1. Department of Biology, Northern Arizona University, Flagstaff, Arizona 86011, Department of Chemistry, Pacific University, Forest Grove, Oregon 97116, Department of Immunology, The Weizmann Institute of Science, Rehovot, Israel, Department of Microbiology and Immunology, Virginia Commonwealth University, Richmond, Virginia 23298, and Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545