1. Contribution from the Institute of Agricultural Chemistry, University of Bologna, Viale Berti Pichat, 10 40127 Bologna, Italy, Department of Biochemistry, Université Laval, Québec City, Québec G1K 7P4, Canada, Department of Chemistry, University of Florence, Via Gino Capponi, 7, 50121 Florence, Italy, and Department of Chemistry, University of Modena, Via Campi 183, 41100 Modena, Italy