Figure 2.

Representative cross-linking results. A) Cross-linked Ms sGC NT13, examined by SDS-PAGE and stained with Coomassie brilliant blue. Individual bands were cut from the gel, digested with trypsin and examined by tandem mass spectrometry. B) Representative cross- linked peptide MS/MS spectrum. BS²G cross-link α₁ K434 to β₁ K366 was found in the +4 charge state due to two amine termini and lysine/arginine at the trypsin cleavage sites, and displayed a 22 ppm error in observed vs. calculated precursor mass (Table 1). The cross-linked peptide was observed and selected for fragmentation in 7 scans in a single experiment. Between 7 and 36 fragments were identified in each scan, each with a mass accuracy of approximately 1 ppm. Representative fragments are labeled in the mass spectrum. Identified fragments in this spectrum included β₁ fragments b_4–10, y_2–6, y_8–15 (which contain the cross-linker and entire α₁ peptide), α₁ fragments y_2–8, and α₁ fragments b_3–5 (which contain the cross-linker and entire β₁ peptide).