1. Department
of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K.
2. Department
of Molecular Biology and Biotechnology, University of Sheffield, Western Bank, Sheffield S10 2TN, U.K.
3. The
Johnson Research Foundation and Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania 10104, United States
4. Department
of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States
5. Cavendish
Laboratory, Dept. of Physics, University of Cambridge, J. J. Thomson
Ave, Cambridge, CB3 0HE, U.K.
6. COMP Centre
of Excellence, Department of Applied Physics, Aalto University, School of Science,
P.O. Box 15100, 00076 Aalto, Finland