1. Department of Chemistry, McMaster University, Hamilton, ON, Canada L8S 4M1, Laboratoire “Lésions des Acides Nucléiques”, INAC/SCIB - UMR-E no. 3 CEA-UJF, CEA Grenoble, rue des Martyrs, F-38 054 Grenoble Cedex 9, France, Departamento de Fisica Geral, Instituto de Fisica, Universidade de Sao Paulo, Cidade Universitaria, S.P. Brazil, and Department of Physics, Faculty of Science, University of Chile, Santiago, Chile