2.5. Thiolysis and reversed-phase HPLC-ESI-MS analysis
Thiolysis was carried out by the method described by Wei et al. (2012). Thiolysis products were analyzed by reversed-phase HPLC-ESI-MS.
Reversed-phase HPLC-ESI-MS analysis was performed using an Agilent 1200 system (Agilent, USA) interfaced to a QTRAP 3200 (Applied Biosystems, USA) with a Hypersil ODS column (4.6 mm × 250 mm, 5 μm) (Elite, China). Two solvents, namely A = 0.5% (v/v) TFA in aqueous and B = CH3CN, were used. The gradient condition was: 0–45 min, 12%–80% B; 45–50 min, 80%–12% B. The column temperature was 25 °C and the flow-rate was set at 1 mL/min. Detection was at a wavelength of 280 nm and the UV spectra were acquired between 200 and 600 nm. Degradation products were identified on chromatograms according to their relative retention times and LC-MS.