1. Department of Chemistry & Biochemistry, Center for Theoretical Biological Physics, Department of Pharmacology, Howard Hughes Medical Institute, University of California at San Diego, La Jolla, California 92093-0365, Interdisciplinary Centre for Mathematical and Computational Modelling, Warsaw University, 02-106 Warsaw, Poland, and NEST Scuola Normale Superiore, Piazza dei Cavalieri, 7 I-56126 Pisa, Italy