1. Biosciences Division, Argonne National Laboratory, 9700 South Cass Avenue, Building 221 Argonne, Illinois 60439, Center for Molecular Modeling, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia, and Department of Biochemistry and Molecular Biology, Gordon Center for Integrative Science, University of Chicago, 929 57th Street, Chicago, Illinois 60637