1. Department of Biochemical Sciences, University of “La Sapienza”, P.le A. Moro 5, I-00185 Roma, Italy, Department of Physics, Drexel University, Philadelphia, Pennsylvania 19104, Institute of Molecular Biology, University of Oregon, Eugene, Oregon 97403, Department of Molecular and Cell Biology and Department of Physics, University of California, Berkeley, California 97420, and Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720