Department of Chemistry, The University of Chicago, Chicago, IL 60637, USA. Grossman Institute of Neuroscience, Quantitative Biology and Human Behavior, The University of Chicago, Chicago, IL 60637, USA. Ben May Department for Cancer Research, The University of Chicago, Chicago, IL 60637, USA. Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP), 13125 Berlin, Germany. Department of Cell & Systems Biology, University of Toronto, Toronto, ON M5S 3G5, Canada. Department of Molecular Physiology and Biophysics, The University of Iowa, Iowa City, IA 52242, USA. Department of Anesthesiology, Weill Cornell Medical School, New York, NY 10065, USA. Department of Physiology and Biophysics, Weill Cornell Medical School, New York, NY 10065, USA. Department of Biochemistry, Weill Cornell Medical School, New York, NY 10065, USA. Department of Physiology, Johns Hopkins University School of Medicine, Baltimore, MD 21218, USA. Solomon H. Snyder Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, MD 21218, USA. Department of Chemistry, The University of Chicago, Chicago, IL 60637, USA yamuna@uchicago.edu.