A 15-ml portion of the filtrate was evaporated to dryness using rotary evaporator and 35°C water bath. The residue was dissolved in 1.5 ml of methanol and filtered through a 0.45 mm filter for organic solvents (Acrodisc LC13 PVDF filter, Gelman Sciences, MI) prior to injection (20 ml) to the HPLC apparatus.