1. Contribution from the National Institute for Medical Research (N.I.M.R.), The Ridgeway, Mill Hill, London NW7 1AA, U.K., Laboratory of Biophysical Chemistry and BIOSON Research Institute, Nijenborgh 4, 9747 AG Groningen, The Netherlands, Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands, and Department of Chemistry, York University, York Y01 5DD, U.K.