Crosslinking of reactive glutamines in fibrinogen αC (233-425) with lysine mimic GEE. Representative MALDI-TOF spectra showing crosslinking reaction between reactive glutamine in αC (233-425) and the lysine mimic GEE catalyzed by FXIIIa. (A) Chymotrypsin digest at time points 0, 5, 10, and 15 minutes show that the fragment peak containing reactive glutamine Q328 (2448.5 m/z) was crosslinked to GEE by FXIIIa to form a product peak Q328-GEE (2534.5 m/z). Each spectrum was obtained from the same starting assay and represents a quenched time point followed by a chymotrypsin digest analyzed using MALDI-TOF mass spectrometry. (B) A similar plot with GluC digest at time points 0, 0.4, 5, and 10 minutes shows reactant and product peaks containing Q237 and Q366, and their respective GEE-crosslinked products (1636.5 m/z and 1435.3 m/z). Q237 is mostly crosslinked to product by 20 seconds as compared with Q366, which was only crosslinked to product by 10 minutes.