[1] Univ Wisconsin, Dept Chem, Madison, WI 53706 USA
[2] Univ Copenhagen, Dept Neurosci & Pharmacol, Fac Hlth Sci, DK-2200 Copenhagen, Denmark
[3] Univ London Imperial Coll Sci Technol & Med, Dept Life Sci, London SW7 2AZ, England
[4] Univ Calif San Francisco, Dept Biochem & Biophys, San Francisco, CA 94158 USA
[5] Stanford Univ, Stanford, CA 94305 USA