1. National Research Council, Steacie Institute for Molecular Sciences, Bldg. 459, Stn. 18, Chalk River, Ontario, K0J 1J0, Canada, Institute of Biophysics and X-Ray Structure Research, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria, and Raman Research Institute, Bangalore 560 080, India