George Lang

Mössbauer spectroscopic study of compound es of cytochrome c peroxidase

Mössbauer spectra of Compound ES of cytochrome c peroxidase have been observed over a range of temperature and applied magnetic field. These have been interpreted in terms of a crystal field model of the iron site in which the iron is assumed to be in the Fe(IV) state with unpaired spin S = 1. Detailed least-squares fitting of the spectra fitting of the spectra indicates that both the electric field gradient choice of a single parameter, the axial crystal field, the magnetic properties are well reproduced. The model also provides the observed positive sign for the electric field gradient interaction, but overestimates its magnitude. This apparent discrepnancy may be caused by the presence of significant electronic charge in filled bonding orbitals, a feature which is in keeping with expected covalent charge compensation of the extreme oxidation state. There is no evidence in the Mössbauer spectra of interaction between the iron and the ESR-visible free radical. This suggests they are well separated.

Mössbauer spectroscopy of hemoglobin model compounds: Evidence for conformational excitation

Zero‐field Mossbauer spectra of a model compound for hemoglobin, capable of undergoing reversible oxygenation, were recorded at various temperatures. A pair of peaks with temperature‐dependent quadrupole splitting and linewidth were observed. The results have been interpreted in terms of a model which is consistent with previous x‐ray studies and which provides for relaxation effects as the molecule assumes two possible conformational states. In terms of the proposed model, the two conformational states have been characterized by their energy separation and their respective electric field gradient tensors. The relaxation rate at each temperature has also been determined.

Mössbauer effect study of the magnetic properties of S=1 ferrous tetraphenylporphyrin

Mossbauer spectra of the polycrystalline form of the square planar compound α,β,γ,δ‐tetraphenylporphinato–iron (II) have been observed at a variety of temperatures. Analysis of the resulting spectra yield a positive electric field gradient interaction with quadrupole splitting ΔE=1.51 mm/s (4.2 K) and 1.52 mm/s (300 K). At low temperature, an external magnetic field induces a negligible internal field along the symmetry axis of the molecule. In the transverse direction, it gives rise to a positive internal field with little temperature dependence up to 30 K. The data can be interpreted in terms of a crystal field model involving a low‐lying 3A2g state which is split by spin–orbit coupling into a ground singlet and a doublet lying 80 cm−1 above it.

Mössbauer effect study of tight spin coupling in oxidized chloro‐5,10,15,20‐tetra(mesityl)porphyrinatoiron(III)

Mossbauer spectra of a polycrystalline form of oxidized chloro‐5,10,15,20‐tetra(mesityl)porphyrin‐ atoiron(III) [Fe(TMP)Cl], compound A, were recorded over a range of temperatures (4.2–195 K) and magnetic fields (0–6 T). These spectra of compound A exhibit magnetic features which are markedly different from those of the analogous protein complexes, horse radish peroxidase compound I (HRP‐I) and compound ES of cytochrome c peroxidase, even though chemical evidence and optical spectroscopy indicate that compound A is similar to the others in comprising a Fe(IV) complex within a porphyrin cation radical. We interpret the data by employing a spin Hamiltonian model in which the central Fe(IV) complex, with S=1, is tightly coupled to a S=1/2 system of the oxidized porphyrin to yield a net S=3/2 system as suggested by the susceptibility measurements. The theoretical treatment yields information on the d‐electron energies which is similar to that more directly available in the peroxidase spectra. The strength of ...

Mössbauer Effect in Low‐Spin (d7) Complex Molecules of Fe

Mossbauer spectra of Fe(CN)5NO3− (complex I), Fe(CN)5NOH2− (complex II), and hemoglobin nitric oxide are presented and compared with theoretical predictions. The spectra of complex I show no magnetic hyperfine interaction, while those of complex II and hemoglobin nitric oxide correspond to calculated spectra based on the assumption of unpaired spin in sigma bonds. The iron orbital components of the sigma wavefunctions are about 75% and 50%, respectively. In order to account for the Mossbauer spectra in zero applied field, transferred hyperfine interaction of the unpaired spin with the nitrosyl nitrogen must be taken into account. Results are in general agreement with ESR measurements.

Low temperature photodissociation studies of ferrous hemoglobin and myoglobin complexes by Mössbauer spectroscopy.

57Fe-enriched complexes of hemoglobin and myoglobin with CO and O2 were photodissociated at 4.2 degrees K, and the resulting spectra were compared with those of the deoxy forms. Differences in both quadrupole splitting and isomer shift were noted for each protein, the photoproducts having smaller isomer shift and larger quadrupole splitting than the deoxy forms. The photoproducts of HbCO and HbO2 had narrow absorption lines, indicating a well-defined iron environment. The corresponding myoglobin species had broader absorption lines, as did both deoxy forms. The weak absorption lines of photodissociated NO complexes appeared to be wide, possibly indicating magnetic interaction with the unpaired electron of the nearby NO.

Mössbauer spectroscopy of cytochrome C.

The cytochrome c of T. utilis grown in a 57‐enriched medium has been investigated by Mossbauer spectrometry over a wide temperature range. The ferric cytochrome, in both lyophilized and frozen‐solution form, shows magnetic hyperfine interaction at low temperatures and resembles some of the low‐spin homoglobins. Spectra are in good agreement with calculations based on g values of horse‐heart ferricytochrome c. The ferrous cytochrome exhibits a simple temperature‐independent quadrupole splitting of 1.18 ± 0.05 mm/sec. Measurements in applied field indicate that Vzz > 0, and η ∼ 0.5.

Mössbauer studies of low‐symmetry crystal fields in low‐spin ferric heme complexes

The Mossbauer spectra of the azide and cyanide complexes of myoglobin and cytochrome c peroxidase as well as the spectra of the synthetic analogues heme bis‐pyridine, heme cyanopyridine, and heme bis‐cyanide were recorded a 4.2 K in small applied magnetic fields. These low‐spin ferric ions exhibit well‐resolved paramagnetic hyperfine structure below 20 K and the results were analyzed by assuming that the iron ion is in a low‐symmetry crystal field. The crystal field parameters and the relative orientations and components of the magnetogyric tensor g, the magnetic hyperfine tensor A, and the quadrupole tensor Ṽ were determined for each sample. Good fits to the experimental data were achieved. In particular the orientation of the g‐tensors obtained from the Mossbauer measurements on frozen solutions of the myoglobin complexes are in good agreement with published results derived from single‐crystal EPR measurements.

Mössbauer investigation of deoxymyoglobin in a high magnetic field. Orientation of the electric field gradient and magnetic tensors

We have examined the Mössbauer spectra of deosymyoglobin in a 6 T magnetic field in the temperature range 4.2-195 K. Spectra were fitted by the least-squares method using a phenomenological model in which the internal magnetic hyperfine field was assumed to be related to the applied field by a temperature dependent tensor õmega. The results indicate that õmega has axial symmetry and a principal axis system aligned with that of the electric field gradient (efg). The principal component of the latter is negative and lies on the symmetry axis of õmega. Our fits indicate that of efg asymmetry parameter is eta = 0.7, with no appreciable temperature dependence. Both axial and transverse components of õmega have the expected 1/T temperature dependence for T greater than 20 K. Our experimentally determined value of eta, combined with published single crystal zero-field measurements, constrains the efg-heme relative orientation to two possibilities. In neither of these is an efg principal axis near to the heme normal.

High magnetic field mössbauer studies of deoxymyoglobin, deoxyhemoglobin, and synthetic analogues

Mössbauer spectra of deoxymyoglobin, deoxyhemoglobin, and the synthetic analogues, iron (II) 2-methylimidazole meso-tetraphenylporphyrin, and iron (II) 1,2-dimethylimidazole meso-tetraphenylporphyrin have been observed in high magnetic fields and over a wide range of temperature. At temperatures greater than 20 K all materials exhibit remarkably similar spectra, with anisotropic internal magnetic fields decreasing as 1/T. All have negative quadrupole interaction, and both this and the magnetic anisotropy imply that the orbital of the odd electron is prolate in the ground quintet, with little unquenched orbital angular momentum. At 4.2 K the spectra differ, suggesting different detailed structure within the quintet. In contrast to the proteins, the 2-methyl model exhibits spectra at 4.2 K which imply that the lowest spin state has high susceptibility in a single direction.