(A) Photochemical labeling of 7 and mechanistic reversal of the labeling process using 1d. Trp residues in 7 are highlighted in cyan. (B) Unmodified 7. (C) 7 + 1a. (D) 7 + 1d, 15 min irradiation time at 302 nm. (E) 7 + 1d, 45 min irradiation time with 320 nmlong pass filter. (F) Confirmation of site selectivity by MS-MS. a Conversion estimated by TIC, average of 2 runs. b 7 mM 1a, 1 mM GSH, 20 mM pH 6.9 NH4OAc buffer. c 3 mM 1d, 6 mM GSH, 20 mM pH 6.9 NH4OAc buffer. d Ratio of +1 to +2 modifications. e 3 mM 1d, 3 mM GSH, 20 mM pH 6.9 NH4OAc buffer, 320 nm long-pass filter used.