1. DuPont Life Sciences Enterprise, Biochemical Science and Engineering, P.O. Box 80328, Experimental Station, Wilmington, Delaware 19880-0328, DuPont Corporate Center for Analytical Sciences, P.O. Box 80228, Experimental Station, Wilmington, Delaware 19880-0328, Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6059