1. Institute of Biomolecular Chemistry of CNR, Padova Unit, via F. Marzolo 1, 35131 Padova, Italy, Department of Biological Chemistry, University of Padova, via G. Colombo 3, 35131 Padova, Italy, Vrije Universiteit Brussel, Organic Chemistry Department, Pleinlann 2, B-1050 Brussel, Belgium, and Department of Chemical Sciences, University of Padova, via F. Marzolo 1, 35131 Padova, Italy