*2.1. Chemicals*

All chemicals were of analytical grade and obtained from Sigma–Aldrich (St. Louis, MO, USA), Fluka (Buchs, Switzerland) or Sigma–Aldrich (Steinheim, Germany). All the solvents used were of HPLC grade and of analytical grade and obtained from Sigma–Aldrich (St. Louis, MO, USA and Steinheim, Germany) or Carlo Erba Reagents (Cornaredo, Milan, Italy).
