1. Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, Basic Research Program, SAIC Frederick, National Cancer Institute at Frederick, Frederick, Maryland 21702, Department of Chemistry, George Mason University, Fairfax, Virginia 22030, and Chemistry Section, Laboratory of Comparative Carcinogenesis, National Cancer Institute at Frederick, Frederick, Maryland 21702 jtoscano@jhu.edu.