1. Contribution from the Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093-0371, Center for Theoretical Biological Physics, University of California, San Diego, La Jolla, CA 92093-0371, Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, AstraZeneca R&D Mlndal, Medicinal Chemistry, S-42183 Mlndal, Sweden, and Department of Physics, University of California, San Diego, La Jolla, CA 92093-0371