1. Joint Research Center for Atom Technology (JRCAT), Angstrom Technology Partnership (ATP) and National Institute for Advanced Interdisciplinary Research (NAIR), Higashi 1-1-4, Tsukuba, Ibaraki 305, Japan; and Department of Biomolecular Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan