[1] Univ Illinois, Beckman Inst, Urbana, IL 61801 USA
[2] Univ Illinois, Dept Phys, Urbana, IL 61801 USA
[3] Univ Washington, Dept Bioengn, Seattle, WA 98195 USA
[4] Univ Paris 06, UMR 7613 CNRS, F-75252 Paris 05, France
[5] Univ Oxford, Dept Biochem, Oxford OX1 3QU, England
[6] Univ Oxford, Oxford Ctr Mol Sci, Oxford OX1 3QU, England