[1] Georgia Inst Technol, Sch Phys, Atlanta, GA 30332 USA
[2] Oak Ridge Natl Lab, Biosci Div, UT ORNL Ctr Mol Biophys, POB 2009, Oak Ridge, TN 37831 USA
[3] Lab Int Associe CNRS, F-54506 Vandoeuvre Les Nancy, France
[4] Univ Illinois, Dept Phys, Urbana, IL 61801 USA