1. B. R. Crane, A. S. Arvai, E. D. Getzoff, and J. A. Tainer are in the Department of Molecular Biology and the Skaggs Institute for Chemical Biology, The Scripps Research Institute, La Jolla, CA 92037, USA. R. Gachhui, C. Wu, D. K. Ghosh, and D. J. Stuehr are in the Department of Immunology, Cleveland Clinic, Cleveland, OH 44106, USA.