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Knowledge of the molecular architecture around the copper atoms in the "type 3" site of hemocyanins is of importance in understanding how these proteins reversibly bind dioxygen. Analysis of the extended X-ray absorption fine structure (EXAFS) has proven to be useful in the structural elucidation of specific metal sites in metalloproteins under noncrystalline conditions.1

Curve fitting methods have been developed which promise to be of considerable utility in extracting structural information from x-ray absorption spectra. In principle, the extended x-ray absorption fine structure (EXAFS) for a given atom can be analyzed to reveal the radial distances and atomic numbers of those atoms surrounding the absorber. The requisite highquality x-ray absorption spectra can now be easily and rapidly obtained on solid or solution samples, using synchrotron radiation from the Stanford electron-positron storage ring SPEAR at the Stanford Linear Accelerator Center. In order to provide the basis for an understanding of metalloprotein absorption fine structure, data for ferrocene, C O ( N H J )~~+ , Co(OH&*+, and a variety of iron porphyrins have been analyzed. The procedure used was to fit the EXAFS of known structures with a parameterized, semiempirical function: ~, C , k -f i~ exp(-2( u 1 2 ) k 2 )sin (2R,k + a ( k ) ) ; where the sum is over all neighboring atoms, the distance information R, is contained in the phase term, and the other variables account for the amplitude behavior as described in detail in the text. These parameters are then used in fits on other known structures, and the accuracy of the calculated distances (R,) is used as a test for the validity of the method. The results of fits on FeIlTPP, Fe1"TPP(SC6H5), and FeT-pivPP(N-Me1m)Oz indicate that radial distances out to four coordination shells in iron porphyrins can be determined to better than 0.1 A. For cases where the interest is only in changes in coordination, difference methods of analysis have been developedwhich allow accurate determination of the geometry and distances to small molecules such as dioxygen which can reversibly bind to the metal ion. For oxy and deoxy FeTpivPP(N-MeIm), the F e -0 distances are determined to better than 0.03 A. At present, the interpretation of the EXAFS from an unknown structure relies heavily on comparison with model compound data. However, the present results suggest that a library of transferable phase shift and amplitude functions may be eventually compiled, and that curve-fitting analysis of the extended fine structure will be useful for determining the local structure around specific atomic constituents in metalloproteins and other complex materials.

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