1. Laboratoire de Biologie Structurale (C.N.R.S.), I.G.B.M.C., 1 rue Laurent Fries, 67404 Illkirch (C.U. de Strasbourg), France, Laboratoire de Chimie Biophysique, Institut Le Bel, 4 rue Blaise Pascal, Université Louis Pasteur, 67000 Strasbourg, France, Department of Natural Sciences, University of Cyprus, P.O. Box 537, CY 1678, Nicosia, Cyprus, and Department of Chemistry & Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138