1. Department of Biochemistry and Center of Neurosciences, University of Coimbra, P.O. Box 3126, Coimbra 3001-401, Portugal
2. Department of Chemistry, The University of Texas at Dallas, P. O. BOX 830688, Richardson, TX 75083-0688, USA
3. The Mary Nell and Ralph B. Rogers Magnetic Resonance Center, Department of Radiology, University of Texas Southwestern Medical Center, 5801 Forest Park Rd., Dallas, TX 75235-9085, USA