Fig. 2.

Typical selected reaction monitoring (SRM) chromatograms of the dilysyl-MDA cross-link. Dilysyl-MDA cross-links were purified by Oasis cartridge and HPLC, after being digested to single amino acid by step digestion with proteases. The fractions containing the radioactivity were pooled, concentrated, and analyzed by LC/MS/MS. SRM of specific transition ions for the precursor ions at m/z 329 (upper panels) or m/z 341 (IS, lower panels) are shown. The IS was prepared from reaction of MDA with lysine and is assumed to be cross-linked to the α-amine, thus yielding a shorter retention time.