Ferric aquo heme complexes comprise the active site of three major families of heme proteins, the peroxidases, the cytochrome P450s, and metmyoglobin. There is ample evidence from a variety of spectroscopic studies of wild type (wt) andmutant enzymes that in these resting state complexes, the sextet, quartet, and doublet states are very close in energy and the predominant spin state observed is a sensitive function of the number, nature, and geometry of the axial ligands. One correlation that is very difficult to determine experimentally is the relationship between spin state and geometry in the same heme complex. This coupling of geometry and spin state in the same complex can be functionally important since spin state changes are often a key part of function, for example, in the enzymatic cycle of the cytochrome P450s. To further explore the relationship between geometry and spin state, we report here for the first time the use of ab initio methods to calculate optimized geometries and electronic structure of a model for the resting state of peroxidases in its sextet, quartet, and doublet states. The sextet state is found to be the lowest energy state in agreement with experimental results reported for a model diaquo heme compound. Although a longer Fe-water distance was obtained in the model compound, the unique feature of these calculations is their ability to monitor changes in geometry in the various spin states in the same complex. While the optimized quartet geometry is similar to the sextet geometry, the doublet state has a considerably shorter Fe-water distance. These results suggest that the environment of the protein can modulate spin state changes by imposing geometric changes in this mobile Fe-ligand interaction by interaction from both the proximal and distal sides. Experimental determination of spin state populations by a number of spectroscopic methods in wt and mutants of the Fe(m) resting form of cytochrome-C peroxidases (CCP) with known Fe-water distances from crystal structures provide strong support for this hypothesis.
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