1. Peptide Synthesis Laboratory, CNR, Institute of Mutagenesis and Differentiation, Via Svezia 2A, I-56124 Pisa, Italy, Menarini Ricerche, Via Sette Santi 3, I-50131 Firenze, Italy, Department of Organic Chemistry, Biopolymer Research Center, CNR, University of Padova, Via Marzolo 1, I-35131 Padova, Italy, and Department of Organic Chemistry “Ugo Schiff”, University of Florence, Via G. Capponi 9, I-50121 Firenze, Italy