Observations and interpretation of x-ray absorption edges in iron compounds and proteins.

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Abstract
X-ray absorption spectra near the K.alpha. edge were measured in various Fe group compounds using the intense synchrotron radiation available at the Stanford Synchrotron Research Project. In the cubic compounds KMF3 where M = Mn2+, Fe2+, Co2+, Ni2+ and Zn2+, well resolved lines were observed and assigned to the 1s .fwdarw. 3d, 1s .fwdarw. 4s, and 1s .fwdarw. 4p transitions. The observed energies agreed rather well with the spectroscopic energy levels of the Z + 1 ion and the intensities agree with those expected on the basis of one electron transitions of the form Z 1s2dn(L,S) .fwdarw. (Z+ 1)1s2dnn''l''(L",S). The energies of the intense 1s .fwdarw. 4p transition increase by about 5 V going from KFeF3 to K2NaFeF6, but only by about 1 V from K4Fe(CN)6 to K3Fe(CN)6. The transitions confirm that upon oxidation of the hexacyanides the Fe electronic structure barely changes. In the Fe-S protein rubredoxin, where the Fe is bound to a tetrahedron of sulfurs, the 1s .fwdarw. 3d transition was about 7 times more intense than the same transition in an octahedrally coordinated compound. These intensities parallel those observed in the d-d transitions of optical spectra, because in both types of spectra the intensities depend upon 4p admixture. In the heme protein cytochrome c, upon oxidation the 1s .fwdarw. 4p transition shifts only about 1 V to higher energies, similar to the Fe hexacyanides. These results are discussed in terms of covalent bonding.

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