the mixture was derivatized by the addition of 2 ml dansylchloride solution (10 mg/ml in acetone). After incubation for 30 min at 40 °C in darkness, 100 ml of aqueous ammonia was added to remove any residual Dns-Cl. The volume was adjusted to 5 ml with acetonitrile. Finally, the supernatant was filtered through 0.22 μm-pore-size filters prior to HPLC analysis.