[1] Univ Calif Berkeley, Biophys Grad Grp, Berkeley, CA 94720 USA
[2] Lawrence Berkeley Natl Lab, Phys Biosci Div, Berkeley, CA 94720 USA
[3] Max Planck Inst Biophys Chem, Dept NanoBiophoton, D-37077 Gottingen, Germany
[4] Univ Calif Berkeley, Dept Phys, Berkeley, CA 94720 USA