1. Contribution from the CERM, University of Florence, Via Luigi Sacconi 6, 50019 Sesto Fiorentino, Florence, Italy, Department of Chemistry, University of Florence, Via della Lastruccia 5, 50019 Sesto Fiorentino, Florence Italy, and Department of Agricultural Biotechnology, University of Florence, P.le delle Cascine 28, 50144 Florence, Italy