced, OT is studied as dianions and radical anions. Radical anions are prepared and photo-fragmented in two-color experiments (UV + VUV) in a linear ion trap. VUV photo-fragmentation patterns are analyzed and compared, and radical-induced mechanisms are proposed. The effect of VUV is principally to ionize but secondary fragmentation is also observed. This secondary fragmentation seems to be considerably enabled by the initial position of the radical on the molecule. In particular, the possibility to form a radical on free cysteines seems to increase the susceptibility to VUV fragmentation. Interestingly, disulfide bridges, which are fundamental for protein structure, could also be responsible for an increased resistance to ionizing radiations. Graphical Abstract.