1. Centro di Risonanze Magnetiche and Department of Chemistry, University of Florence, Via Luigi Sacconi 6, 50019, Sesto Fiorentino, Italy, Department of Chemistry, University of Siena, Via Aldo Moro, I-53100 Siena, Italy, and EMBL-Hamburg Outstation, Building 25A, DESY, Notkestrasse 85, 22603 Hamburg, Germany