Mössbauer studies on O2and CO binding to the heme iron in myoglobin

Studies on the binding of CO to low-spin [Fe(II)(Por)L2] complexes: an aid to understanding the binding of CO to haemoglobin and myoglobin

The visible and Mössbauer spectra of [Fe(II)(Por)L₂] and [Fe(II)(Por)L(CO)] complexes (where Por = protoporphyrin IX (PPIX) or tetra(p-sulfophenyl)porphyrin (TPPS) and L = an aliphatic or aromatic nitrogenous base) are reported and discussed. The results are compared to those of previously reported [Fe(II)(Por)L(CO)] complexes (where Por = PPIX, TPPS, PMXPP, TPP, OMTBP and OEP; L = a nitrogenous aromatic ligand) and HbCO (where Hb = haemoglobin) and MyCO (where My = myoglobin). A new approach, to extracting information from the Mössbauer parameters has been developed by plotting those of the [Fe(II)(Por)L₂] complexes against those of [Fe(II)(Por)L(CO)] complexes for the same ligands, has yielded a series of trend lines that show a significant dependence on both the nature of the porphyrin and also of the nitrogenous ligand. Different trend lines were found for aromatic nitrogenous ligands to aliphatic nitrogenous ligands showing that the porphyrins could donate different amounts of charge to the Fe(II) cations as the L ligand changed, and hence, they display electron sink properties. From the plots, it was shown that haemoglobin and myoglobin both bind CO very strongly compared to the model complexes studied herein. Using the reported structural and Mössbauer data for the [Fe(II)(Por)L₂] and [Fe(II)(Por)L(CO)] complexes, it proved possible and instructive to plot the Mössbauer parameters against a number of the bond lengths around the Fe(II) cations. The interpretation of the resulting trend lines both supported and facilitated the extension of our findings enabling further understanding of the geometry of the bonding in CO haemoglobin and CO myoglobin.

Biomimetic O2 adsorption in an iron metal-organic framework for air separation

Bio-inspired motifs for gas binding and small molecule activation can be used to design more selective adsorbents for gas separation applications. Here, we report an iron metal–organic framework, Fe-BTTri (Fe₃[(Fe₄Cl)₃(BTTri)₈]₂·18CH₃OH, H₃BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene), that binds O₂ in a manner similar to hemoglobin and therefore results in highly selective O₂ binding. As confirmed by gas adsorption studies and Mössbauer and infrared spectroscopy data, the exposed iron sites in the framework reversibly adsorb substantial amounts of O₂ at low temperatures by converting between high-spin, square-pyramidal Fe(ii) centers in the activated material to low-spin, octahedral Fe(iii)–superoxide sites upon gas binding. This change in both oxidation state and spin state observed in Fe-BTTri leads to selective and readily reversible O₂ binding, with the highest reported O₂/N₂ selectivity for any iron-based framework.

Bio-inspired motifs for gas binding and small molecule activation can be used to design more selective adsorbents for gas separation applications.

A reaction pathway to compound 0 intermediates in oxy-myoglobin through interactions with hydrogen sulfide and His64

Myoglobin (Mb) binds oxygen with high affinity as a low spin singlet complex and thus functions as an oxygen storage protein. Yet, hybrid Density Functional Theory/Molecular Mechanical (DFT/MM) calculations of oxy-Mb models predict that the O₂ bond is much less resistant to breaking in the presence of hydrogen sulfide (H₂S) compared with water. Specifically, a hydrogen atom from H₂S can be transferred to the distal oxygen atom through homolytic cleavage of the S-H bond to form the intermediate Compound (Cpd) 0 structure and a thiyl radical. In the presence of a neutral His64 (Nε protonation, His64-ε) and H₂S, only a metastable Cpd 0 would be formed as the active site is devoid of any additional proton donor to fully break the O₂ bond. In contrast, the calculations predict that the triplet state is significantly favored over the open shell singlet diradical state throughout the entire reaction coordinate in the presence of H₂S and a positively charged His64. Furthermore, a positively charged His64 can readily donate a proton to Cpd 0 to fully break the O₂ bond resulting in a configuration analogous to reported reaction models of a hemoglobin mutant bound to H₂O₂ with H₂S present. Typically, exotic techniques are required to generate Cpd 0 but under the conditions just described the intermediate is readily detected in UV-Vis spectra at room temperature. The effect is observed as a 2 nm red shift of the Soret band from 414 nm to 416 nm (pH 5.0, His64-εδ) and from 416 nm to 418 nm (pH 6.6, His64-ε).

NMR, IR, Mössbauer and quantum chemical investigations of metalloporphyrins and metalloproteins

We review contributions made towards the elucidation of CO and O2binding geometries in respiratory proteins. Nuclear magnetic resonance, infrared spectroscopy, Mössbauer spectroscopy, X-ray crystallography and quantum chemistry have all been used to investigate the Fe –ligand interactions. Early experimental results showed linear correlations between17O chemical shifts and the infrared stretching frequency (νCO) of the CO ligand in carbonmonoxyheme proteins and between the17O chemical shift and the13CO shift. These correlations led to early theoretical investigations of the vibrational frequency of carbon monoxide and of the13C and17O NMR chemical shifts in the presence of uniform and non-uniform electric fields. Early success in modeling these spectroscopic observables then led to the use of computational methods, in conjunction with experiment, to evaluate ligand-binding geometries in heme proteins. Density functional theory results are described which predict57Fe chemical shifts and Mössbauer electric field gradient tensors,17O NMR isotropic chemical shifts, chemical shift tensors and nuclear quadrupole coupling constants (e2qQ/h) as well as13C isotropic chemical shifts and chemical shift tensors in organometallic clusters, heme model metalloporphyrins and in metalloproteins. A principal result is that CO in most heme proteins has an essentially linear and untilted geometry (τ = 4 °, β = 7 °) which is in extremely good agreement with a recently published X-ray synchrotron structure. CO / O2discrimination is thus attributable to polar interactions with the distal histidine residue, rather than major Fe–C–O geometric distortions.

Heme carbonyls: environmental effects on nu(C-O) and Fe-C/C-O bond length correlations

The synthesis and characterization of four low-spin (carbonyl)iron(II) tetraphenylporphyrinates, [Fe(TPP)(CO)(L)], where L = 1-methylimidazole, 2-methylimidazole, 1,2-dimethylimidazole (unsolvated), and 1,2-dimethylimidazole (toluene solvate) are reported. The complexes show nearly the same value of nu(C-O) in toluene solution (1969-72 cm(-1)) but a large range of CO stretching frequencies in the solid-state (1926-1968 cm(-1)). The large solid-state variation results from CO interactions in the solid state, as shown by an examination of the crystal structures of the four complexes. The high precision of the four structures obtained allows us to make a number of structural and spectroscopic correlations that describe the Fe-C-O and N(Im)-Fe-CO units. The values of nu(C-O) and the Fe-C and C-O bond distances are strongly correlated and provide a structural, as well as a spectroscopic, correlation of the pi back-bonding model. The interactions of CO described are closely related to the large range of CO stretching frequencies observed in heme proteins and specific interactions observed in carbonylmyoglobin (MbCO).

Variable pi-bonding in iron(II) porphyrinates with nitrite, CO, and tert-butyl isocyanide: characterization of [Fe(TpivPP)(NO2)(CO)]-

The addition of the strongly pi-bonding ligands CO or tert-butyl isocyanide to the low-spin five-coordinate iron(II) nitrite species [Fe(TpivPP)(NO2)]- (TpivPP = picket fence porphyrin) gives two new six-coordinate species [Fe(TpivPP)(NO2)(CO)]- and [Fe(TpivPP)(NO2)(t-BuNC)]-. These species have been characterized by single-crystal structure determinations and by UV-vis, IR, and Mössbauer spectroscopies. All evidence shows that in the mixed-ligand iron(II) porphyrin species, [Fe(TpivPP)(NO2)(CO)]-, the two trans, pi-accepting ligands CO and nitrite compete for pi density. The CO ligand however dominates the bonding. The Fe-N(NO2) bond lengths for the two independent anions in the unit cell at 2.006(4) and 2.009(4) A are lengthened compared to other nitrite species with either no trans ligands or non-pi-accepting trans ligands to nitrite. The Fe-C(CO) bond lengths are 1.782(4) A and 1.789(5) A for the two anions. The two Fe-C-O angles at 175.5(4) and 177.5(4) degrees are essentially linear in both anions. The quadrupole splitting for [Fe(TpivPP)(NO2)(CO)]- was determined to be 0.32 mm/s, and the isomer shift was 0.18 mm/s at room temperature in zero applied field. Both of the Mössbauer parameters are much smaller than those found for six-coordinate low-spin iron(II) porphyrinates with neutral nitrogen-donating ligands as well as iron(II) nitro complexes. However, the Mössbauer parameters are typical of other six-coordinate CO porphyrinates signifying that CO is the more dominant ligand. The CO stretching frequency of 1974 cm(-1) is shifted only slightly to higher energy compared to six-coordinate CO complexes with neutral nitrogen-donor ligands trans to CO. Crystal data for [K(222)][Fe(TpivPP)(NO2)(CO)].1/2C6H5Cl: monoclinic, space group P2(1)/c, Z = 8, a = 33.548(6) A, b = 18.8172(15) A, c = 27.187(2) A, beta = 95.240(7) degrees, V = 17091(4) A3.

Unexpected nitrosyl-group bending in six-coordinate [M(NO)](6) sigma-bonded aryl(iron) and -(ruthenium) porphyrins

The six-coordinate nitrosyl sigma-bonded aryl(iron) and -(ruthenium) porphyrin complexes (OEP)Fe(NO)(p-C(6)H(4)F) and (OEP)Ru(NO)(p-C(6)H(4)F) (OEP = octaethylporphyrinato dianion) have been synthesized and characterized. Single-crystal X-ray structure determinations reveal an unprecedented bending and tilting of the MNO group for both [MNO](6) species as well as significant lengthening of trans axial bond distances. In (OEP)Fe(NO)(p-C(6)H(4)F) the Fe-N-O angle is 157.4(2) degrees, the nitrosyl nitrogen atom is tilted off of the normal to the heme plane by 9.2 degrees, Fe-N(NO) = 1.728(2) A, and Fe-C(aryl) = 2.040(3) A. In (OEP)Ru(NO)(p-C(6)H(4)F) the Ru-N-O angle is 154.9(3) degrees, the nitrosyl nitrogen atom is tilted off of the heme normal by 10.8 degrees, Ru-N(NO) = 1.807(3) A, and Ru-C(aryl) = 2.111(3) A. We show that these structural features are intrinsic to the molecules and are imposed by the strongly sigma-donating aryl ligand trans to the nitrosyl. Density functional-based calculations reproduce the structural distortions observed in the parent (OEP)Fe(NO)(p-C(6)H(4)F) and, combined with the results of extended Hückel calculations, show that the observed bending and tilting of the FeNO group indeed represent a low-energy conformation. We have identified specific orbital interactions that favor the unexpected bending and tilting of the FeNO group. The aryl ligand also affects the Fe-NO pi-bonding as measured by infrared and (57)Fe Mössbauer spectroscopies. The solid-state nitrosyl stretching frequencies for the iron complex (1791 cm(-)(1)) and the ruthenium complex (1773 cm(-)(1)) are significantly reduced compared to their respective [MNO](6) counterparts. The Mössbauer data for (OEP)Fe(NO)(p-C(6)H(4)F) yield the quadrupole splitting parameter +0.57 mm/s and the isomer shift 0.14 mm/s at 4.2 K. The results of our study show, for the first time, that bent Fe-N-O linkages are possible in formally ferric nitrosyl porphyrins.

The photoexcited triplet state as a probe of chromophore-protein interaction in myoglobin

The photoexcited metastable triplet state of Mg(2+)-mesoporphyrin IX (MgMPIX) or Mg(2+)-protoporphyrin IX (MgPPIX) located in the heme pocket of horse myoglobin (Mb) was investigated by optical and electron paramagnetic resonance (EPR) spectroscopy, and its properties were compared with the model complexes, MgMPIX, MgPPIX, and Mg2+ etioporphyrin I (MgETIOI), in noncoordinating and coordinating organic glasses. Zero-field splitting parameters, line shape, and Jahn-Teller distortion in the temperature range of 3.8-110 K are discussed in terms of porphyrin-protein interactions. The triplet line shapes for MgMPIXMb and MGPPIXMb show no temperature-dependent spectral line shape changes suggestive of Jahn-Teller dynamics, and it is concluded that the energy splitting is >> 150 cm-1, suggesting symmetry breaking from the anisotropy of intermal electric fields of the protein, and consistent with previous predictions (Geissinger et al. 1995. J. Phys. Chem. 99:16527-16529). Both MgMPIXMb and MgPPIXMb demonstrate electron spin polarization at low temperature, and from the polarization pattern it can be concluded that intersystem crossing occurs predominantly into in-plane spin sublevels of the triplet state. The splitting in the Q0.0 absorption band and the temperature dependence and splitting of the photoexcited triplet state of myoglobin in which the iron was replaced by Mg2+ are interpreted in terms of effects produced by electric field asymmetry in the heme pocket.

Synthesis, structure and spectroscopic properties of two models for the active site of the oxygenated state of cytochrome P450 [corrected]

Two dioxygen adducts of thiolato-iron(II) porphyrins, [K(222)][Fe(TPpivP)(SC6HF4)(O2)] 1a and [Na(18c.6)][Fe(TPpivP)(SC6HF4)(O2)] 2 were synthesized by reaction of O2 with five-coordinate, high-spin, cryptated alkali metal thiolato-iron(II) 'picket fence' porphyrinate. They were characterized by visible and infrared spectroscopy: lambda max (log epsilon) = 360 nm (4), 427 nm (4.69), 560 nm (3.69), 610 nm (3.40) for both compounds; v(16O-16O) = 1139 cm-1 in chlorobenzene and fluorobenzene for 1a and 2. Single crystals of composition [K(222)][Fe(TPpivP)(SC6HF4)(O2)].[K(222)](SC6HF4)(C 6H5Cl)(H2O) 1b were obtained by diffusion of pentane/xylene mixtures into chlorobenzene solutions of 1a at -5 degrees C. Single crystals of composition [Na(18c.6)][Fe(TPpivP)(SC6HF4)(O2)] were obtained by slow diffusion of pentane into benzene solutions of 2. Structures of 1b and 2 were studied at 20 degrees C (1b) and -100 degrees C (1b and 2). 1b: space group P2(1)/c (monoclinic), a = 16.806(5) A (1.6806 nm), b = 14.331(4) A (1.4331 nm), c = 52.000(15) A (5.2000 nm), beta = 92.95(2) degrees, V = 12.507 A3 (12.507 nm3), Z = 4, Dcal = 1.28 g.cm-3 (t = 20 degrees C). The final R1 factor was 0.085 for 5238 reflections having I greater than 3 sigma(I). 2: space group P2(1)/c (monoclinic), a = 13.107(3) A (1.3107 nm), b = 27.055(4) A (2.7055 nm), c = 25.029(4) A (2.5029 nm), beta = 96.84(2) degrees, V = 8812 A3 (8.812 nm3), Z = 4, Dcal = 1.18 g.cm-3 (t = -100 degrees C). The final R1 factor was 0.088 for 6587 reflections having I greater than 3 sigma(I). The iron atom is, in both compounds, bonded to the four porphyrinato nitrogens (Np), the sulfur atom of the axial thiolate and one oxygen atom of the axially end-on bonded dioxygen molecule. The average Fe-Np distance found in 1b [1.994(4) A, 0.1994 nm] is not significantly different from that found in 2 [1.993(3) A, 0.1993 nm]. The Fe-S bond length is 2.367(3) A (0.2367 nm) in 1b and 2.365(2) A (0.2365 nm) in 2. The Fe-O1 distances with the oxygen atom of O2 bonded to iron are respectively 1.837(9) A (0.1837 nm) and 1.850(4) A (0.1850 nm). The end-on bonded O2 molecule is disordered in both complexes 1b and 2.(ABSTRACT TRUNCATED AT 400 WORDS)