1. Contribution from the Department of Biochemistry. University of Illinois at Urbana-Champaign, 600 South Mathews Street, Urbana, Illinois 61801, Illinois EPR Research Center, 506 South Mathews Street, Urbana, Illinois 61801, and Department of Chemistry and Center for Biological Macromolecules, University at Albany, State University of New York, Albany, New York 12222