The
concentration of PBS was calculated on the basis of the molar
absorptivity formula.15 The cross-linking experiment and LC−
MS/MS were used as previously described with minor
changes.33 The PBS−OCP sample was resuspended at 0.1
μM based on the PB concentration calculation. BS3 and DSS
cross-linking was performed according to the manufacturer’s
protocol followed by desalting using Zeba columns (Thermo
Scientific, Waltham, MA) with minor modifications. Modified
samples were precipitated using acetone, and the resuspended
samples were directly subjected to trypsin digestion.