1. Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, 108 North Greene Street, Baltimore, Maryland 21201, Center for Fluorescence Spectroscopy, University of Maryland, 725 West Lombard Street, Baltimore, Maryland 21201, Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, and Departments of Chemistry and Biochemistry, University of Michigan, Ann Arbor, Michigan 48109-1055