Computer Graphics Laboratory, Department of Pharmaceutical Chemistry, University of California at San Fransisco, San Fransisco, CA 94143-0446, USA.|Computer Graphics Laboratory, Department of Pharmaceutical Chemistry, University of California at San Fransisco, San Fransisco, CA 94143-0446, USA.|Computer Graphics Laboratory, Department of Pharmaceutical Chemistry, University of California at San Fransisco, San Fransisco, CA 94143-0446, USA.