1. Contribution from the Departments of Biomolecular Sciences, Physics, and Chemical Engineering, UMIST, Manchester M60 1QD, U.K., School of Biological and Chemical Sciences, University of Exeter, Exeter EX4 4QG, U.K., and ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, U.K.