1. Laboratory of Molecular Biophysics, Rex Richards Building, South Parks Road, Oxford OX1 3QU, U.K., Rega Institute for Medical Research, K. U. Leuven, Minderbroedersstraat 10, B-3000 Leuven, Belgium, and Oxford Centre for Molecular Sciences, New Chemistry Building, South Parks Road, Oxford OX1 3QT, U.K.