Department of Chemistry, Stanford University, Stanford, CA 94305, USA Department of Zoology and Genetics, Iowa State University, 2112 Molecular Biology Building, Ames, IA 50011-3260, USA.|Department of Chemistry, Stanford University, Stanford, CA 94305, USA Department of Zoology and Genetics, Iowa State University, 2112 Molecular Biology Building, Ames, IA 50011-3260, USA.|Department of Chemistry, Stanford University, Stanford, CA 94305, USA Department of Zoology and Genetics, Iowa State University, 2112 Molecular Biology Building, Ames, IA 50011-3260, USA.