Trypsin Digestion
The in-solution trypsin digestion of rErAII was performed using an adapted protocol (Klammer and Maccoss, 2006). Desalted and dried samples of rErAII containing 35 µg of protein (1 nmol) were resuspended in 50 µL of 0.1% (w/v) RapiGest SF acid labile surfactant (Waters Corp.) diluted in 50 mM ammonium bicarbonate, pH 7.8. The samples were heated to 99 ºC for 2 min, and DTT was added for a final concentration of 5 mM. After incubation at 60 ºC for 30 min, iodoacetamide was added to a final concentration of 15 mM and the samples were incubated for 30 min at room temperature while protected from light. Trypsin was added at a ratio of 1:100 enzyme/protein along with CaCl2 at a final concentration of 1 mM, and the solution was incubated for 1 h at 37 ºC. For surfactant degradation, HCl was added at a final concentration of 100 mM. The samples were centrifuged at 20,800 x g at 4 ºC for 10 min, and the supernatants were transferred to clean tubes.