Axial Ligand Effects on Vibrational Dynamics of Iron in Heme Carbonyl Studied by Nuclear Resonance Vibrational Spectroscopy

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• 作者：Takehiro Ohta ; Jin-Gang Liu ; Makina Saito ; Yasuhiro Kobayashi ; Yoshitaka Yoda ; Makoto Seto ; Yoshinori Naruta
• 刊名：The Journal of Physical Chemistry B
• 出版年：2012
• 出版时间：November 29, 2012
• 年：2012
• 卷：116
• 期：47
• 页码：13831-13838
• 全文大小：365K
• 年卷期：v.116,no.47(November 29, 2012)
• ISSN：1520-5207

文摘

Nuclear resonance vibrational spectroscopy (NRVS) and density functional theory calculation (DFT) have been applied to illuminate the effect of axial ligation on the vibrational dynamics of iron in heme carbonyl. The analyses of the NRVS data of five- (5c) and six-coordinate (6c) heme鈥揅O complexes indicate that the prominent feature of ⁵⁷Fe partial vibrational density of state (⁵⁷FePVDOS) at the 250鈥?00 cm^鈥? region is significantly affected by the association of the axial ligand. The DFT calculations predict that the prominent ⁵⁷FePVDOS is composed of iron in-plane motions which are coupled with porphyrin pyrrole in-plane (谓₄₉, 谓₅₀, and 谓₅₃), an out-of-plane (纬₈) (two of four pyrrole rings include the in-plane modes, while the rest of pyrrole rings vibrate along the out-of-plane coordinate), and out-of-phase carbonyl C and O atom displacement perpendicular to the Fe鈥揅鈥揙 axis. Thus, in the case of the 5c CO鈥揾eme the prominent ⁵⁷FePVDOS shows sharp and intense feature because of the degeneracy of the e symmetry mode within the framework of C_4v symmetry molecule, whereas the association of the axial imidazole ligand in the 6c complex with the lowered symmetry results in split of the degenerate vibrational energy as indicated by broader and lower intensity features of the corresponding NRVS peak compared to the 5c structure. The vibrational energy of the iron in-plane motion in the 6c complex is higher than that in 5c, implying that the iron in the 6c complex includes stronger in-plane interaction with the porphyrin compared to 5c. The iron in-plane mode above 500 cm^鈥?, which is predominantly coupled with the out-of-phase carbonyl C and O atom motion perpendicular to Fe鈥揅鈥揙, called as Fe鈥揅鈥揙 bending mode (未_{Fe鈥揅鈥揙}), also suggests that the 6c structure involves a larger force constant for the e symmetry mode than 5c. The DFT calculations along with the NRVS data suggest that the stiffened iron in-plane motion in the 6c complex can be ascribed to diminished pseudo-Jahn鈥揟eller instability along the e symmetry displacement due to an increased a₁鈥?i>e orbital energy gap caused by 蟽* interaction between the iron d_z² orbital and the nitrogen p orbital from the axial imidazole ligand. Thus, the present study implicates a fundamental molecular mechanism of axial ligation of heme in association with a diatomic gas molecule, which is a key primary step toward versatile biological functions.