Fig. 4.

Detection of persulfide species of FnrP by LC–MS. a ESI-TOF mass spectra of [4Fe–4S] FnrP (2.3 μM) before and after the addition of increasing amounts of O₂ are shown, as indicated. The peak at 28,969 Da corresponds to the monomer molecular ion peak of FnrP, and the peaks at +32, +64, +96 and +128 Da correspond to the addition of one, two, three and four covalently bound sulfur atoms, respectively, as indicated. Treatment of an aerobic ([O₂]:[4Fe–4S] = 5) sample, post-O₂ exposure, with DTT led to loss of persulfide adducts. b During the titration, the mass of the FnrP decreased from 28,969 to 28,967 Da, corresponding to the loss of 2 protons and indicative of O₂-induced disulfide bond formation