1. Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (N.T.A., F.E.L.); Department of Botany and Plant Pathology, Purdue University, West Lafayette, Indiana 47907 (D.M.H.); and Molecular Probes, Inc., 4849 Pitchford Ave., Eugene, Oregon 97402 (J.E.W., R.P.H.)