1. Department of Chemistry, University of Texas at Dallas, P.O. Box 830688, Richardson, Texas 75083-0688, Department of Chemistry, School of Sciences, University of Minho, 4714 Braga, Portugal, Department of Biochemistry and Center of Neurosciences, Faculty of Science and Technology, University of Coimbra, Apartado 3126, 3000 Coimbra, Portugal, and Department of Radiology, Rogers Magnetic Resonance Center, University of Texas Southwestern Medical Center, Dallas, Texas 75235-9085