1. Department of Chemistry, The University of Newcastle, Newcastle upon Tyne, NE1 7RU, U.K., Magnetic Resonance Centre, University of Florence, Via L. Sacconi 6, 50019 Sesto Fiorentino, Italy, Department of Chemistry, University of Florence, Via G. Capponi 7, 50121 Florence, Italy, Department of General Physiology and Biochemistry, University of Milan, Via Celoria 26, 20133 Milan, Italy, and Department of Soil Science and Plant Nutrition, University of Florence, P. delle Cascine 28, 50144 Florence, Italy