[1] Stanford Univ, Biophys Program, Stanford, CA 94305 USA
[2] Univ Calif San Diego, Dept Phys, San Diego, CA 92103 USA
[3] Stanford Univ, Chem Engn Appl Phys Mat Sci & Engn, Biophys Program, Stanford, CA 94305 USA
[4] Stanford Univ, Howard Hughes Med Inst, Dept Biochem, Dept Microbiol & Immunol, Stanford, CA 94305 USA