1. Department
of Chemistry, University of Cambridge, Cambridge CB2 1EW, U.K.
2. Department
of Molecular Biology and Biophysics, University of Connecticut Health Center, Farmington, Connecticut 06030, United States
3. Technische Universität Mun̈chen Institute for Advanced Study & Department of Chemistry, Lichtenbergstr. 4, 85748 Garching, Germany
4. Istituto
Pasteur - Fondazione Cenci Bolognetti and Istituto di Biologia e Patologia
Molecolari del CNR, Dipartimento di Scienze Biochimiche “A.
Rossi Fanelli”, Sapienza Università di Roma, Rome 00185, Italy