School of Mathematical Sciences, Nankai University, Tianjin, China.|Center for Applied Mathematics, Tianjin University, Tianjin, China.|Department of Biochemistry and Institute for Protein Design, University of Washington, Seattle, WA, USA.|Department of Biochemistry and Institute for Protein Design, University of Washington, Seattle, WA, USA.|Department of Biochemistry and Institute for Protein Design, University of Washington, Seattle, WA, USA.|School of Mathematical Sciences, Nankai University, Tianjin, China.