1. a Department of Biology , Johns Hopkins University , Baltimore, MD, 21218-2685, U.S.A.
2. b Departement de Chimie , Institut National Polytechnique , Houphouet Boigny, Yamoussokro, BP 1083, Ivory Coast
3. c Laboratory of Biophysical Chemistry , National Institutes of Health , 50 South Drive, Building 50, Room # 3334, Bethesda, MD, 20892, U.S.A.