Collaborative chemical and spectroscopic work from several laboratories resulted in a qualitative structure for the active center in the two-iron ferredoxins, with each iron being in a distorted tetrahedron of sulfur atoms (two acid-labile sulfurs bridging the two iron atoms and the other two from cysteine sulfurs). Subsequent X-ray data from other laboratories confirmed this structure. Detailed EPR spectral syntheses showed that there is a distribution of structures in any given protein (even in a single crystal) resulting in a distribution of the principal values of the g tensor, which may be described by a statistical distribution of still another tensor whose principal axes are, in general, not coincident with the principal-axis frame of the g tensor. (C) 2003 Elsevier Inc. All rights reserved.