1. Department of Pharmaceutical Sciences, University of Padova, via Marzolo 5, 35131 Padova, Italy, Department of Chemistry and Biochemistry, University of Maryland, Baltimore County, Baltimore, Maryland 21250, Menarini Ricerche, via Tito Speri 10, 00040, Pomezia, Italy, and G. Moruzzi Department of Biochemistry, University of Bologna, via Irnerio 48, 40126 Bologna, Italy