Trypsin digest and MS analysis of modified peptides. After incubation with a 5-fold excess of compound 4 for one hour as described above, 1. to quench excess inhibitor. 2 μL of Pierce TM trypsin protease (1 mg/mL) was added to give a final protein: trypsin ratio of 10:1. Overnight incubation at 37 ℃ afforded a series of peptides which were prepared using PierceTM C18 spin columns according to the manufacturer's procedure.