1. Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati, College of Medicine, 231 Bethesda Avenue, Cincinnati, Ohio 45267, Department of Physiology and Biophysics, College of Medicine, University of Illinois, Chicago, Illinois 60612, and Departamento de Quimica, Faculdade de Ciencias e Tecnologia, Universidade de Coimbra, 3049 Coimbra, Portugal