The high performance liquid chromatography (HPLC) apparatus consists of a Waters (Milford, MA, USA) Model 600E pump associated with a Waters Model 600 gradient controller and a Rheodyne (Cotati, CA, USA) Model 7725i and it is coupled with a Waters 486 Tunable UV Absorbance Detector, (Milford, MA, USA). The system is operated with Millenium software.