Definitive Assignment of the g Tensor of [Fe(OEP)(NO)] by Single-Crystal EPR

Abstract

Single-crystal EPR measurements have been performed on the triclinic form of [Fe(OEP)(NO)] (Ellison, M. K.; Scheidt, W. R. J. Am. Chem. Soc. 1997, 119, 7404) and on the isomorphous cobalt derivative [Co(OEP)(NO)] (Ellison, M. K.; Scheidt, W. R. Inorg. Chem. 1998, 37, 382) which has been doped with [Fe(OEP)(NO)]. Principal values of the g tensor determined at room temperature are gmax = 2.106, gmid = 2.057, and gmin = 2.015. The principal direction associated with the minimum g value lies 8 degrees from the Fe-N(NO) direction, 2 degrees from the normal to the heme plane, and 42 degrees from the N-O direction. The direction associated with the maximum g value lies 9 degrees from the normal to the Fe-N-O plane. The fact that the direction of gmin is near the Fe-N(NO) direction is consistent with the dominant role of spin-orbit coupling at the iron atom in determining the g tensor and with the picture of the electronic structure of the compound from restricted calculations, which makes the half-filled orbital mostly dz2 on the iron atom. The hyperfine tensor is nearly isotropic and was only resolved in the doped samples. Principal values of the A tensor determined at room temperature are 40.9, 49.7, and 42.7 MHz. Principal values of the g tensor determined from the doped samples at 77 K are gmax = 2.110, gmid = 2.040, and gmin = 2.012. Principal values of the A tensor are 42.5, 52.8, and 44.1 MHz at 77 K. The small change in g values with temperature is in contrast to the large temperature dependence on g values observed in samples of MbNO (Hori et al. J. Biol. Chem. 1981, 256, 7849).

Synthesis, X-ray Structures, Electronic Properties, and O2/NO Reactivities of Thiol Dioxygenase Active-Site Models

“…The EPR parameters of 1 NO -LS and 2 NO -LS closely align with those computed for S =1/2 [ 2-NO ] 5C , as well as those observed for 5C NO adducts of ferrous-porphyrin complexes. 97–100 Thus, we conclude that the observed low-spin signal is due to dissociation of the trans imidazole donor in a fraction of the {FeNO} 7 adducts prepared at low temperature.…”

“…Importantly, the calculations are consistent with prior studies of low-spin {FeNO} 7 complexes, both heme and nonheme, which have revealed that certain spin-Hamiltonian parameters are diagnostic of coordination number. 97–100 Specifically, the g x -values of 6C species are always less than the free-electron value ( g e = 2.002), whereas g x > g e for 5C {FeNO} 7 species. Additionally, the A -tensors of 6C complexes exhibit relatively large anisotropy and a dominant A y -value, whereas 5C complexes possess relatively isotropic 14 N A -tensors in which the A x -value is the largest component (Table 3).…”

“…Thus, spectral simulations with the computed Euler angles were used to determine the A-value corresponding to each g-value. b Reference 34. c OEP = octaethylporphyrin(2−); references 97 and 99. d Reference 88. …”

Isocyanide or nitrosyl complexation to hemes with varying tethered axial base ligand donors: synthesis and characterization

“…For 5C complexes, the N–O stretch typically lies between 1675 and 1700 cm −1 , whereas for 6C complexes, the N–O stretch occurs at ~1630 cm −1 . Low-temperature EPR spectroscopy studies conducted by several authors reveal interesting differences between the 5C and 6C iron(II) porphyrin NO adducts [60, 61]. Hyperfine lines resulting from the bound nitrogen of NO are observed at the lowest g value (g min) in 5C ferrous-heme nitrosyls.…”

Thiolate coordination to Fe(II)–porphyrin NO centers

“…Scheme 1) and tetrahydrothiophene (a thioether) are presented in this study. The obtained data are correlated to the EPR spectra of 1 itself and of the corresponding adduct with 1-methylimidazole (1-MI) [21][22][23][24][25][26] as well as EPR data of NO adducts of ferrous P450 and P450nor [15,16]. Density functional (DFT) calculations on the simplified model [Fe(P)(SMe)(NO)] À (P = simplified Porphyrin ligand) are then used to explore the general properties of Fe(II)-NO complexes with trans S-donor ligands as they occur in the P450 enzymes (bacterial P450cam, liver microsomal P450 LM and P450nor) in detail.…”