1. Department of Chemical and Life Sciences, The University of Greenwich, Woolwich Campus, Wellington Street, London SE18 9PF, U.K., Department of Biological and Chemical Sciences, Central Campus, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, U.K., and Roche Discovery Welwyn, P.O. Box 8, Welwyn Garden City, Herts Al7 2AY, U.K.