EPR of Mononuclear Non-Heme Iron Proteins

Flexible geometry of three- to six-protein side-chain ligands to non-heme iron in proteins is the basis for widely diverse reactivites ranging from iron transport to redox chemistry. The gap between fixed states determined by x-ray analysis can be filled by spectroscopic study of trapped intermediates. EPR is a versatile and relatively quick approach to defining intermediate states in terms of the geometry and electronic structures of iron. A number of examples in which the iron chemistry of non-heme proteins is understood through x-ray structures at subbond length resolution, refined calculations, and spectroscopy exist now. Some examples in which EPR has provided unique insight are summarized in Table 1. Assignment and quantitative evaluation of the EPR resonances in ferric, non-heme iron sites is the focus of the first section of this review. An earlier chapter in this series provides more background on the theory specific to EPR of S = 5/2 metal ions [1]. Besides EPR spectra of ferric mononuclear sites, EPR of ferrous iron coupled to a spin 1/2 radical, as it pertains to the categories mononuclear and non-heme, will also be covered, in the second half of this chapter. Examples include the quinone-ferrous interactions in photosynthetic reaction centers and nitric oxide complexes with non-heme ferrous iron. Other recent reviews of the biochemistry and spectroscopy of non-heme iron proteins provide additional background [2-6].