1. Departments of Biomedical Engineering and Chemistry, Boston University, Boston, Massachusetts 02215, Departement de Chimie, Université d'Avignon, F-84000 Avignon, France, Department of Cell Biology, Duke University Medical Center, Durham, North Carolina 27710, Department of Biological and Environmental Engineering, Cornell University, Ithaca, New York 14853, and Army Research Office, RTP, North Carolina 27709