1. Department of Chemistry, Brown University, Box H, Providence, Rhode Island 02912-9108, Kitasato Institute for Life Sciences, Kitasato University, 1-151-1, Kitasato, Sagamihara, Kanagawa 228-8555, Japan, and The Kitasato Institute, 9-1, Shirokane 5-chome, Minato-ku, Tokyo 108-8642, Japan