Fig. 4.
(A) Representative kinetic traces that illustrate the time-course of reactivity of C9S SqrR toward various oxidants under various conditions. LC-ESI-MS spectra of C9S SqrR using a 20-fold excess of glutathione persulfide (GSSH), cysteine persulfide (CysSSH), each prepared as described (Table S3), and impure cystine trisulfide (CysSSSCys + CysSO), for variable times followed by addition of excess IAM to cap both thiols and persulfides. Solution conditions: 30 μM protomer C9S SqrR, 150 mM phosphate buffer pH 7.4, 1 mM EDTA. Vertical dashed lines depict the intact masses (amu) of the different oxidation states of the SqrR protomer (12295, reduced, dot-dashed black; 12357, tetrasulfide, purple; 12389, pentasulfide, blue; 12409, AM capped reduced, black; 12471, AM capped persulfide, salmon; 12441, AM capped and mixed disulfide with Cys, tan; 12533, two mixed disulfides with Cys, green). Summary of intermediates and final products with mechanistic proposal compatible with the results for GSSH (B) and CSSH (C) treatment. The pKa of the different thiols are from prior reports (35, 68).