1. Institute of Biophysics, Biological Research Center of the Hungarian Academy of Sciences, P. O. Box 521, Szeged, Hungary, H-6701, Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, Department of Physics, University of Guelph, Guelph, ON, Canada, and Department of Physiology and Biophysics, University of California, Irvine, California