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Title: Interplay of structure and vibrational dynamics in six-coordinate heme nitrosyls.

Abstract

The isolation of two polymorphic forms of nitrosyl(1-methylimidazole)(tetra-p-fluorophenylporphinato)iron(II) provides a unique opportunity to explore the interplay of structure and vibrational dynamics in six-coordinate {l_brace}FeNO{r_brace}{sup 7} nitrosyliron porphyrinates. The two species display differing vibrational spectroscopic properties both in {nu}{sub NO} (IR) and the iron vibrational modes obtained through the use of nuclear resonance vibrational spectroscopy. Structural characterization of the two complexes has yielded extremely high-quality structures that further confirm that coordination of NO leads to ligand tilting and asymmetry in the equatorial Fe-Np bond distances. The two polymorphic structures (monoclinic and triclinic crystal systems) differ in the relative orientations of the two axial ligands and small but significant differences in coordination group bond distances. Although DFT calculations suggest that the NO/imidazole orientations should be indistinguishable, real experimental (structural and vibrational) differences between the two are found. The observed variation in the axial and equatorial Fe-N bonds is strongly correlated to the dynamics of the Fe-NO unit and other motions of iron. Other structural differences appear to have little effect on the vibrational properties of the two forms. The in-plane motions of iron in CO versus NO heme complexes illustrate distinct dynamic differences.

Authors:
; ; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC); National Institutes of Health (NIH)
OSTI Identifier:
914977
Report Number(s):
ANL/XSD/JA-58813
TRN: US200817%%45
DOE Contract Number:
DE-AC02-06CH11357
Resource Type:
Journal Article
Resource Relation:
Journal Name: J. Am. Chem. Soc. Commun.; Journal Volume: 129; Journal Issue: 8 ; Feb. 2, 2007
Country of Publication:
United States
Language:
ENGLISH
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; HEME; IRON COMPLEXES; NITROSO COMPOUNDS; MORPHOLOGY; VIBRATIONAL STATES; PORPHYRINS; NITRIC OXIDE; IMIDAZOLES; DENSITY FUNCTIONAL METHOD

Citation Formats

Silvernail, N. J., Barabanschikov, A., Pavlik, J. W., Noll, B. C., Zhao, J., Alp, E. E., Sturhahn, W., Stage, J. T., Scheidt, W. R., Univ. of Notre Dame, and Notheastern Univ. Interplay of structure and vibrational dynamics in six-coordinate heme nitrosyls.. United States: N. p., 2007. Web. doi:10.1021/ja066869k.
Silvernail, N. J., Barabanschikov, A., Pavlik, J. W., Noll, B. C., Zhao, J., Alp, E. E., Sturhahn, W., Stage, J. T., Scheidt, W. R., Univ. of Notre Dame, & Notheastern Univ. Interplay of structure and vibrational dynamics in six-coordinate heme nitrosyls.. United States. doi:10.1021/ja066869k.
Silvernail, N. J., Barabanschikov, A., Pavlik, J. W., Noll, B. C., Zhao, J., Alp, E. E., Sturhahn, W., Stage, J. T., Scheidt, W. R., Univ. of Notre Dame, and Notheastern Univ. Fri . "Interplay of structure and vibrational dynamics in six-coordinate heme nitrosyls.". United States. doi:10.1021/ja066869k.
@article{osti_914977,
title = {Interplay of structure and vibrational dynamics in six-coordinate heme nitrosyls.},
author = {Silvernail, N. J. and Barabanschikov, A. and Pavlik, J. W. and Noll, B. C. and Zhao, J. and Alp, E. E. and Sturhahn, W. and Stage, J. T. and Scheidt, W. R. and Univ. of Notre Dame and Notheastern Univ.},
abstractNote = {The isolation of two polymorphic forms of nitrosyl(1-methylimidazole)(tetra-p-fluorophenylporphinato)iron(II) provides a unique opportunity to explore the interplay of structure and vibrational dynamics in six-coordinate {l_brace}FeNO{r_brace}{sup 7} nitrosyliron porphyrinates. The two species display differing vibrational spectroscopic properties both in {nu}{sub NO} (IR) and the iron vibrational modes obtained through the use of nuclear resonance vibrational spectroscopy. Structural characterization of the two complexes has yielded extremely high-quality structures that further confirm that coordination of NO leads to ligand tilting and asymmetry in the equatorial Fe-Np bond distances. The two polymorphic structures (monoclinic and triclinic crystal systems) differ in the relative orientations of the two axial ligands and small but significant differences in coordination group bond distances. Although DFT calculations suggest that the NO/imidazole orientations should be indistinguishable, real experimental (structural and vibrational) differences between the two are found. The observed variation in the axial and equatorial Fe-N bonds is strongly correlated to the dynamics of the Fe-NO unit and other motions of iron. Other structural differences appear to have little effect on the vibrational properties of the two forms. The in-plane motions of iron in CO versus NO heme complexes illustrate distinct dynamic differences.},
doi = {10.1021/ja066869k},
journal = {J. Am. Chem. Soc. Commun.},
number = 8 ; Feb. 2, 2007,
volume = 129,
place = {United States},
year = {Fri Feb 02 00:00:00 EST 2007},
month = {Fri Feb 02 00:00:00 EST 2007}
}
  • This Communication addresses a long-standing problem: the exact vibrational assignments of the low-energy modes of the Fe-N-O subunit in six-coordinate ferrous heme nitrosyl model complexes. This problem is addressed using nuclear resonance vibrational spectroscopy (NRVS) coupled to {sup 15}N{sup 18}O isotope labeling and detailed simulations of the obtained data. Two isotope-sensitive features are identified at 437 and 563 cm{sup -1}. Normal coordinate analysis shows that the 437 cm{sup -1} mode corresponds to the Fe-NO stretch, whereas the 563 cm{sup -1} band is identified with the Fe-N-O bend. The relative NRVS intensities of these features determine the degree of vibrational mixingmore » between the stretch and the bend. The implications of these results are discussed with respect to the trans effect of imidazole on the bound NO. In addition, a comparison to myoglobin-NO (Mb-NO) is made to determine the effect of the Mb active site pocket on the bound NO.« less
  • This study presents Nuclear Resonance Vibrational Spectroscopy (NRVS) data on the five-coordinate (5C) ferrous heme-nitrosyl complex [Fe(OEP)(NO)] (1, OEP{sup 2-} = octaethylporphyrinato dianion) and the corresponding {sup 15}N{sup 18}O labeled complex. The obtained spectra identify two isotope sensitive features at 522 and 388 cm{sup -1}, which shift to 508 and 381 cm{sup -1}, respectively, upon isotope labeling. These features are assigned to the Fe-NO stretch v(Fe-NO) and the in-plane Fe-N-O bending mode {delta}{sub ip}(Fe-N-O), the latter has been unambiguously assigned for the first time for 1. The obtained NRVS data were simulated using our quantum chemistry centered normal coordinate analysismore » (QCC-NCA). Since complex 1 can potentially exist in 12 different conformations involving the FeNO and peripheral ethyl orientations, extended density functional theory (DFT) calculations and QCC-NCA simulations were performed to determine how these conformations affect the NRVS properties of [Fe(OEP)NO]. These results show that the properties and force constants of the FeNO unit are hardly affected by the conformational changes involving the ethyl substituents. On the other hand, the NRVS-active porphyrin-based vibrations around 340-360, 300-320, and 250-270 cm{sup -1} are sensitive to the conformational changes. The spectroscopic changes observed in these regions are due to selective mechanical couplings of one component of Eu-type (in ideal D4h symmetry) porphyrin-based vibrations with the in-plane Fe-N-O bending mode. This leads to the observed variations in Fe(OEP) core mode energies and NRVS intensities without affecting the properties of the FeNO unit. The QCC-NCA simulated NRVS spectra of 1 show excellent agreement with experiment, and indicate that conformer F is likely present in the samples of this complex investigated here. The observed porphyrin-based vibrations in the NRVS spectra of 1 are also assigned based on the QCC-NCA results. The obtained force constants of the Fe-NO and N-O bonds are 2.83-2.94 (based on the DFT functional applied) and about 12.15 mdyn/{angstrom}, respectively. The electronic structures of 5C ferrous heme-nitrosyls in different model complexes are then analyzed, and variations in their properties based on different porphyrin substituents are explained. Finally, the shortcomings of different DFT functionals in describing the axial FeNO subunit in heme-nitrosyls are elucidated.« less
  • No abstract prepared.
  • The coordination structure about Ni{sup 2+} in water at temperatures up to 525 C was measured by the X-ray absorption fine structure (XAFS) technique. Solutions containing 0.2 m NiBr{sub 2} and 0.2 m NiBr{sub 2}/0.8 m NaBr were explored at pressures up to 720 bar. For certain systems, both Ni and Br XAFS data were acquired and a global model was used to fit the two independent sets of SAFS data. These two independent measurements gave excellent agreement on the coordination structure of the Ni{sup 2+}/Br{sup {minus}} contact-ion pairing. The result is a complete picture of the coordination structure ofmore » the contact-ion pairing including the coordination numbers, distances for the water-ion and ion-ion associations, and also a high-quality measurement of the binding strength and amount of disorder (Debye-Waller factor and the anharmonicity) of the Ni{sup 2+}/Br{sup {minus}} association. The SAFS measurements strongly indicate a transitional change in the coordination of Ni{sup 2+} from the octahedral Ni{sup 2+}(H{sub 2}O){sub 6} species at room temperature to the 4-coordinate structures at supercritical conditions (e.g., T > 375 C). Specifically, the equilibrium structure at 425 C is Ni{sup 2+}(Br{sup {minus}}){sub 3.3}(H{sub 2}O){sub 1.0} for the aqueous solution of 0.2 m NiBr{sub 2} with 0.8 m NaBr. At 325 C, the octahedral species still exists but it is in equilibrium with new species of lower coordination. Above 425 C, at moderate pressures up to 700 bar, the stable structures are a family of 4-coordinated species (NiBr(H{sub 2}O){sub 3}{sm{underscore}bullet}Br, NiBr{sub 2}(H{sub 2}O){sub 2}, NiBr{sub 3}(H{sub 2}O){sm{underscore}bullet}Na), where the degree of Br{sup {minus}} adduction and replacement of H{sub 2}O in the inner shell depends on the overall Br{sup {minus}} concentration. The most likely symmetry of these species is a distorted tetrahedron. Measurements of the Ni pre-edge 1s {r{underscore}arrow} 3d and to 1s {r{underscore}arrow} 4d transitions using X-ray absorption spectroscopy confirm that a symmetry change occurs at high temperature, and the results are consistent with the SAFS and molecular dynamics (MD) conclusion of a distorted tetrahedral structure. This observation is further confirmed by near-infrared (NIR) spectra of the same system. The MD simulations under identical conditions were used to verify the experimental findings. Although the authors found qualitative agreement between the experimental and simulated first-shell coordination structure, it is clear that refinements of the intermolecular potentials are required to quantitatively capture the true high-temperature structure.« less