1. Magnetic Resonance Center (CERM) and Department of Chemistry, University of Florence, Via L. Sacconi, 6, I-50019 Sesto Fiorentino, Italy, and Magnetic Resonance Center (CERM) and Department of Agricultural Biotechnology, University of Florence, Via L. Sacconi, 6, I-50019 Sesto Fiorentino, Italy