Characterization of Electronic Structure and Properties of a Bis(histidine) Heme Model Complex
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- 作者：Dayle M. A. Smith ; Michel Dupuis ; Erich R. Vorpagel ; and T. P. Straatsma
- 刊名：Journal of the American Chemical Society
- 出版年：2003
- 出版时间：March 5, 2003
- 年：2003
- 卷：125
- 期：9
- 页码：2711 - 2717
- 全文大小：121K
- 年卷期：v.125,no.9(March 5, 2003)
- ISSN：1520-5126
文摘
Ferric and ferrous hemes, such as those present in electron transfer proteins, often have low-lying spin states that are very close in energy. To explore the relationship between spin state, geometry,and cytochrome electron transfer, we investigate, using density functional theory, the relative energies,electronic structure, and optimized geometries for a high- and low-spin ferric and ferrous heme modelcomplex. Our model consists of an iron-porphyrin axially ligated by two imidazoles, which model theinteraction of a heme with histidine residues. Using the B3LYP hybrid functional, we found that, in theferric model heme complex, the doublet is lower in energy than the sextet by 8.4 kcal/mol and the singletferrous heme is 6.7 kcal/mol more stable than the quintet. The difference between the high-spin ferric andferrous model heme energies yields an adiabatic electron affinity (AEA) of 5.24 eV, and the low-spin AEAis 5.17 eV. Both values are large enough to ensure electron trapping, and electronic structure analysisindicates that the iron d
orbital is involved in the electron transfer between hemes. Mössbauer parameterscalculated to verify the B3LYP electronic structure correlate very well with experimental values. Isotropichyperfine coupling constants for the ligand nitrogen atoms were also evaluated. The optimized geometriesof the ferric and ferrous hemes are consistent with structures from X-ray crystallography and reveal thatthe iron-imidazole distances are significantly longer in the high-spin hemes, which suggests that the proteinenvironment, modeled here by the imidazoles, plays an important role in regulating the spin state. Iron-imidazole dissociation energies, force constants, and harmonic frequencies were calculated for the ferricand ferrous low-spin and high-spin hemes. In both the ferric and the ferrous cases, a single imidazoleligand is more easily dissociated from the high-spin hemes.
orbital is involved in the electron transfer between hemes. Mössbauer parameterscalculated to verify the B3LYP electronic structure correlate very well with experimental values. Isotropichyperfine coupling constants for the ligand nitrogen atoms were also evaluated. The optimized geometriesof the ferric and ferrous hemes are consistent with structures from X-ray crystallography and reveal thatthe iron-imidazole distances are significantly longer in the high-spin hemes, which suggests that the proteinenvironment, modeled here by the imidazoles, plays an important role in regulating the spin state. Iron-imidazole dissociation energies, force constants, and harmonic frequencies were calculated for the ferricand ferrous low-spin and high-spin hemes. In both the ferric and the ferrous cases, a single imidazoleligand is more easily dissociated from the high-spin hemes.