1. Department of Chemistry, University of Texas at Dallas, P.O. Box 830688, Richardson, Texas 75083-0688, The Rogers Magnetic Resonance Center, Department of Radiology, University of Texas Southwestern Medical Center, 5801 Forest Park Road, Dallas, Texas 75235-9085, and Department of Radiology, Beth Israel Deaconess Medical Center and Harvard Medical School, 1 Deaconess Road, Boston, Massachusetts 02115