Fig. 8. Replacement of Glu269 with Asp abolishes substrate protection against DiPC reactivity. Purified E269D/C148 LacY (∼20 µM) in DDM at pH 6.0 was incubated for 30 min at 30°C with 2% DMSO (A) and 20 mM DiPC (B), 20 mM DiPC in the presence of NPGal (C) or 20 mM DiPC in the presence of NPGlc (D). Data are displayed as selected ion chromatograms of unmodified and DiPC-modified peptide ²⁶⁸-GDLLNASIM-²⁷⁶ (m/z 885.6 and 1011.6, respectively). The chromatograms are representative of three independent experiments. Maximum peak height in the absence of modification was typically 5 × 10⁵ counts.