1. Departments of Chemistry, University College London, Christopher Ingold Laboratories, 20 Gordon Street, London WC1H 0AJ, Wolfson Centre for Gene Therapy of Childhood Disease, UCL Institute of Child Health, 30 Guilford Street, London WC1N 1EH, and Biochemical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom