1. Contribution from the Department of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel, Department of Molecular Biology, The Hebrew University of Jerusalem - Hadassah Medical School, Jerusalem 91120, Israel, Department of Chemistry, Nuclear Chemistry, The Royal Institute of Technology, S-10044 Stockholm 70, Sweden, and Radiation Biology Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA