1. Department of Chemistry, Structural Biology Laboratory, University of York, Heslington, York YO10 5DD, U.K., Department of Biological and Nutritional Sciences, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, U.K., and CIISA-Faculdade de Medicina Veterinária, Rua Gomes Freire, 1199 Lisboa Codex, Portugal