1. Department of Clinical Biochemistry, Institute of Child Health (University of London), 30 Guilford Street, London WC1N 1EH, U.K.
2. Department of Chemistry, King's College London, The Strand, London WC2R 2LS, U.K.
3. Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, U.K.
4. Dyson Perrins Laboratory and Oxford Centre for Molecular Sciences, Oxford University, South Parks Road, Oxford OXI 3QY, U.K.