1. Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300, the Program in Biophysics and Structural Biology, Departments of Biochemistry and Chemistry and the Rosenstiel Basic Medical Sciences Research Center, Brandeis University, 415 South Street, Waltham, Massachusetts 02254, and the National Biomedical EPR Center, Department of Biophysics, Medical College of Wisconsin, Milwaukee, Wisconsin 53226-0509