1. Contribution from the Departamento de Química Orgánica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, PROPLAME-CONICET, Ciudad Universitaria-Pabellón II, 1428 Buenos Aires, Argentina, Department of Molecular Biology, Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, D-37077 Göttingen, Germany, and Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Higashi-ku, 812, Japan