1. a Department of Molecular Biology , Max Planck Institute for Biophysical Chemistry , P.O.B. 2841, D-37018, Göttingen , Germany
2. b Department of Molecular Biology , Institute of Organic Chemistry University of , Göttingen , Germany
3. c Department of Molecular Biology , Institute for Drug Development , TX 78245-3217, San Antonio , U.S.A.
4. d Department of Chemistry , New York University , New York , NY , 10003 , U.S.A.