1. Center for Biomolecular Science and Engineering, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, D.C. 20375, Advanced Materials and Processing Branch, NASA Langley Research Center, 6A W Taylor Street, MS 226, Hampton, Virginia 23681-0001, and Laboratorie Physico-Chemie Curie, CNRS UMR No. 168, Institut Curie-Section de Recherche, 11 rue Pierre et Marie Curie, 75231 Paris, Cedex 05 France