1. Kimika Fakultatea, Euskal Herriko Unibertsitatea, P.K. 1072, 20080 Donostia, Euskadi, Spain, Department of Chemistry, The University of Sheffield, Sheffield S3 7HF, England, Laboratoire de Chimie Biophysique, Institut le Bel, Université Louis Pasteur, 67000 Strasbourg, France, and Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138