1. Contribution from the Departments of Chemical Physics and Immunology, Weizmann Institute of Science, Rehovot, Israel, Faculty of Biology, University of Konstanz, D-78457 Konstanz, Germany, Lehrstuhl für Mikrobiology, Universität Fridericiana, D-76128 Karlsruhe, Germany, Royal Danish School of Pharmacy, Institute of Analytical & Pharmaceutical Chemistry, DK-2100 Copenhagen, Denmark, and Department of Chemistry, University of Illinois, Urbana, Illinois 61801