1. Department of Biological and Molecular Sciences, University of Stirling, Stirling FK9 4LA, Scotland
2. Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, Scotland
3. Department of Protein Chemistry, Celltech Ltd., 216 Bath Road, Slough, Berks. SLI 4DY
4. Department of Biochemistry, University of Edinburgh, George Square, Edinburgh EH8 9XD, Scotland, U.K.