X-Ray methods based on synchrotron technology have the promise of providing time-resolved structural data based on the high flux and brightness of the X-Ray beams. One of the most closely examined problems in this area of time-resolved structure determination has been the examination of intermediates in ligand binding to myoglobin. Recent crystallographic experiments using synchrotron radiation have identified the protein tertiary and heme structural changes that occur upon photolysis of the myoglobin-carbon monoxide complex at cryogenic temperatures (Schlichting, I., Berendzen, J., Phillips, G, & Sweet, R. Nature, 371, p.808-812 (1994)). However, the precision of protein crystallographic data (~0.2 ?) is insufficient to provide precise metrical details of the iron-ligand bond lengths. Since bond length changes on this scale can trigger reactivity changes of several orders of magnitude, such detail is critical to a full understanding of metalloprotein structure-function relationships. Extended X-Ray Absorption Fine Structure (EXAFS) spectroscopy has the potential for analyzing bond distances to a precision of 0.02 ?, but is hampered by its relative insensitivity to the geometry of the backscattering atoms. Thus, it is often unable to provide a unique solution to the structure without ancillary structural information. We have developed a suite of computer programs that incorporate this ancillary structural information and compute the expected experimental spectra for a wide ranging series of Cartesian Coordinate sets. The programs systematically increment the distance of the metal to various coordinating ligands (along with their associated higher shells). Then, utilizing the ab initio EXAFS code FEFF 6.01, simulated spectra are generated, compared to the actual experimental spectra, and the differences computed. Thus, the details of the local minimum as well as the global problem are revealed in a single plot. This allows direct evaluation of the adequacy of competing solutions. The power of these programs are demonstrated in the examination of high signal to noise EXAFS data from a photolyzed solution sample of the myoglobin-carbon monoxide complex at 10 K. Evaluation of this data using our global mapping procedures placed the iron to pyrrole nitrogen average distances close to the value for deoxy myoglobin (2.05 0.01 ?), while the distance from iron to the proximal histidine nitrogen is seen to be 2.20 0.04 ?. It is also shown that one cannot uniquely position the CO ligand based on the EXAFS data alone, as a number of reasonable minima (from the perspective of the EXAFS) are observed. This provides a reasonable explanation for the multiplicity of solutions that have been previously reported. The results presented here are seen to be in complete agreement with the crystallographic results within the respective errors of the two techniques however, the Extended X-Ray Absorption Fine Structure data allows the iron-ligand bond lengths to be precisely defined.
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