1. Optical Spectroscopy Section, Laboratory of Molecular Biophysics, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892-1412, Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, Department of Biology, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, and Department of Chemistry & Biochemistry, Montana State University, Bozeman, Montana 59717