1. Chemistry Department, Reading University, Whiteknights, Reading RG6 6AD, U.K., CRC Centre for Cancer Therapeutics, Institute of Cancer Research, Sutton, Surrey SM2 5NG, U.K., and CRC Biomolecular Structure Unit, Chester Beatty Laboratories, Institute of Cancer Research, 237 Fulham Road, London SW3 6JB, U.K.