1. Department of Medical Biochemistry and Biophysics, Umeå University , Umeå 90187 , Sweden
2. Department of Chemistry - BMC, Uppsala University , Box 576 , Uppsala S-751 23 , Sweden
3. Department of Drug Design and Pharmacology, University of Copenhagen , Universitetsparken 2 , 2100 Copenhagen, Denmark
4. School of Chemistry and Biochemistry, Georgia Institute of Technology , 901 Atlantic Drive NW , Atlanta , GA 30332-0400 , USA