Fig. 3.

Tryptic peptides of SeO₃²⁻-reacted CstR identify a selenotrisulfide. The peptides involved in the disulfide cross-link are denoted peptide “A” (red) for the Cys31 peptide ²⁴MMEEGKDCKDVITQISASK⁴² and peptide “B” (blue) for Cys60′, ^48′LMGIIISENLIECVK^62′. Fragmentation ions, b and y, are assigned relative to the respective peptide, “A” or “B”, and cross-linked peptides are denoted as AB_yn. Peptide “A” remained intact for all cross-linked ions identified and all fragmentation occurred on peptide “B” (A) Accurate mass and isotopic distribution of the RS-SR′ CstR disulfide in the +4 charge state. The monoisotopic mass is 946.980 Da (Table 1) (B) Fragmentation pattern of RS-SR′ disulfide (C) Isotopic distribution of RS-Se-SR′ selenotrisulfide in the +4 charge state. The monoisotopic mass is 966.710 Da (all ¹²C and ⁸⁰Se) and among the preceding peaks, ⁷⁶Se, ⁷⁷Se and ⁷⁸Se were identified. (D) Fragmentation of the RS-Se-SR′ cross-link.