1. Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health (NIH), Bethesda, MD 20892.
2. Department of Biochemistry and Genetics, University of Newcastle-upon-Tyne, Newcastle NE2 4HH, United Kingdom.
3. Protein Expression Laboratory, Office of the Director, NIH, Bethesda, MD 20892.