1. Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma Tor Vergata, Via della Ricerca Scientifica, 00133, Rome, Italy, Department of Chemistry and Biochemistry and Interdepartmental Program in Biomolecular Science and Engineering, University of California, Santa Barbara, Santa Barbara, California 93106, and Department of Biochemistry, Carver College of Medicine, University of Iowa, Iowa City, Iowa 52242-1109