1. Center for Computational Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba Science City, Ibaraki 305-8577, Japan, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8571, Japan, Laboratoire de Dynamique Moléculaire, Insitut de Biologie Structurale Jean-Pierre Ebel, 41 rue Jules Horowitz, 38027 Grenoble Cedex 1, France, and Institute of Medical Science, University of Tokyo, Shirokanedai 4-6-1, Minatoku, Tokyo 108-8639, Japan