1. Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, D.C. 20375, Laboratorie Physico-Chimie Curie CNRS UMR No. 168, Institut Curie-Section de Recherche, 11 Rue Pierre et Marie Curie, 75231 Paris, Cedex 05 France, and Geo-Centers, Inc., c/o Naval Research Laboratory, 54555 Overlook Avenue SW, Washington, D.C. 20375