1. ISIS Facility, STFC Rutherford Appleton Laboratory, Harwell Campus, Didcot OX11 0QX, U.K.
2. Dipartimento di Scienze, Università degli Studi “Roma Tre”, via della Vasca Navale 84, 00146 Roma, Italy
3. Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K.