1. Center for Molecular Modeling, Office of Intramural Research Center for Information Technology National Institutes of Health Bethesda MD 20892 USA
2. Present address: Bioinformatics and Computational Bioscience Branch National Institute of Allergy and Infectious Diseases National Institutes of Health Bethesda MD 20892 USA
3. Present address: Intramural Research Program, National Library of Medicine National Institutes of Health Bethesda MD 20892 USA
4. Chemistry and Synthesis Center National Heart, Lung, and Blood Institute National Institutes of Health Rockville MD 20850 USA