Methodology to Determine Heme Electronic Structure by Means of Mössbauer Spectrometry

Abstract

Functions of heme proteins are finely tuned by the electronic and coordination structure of the heme (iron porphyrin complex) placed in the active site of protein. Therefore, it is quite important to reveal the physicochemical properties of a wide variety of synthetic model heme complexes for better understanding the functions and reaction mechanisms of heme proteins. The heme electronic structures have been determined by the combined analysis of NMR, ESR, Mössbauer, and IR spectrometry as well as SQUID(superconducting quantum interference device) magnetometry and X-ray crystallography. Specifically, two kinds of electronic ground states observed in low-spin iron(III) porphyrin complexes, i.e. commonly observed(d_xy)²(d_xz, d_yz)³ and less common(d_xz, d_yz)⁴(d_xy)¹, as well as the novel spin crossover phenomena involving the intermediate-spin state are described. Special interest is the mono-imidazole complexes showing the spin crossover among all the possible spin states in iron(III) porphyrin complexes, i.e., S＝1/2, 3/2, and 5/2. The novel electronic structures mentioned above are recently pointed out to involve in the catalytic process of some naturally occurring heme proteins.