1. Department of Physics; University of Houston; Houston TX 77204 USA
2. Center for Theoretical Biological Physics; Rice University; Houston TX 77005 USA
3. Sealy Center for Structural Biology and Molecular Biophysics; University of Texas Medical Branch; Galveston TX 77555 USA
4. Department of Neurobiology and Anatomy; University of Texas Health Science Center; Houston TX 77030 USA