EXAFS STUDIES OF PROTEINS AND MODEL COMPOUNDS CONTAINING DIMERIC AND TETRAMERIC IRON-SULFUR CLUSTERS

Proteins and model compounds containing dimeric and tetrameric iron-sulfur clusters have been studied by extended X-ray absorption fine structure (EXAFS) spectroscopy in fluorescence and transmission modes. Iron-sulfur and iron-iron distances have been obtained for both proteins and models. Debye-Waller factors, accurate to within 10%, provide an indirect but reliable measure of the spread of these distances. Detailed comparisons of these molecular parameters (average interatomic distances and Debye-Waller factors) are made between (1) proteins and models (which provides direct structural evidence that Hol's model compounds are excellent representations of the active sites of the proteins); (2) solid and solution states (which shows no drastic structural changes at the active site upon dissolution of the proteins); (3) oxidized and reduced states (which reveals small but significant structural changes upon redox). Thus, the present study bridges the two reservoirs of structural information on these important nonheme iron-sulfuT proteins-the intact protein on one hand and the bare inorganic clusters on the other. It also provides strong evidence that structural changes at the active site(s) are energetically insignificant as judged by comparing the protein structures with those of the model compounds. © 1979, American Chemical Society. All rights reserved.