1. Center for Bio/Molecular Science and Engineering, Naval Research Lab, 4555 Overlook Avenue SW, Code 6950, Washington, D.C. 20375, and Laboratoire Physico-Chimie Curie, CNRS-UMR 168, Institut Curie-Section de Recherche, 11 rue P. et M. Curie, 75231 Paris Cedex 05, France