1. Department of Chemistry and Biochemistry and; Centre for Research in Molecular Modeling (CERMM); Concordia University; 7141 Sherbrooke St. West Montréal Québec H4B 1R6 Canada
2. Present address: Howard Hughes Medical Institute; Department of Molecular Biology and; Center for Computational and Integrative Biology; Massachusetts General Hospital; 185 Cambridge Street Boston MA 02114 USA