All eluents used were HPLC grade (Baker, Gross-Gerau, Germany). The analysis via ESI-MSn was done on a Bruker Daltonics (Bremen, Germany) HCT PTM Discovery System electrospray ionization ion trap mass spec- trometer using the following parameters: negative ion mode; scan, 100–1000 m/z; nebulizer, 60.00 psi; dry gas, 11 L/min; 365 C; capillary, ±3500 V.