before injection into an HPLC system consisting of a Hitachi L-2130 pump, a Hitachi L-2400 UV detector, and a reverse-phase column (300 mm 4.6 mm 5 lm, Phenomenex, Torrance, CA). The mobile phase was 0.1% trifluoroacetic anhydride/ acetonitrile, 86:14 (v/v) at a flow rate of 1.0 ml/min, and UV detection was at 280 nm.