1. Centre for Metalloprotein Spectroscopy and Biology, School of Biological Sciences and School of Chemical Sciences, University of East Anglia, Norwich, NR4 7TJ, U.K., C.L.R.C. Daresbury Laboratory, Warrington, WA4 4AD, U.K., Department of Biochemistry, University of Oxford, Oxford, OX1 3QU, U.K., Department of Chemistry, University of Manchester, Manchester, M13 9PL, U.K., and Nitrogen Fixation Laboratory, John Innes Centre, Colney Lane, Norwich, NR4 7UH, U.K.