1. Contribution from the Biological Physics Group, Department of Physics, UMIST, PO Box 88, Manchester M60 1QD United Kingdom, Centre for Biomedical Engineering NE47-379, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, and ISIS Neutron Facility, CLRC, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QZ, United Kingdom