1. Department
of Biochemistry, University of Oxford, Oxford OX1 3QU, U.K.
2. Unidad
de Biofísica (CSIC, UPV-EHU) and Departamento de Bioquímica, Universidad del País Vasco, P.O. Box 644, 48080 Bilbao, Spain
3. Department
of Physics, King’s College London, London WC2R 2LS, U.K.