1. Contribution from the Department of Biology, University College London, Gower Street, London WC1E 6BT, U.K., Centre for Biomolecular Sciences and Department of Physics and Astronomy, University of St. Andrews, North Haugh, Saint Andrews, Fife KY16 9ST, U.K., and Department of Biotechnology and Molecular Sciences, University of Insubria, 21100 Varese, Italy