Graeme R. A. Wyllie

Nuclear resonance vibrational spectroscopy of a protein active-site mimic

For many years, Mossbauer spectroscopy has been applied to measure recoilless absorption of x-ray photons by nuclei. Recently, synchrotron radiation sources have enabled the observation of weaker features separated from the recoilless resonance by the energy of vibrational quanta. This enables a form of vibrational spectroscopy with a unique sensitivity to the probe nucleus. Biological applications are particularly promising, because it is possible to selectively probe vibrations of a single atom at the active site of a complex biomolecule, while avoiding interference from the vibrations of thousands of other atoms. In contrast with traditional site-selective vibrational spectroscopies, nuclear resonance vibrational spectroscopy (NRVS) is not hampered by solvent interference and faces selection rule limitations only if the probe nucleus lies on a symmetry element. Here, we formulate a mathematical language appropriate for understanding NRVS measurements on molecular systems and apply it to analyse NRVS data recorded on ferrous nitrosyl tetraphenylporphyrin, Fe(TPP)(NO). This compound mimics the haem group found at the active site of many proteins involved in the biological usage of oxygen and nitric oxide. Measurements on such model compounds provide a baseline for evaluating the extent to which vibrations are localized at the active site of a protein, with the goal of elucidating the mechanisms of biological processes, such as intersite communication in allosteric proteins.

Iron normal mode dynamics in (nitrosyl)iron(II)tetraphenylporphyrin from X-ray nuclear resonance data.

The complete iron atom vibrational spectrum has been obtained by refinement of normal mode calculations to nuclear inelastic x-ray absorption data from (nitrosyl)iron(II)tetraphenylporphyrin, FeTPP(NO), a useful model for heme dynamics in myoglobin and other heme proteins. Nuclear resonance vibrational spectroscopy (NRVS) provides a direct measurement of the frequency and iron amplitude for all normal modes involving significant displacement of (57)Fe. The NRVS measurements on isotopically enriched single crystals permit determination of heme in-plane and out-of-plane modes. Excellent agreement between the calculated and experimental values of frequency and iron amplitude for each mode is achieved by a force-field refinement. Significantly, we find that the presence of the phenyl groups and the NO ligand leads to substantial mixing of the porphyrin core modes. This first picture of the entire iron vibrational density of states for a porphyrin compound provides an improved model for the role of iron atom dynamics in the biological functioning of heme proteins.

Metalloporphyrin mixed-valence π-cation radicals: [Fe(oxoOEC(•/2))(Cl)]2SbCl6, structure, magnetic properties, and near-IR spectra.

The preparation and characterization of a mixed-valence π-cation radical derivative of an iron(III) oxochlorinato complex is reported. The new complex has been synthesized by the one-electron oxidation of a pair of [Fe(oxoOEC)(Cl)] molecules to form the dimeric cation [Fe(oxoOEC)(Cl)]₂⁺. The cation has been characterized by X-ray analysis, Mössbauer spectroscopy, UV-vis and near-IR spectroscopy, and magnetic susceptibility measurements from 6-300 K. The crystal structure shows that the two rings have a smaller overlap area than those of the formally related nickel and copper octaethylporphinate derivatives, reflecting the larger steric congestion at the periphery in part of the oxochlorin rings. The Mössbauer data is consistent with two equivalent iron(III) centers. The unpaired electron is delocalized over the two oxochlorin rings and mediates a strong antiferromagnetic interaction between the high-spin iron(III) centers.

Structural and magnetic effects of meso-substitution in alkyl-substituted metalloporphyrinate pi-cation radicals: characterization of [Fe(TalkylP*)(Cl)]SbCl6 (alkyl = ethyl and n-propyl).

We report the preparation and characterization of two meso-alkyl substituted porphyrin pi-cation radical derivatives, [Fe(TalkylP(*))(Cl)]SbCl(6) (alkyl = ethyl or propyl). Both complexes have been characterized by UV/vis/near-IR, IR, and Mossbauer spectroscopy, temperature-dependent solid-state magnetic susceptibility measurements, and X-ray structure determinations. All data for both oxidized species are consistent with the formulation of the complexes as ring-oxidized iron(III) porphyrin species. The molecular structures of the two five-coordinate species have the typical square-pyramidal coordination group of high-spin iron(III) derivatives. The crystal structures also reveal that the species form cofacial pi-pi dimers with lateral shifts of 1.44 A and 3.22 A, respectively, for the propyl and ethyl radical derivatives. Both radicals exhibit porphyrin cores with alternating bond distance patterns in the inner 16-membered ring. In addition, [Fe(TEtP(*))(Cl)]SbCl(6) and [Fe(TPrP(*))(Cl)]SbCl(6) have been characterized by temperature-dependent (6-300 K) magnetic susceptibility studies, the best fitting of the temperature-dependent moments reveal strong coupling between iron spins and porphyrin radical, and a smaller magnitude of antiferromagnetic coupling between ring radicals, which are opposite to those found in the five-coordinate iron(III) OEP radicals. The differences in structure and properties of the cation radical meso-alkyl and beta-alkyl derivatives possibly reflect differences in properties of a(1u)- and a(2u)-forming radicals.

Structure and magnetic properties of [Cr(en)2(ox)][Cr(en)(ox)2]·2H2O,Δ-[Cr(en)3]Δ-[Cr(ox)3] and Δ-[Co(en)3]Δ-[Cr(ox)3]

Abstract Interactions between metal ion complexes in the compounds Δ-[Cr(en)3]Δ-[Cr(ox)3] and Δ-[Co(en)3]Δ-[Cr(ox)3] have been investigated by X-ray crystallography and magnetic susceptibility measurements. Crystals of both compounds are hexagonal and are isostructural with the known Δ-[Co(en)3]Δ-[Rh(ox)3] [Inorg. Chem. 30 (1991) 4954] with chains of alternating anions and cations parallel to the crystallographic c-axis. The susceptibility behavior suggests that magnetic interactions are stronger in Δ-[Cr(en)3]Δ-[Cr(ox)3] where both anion and cation are paramagnetic than in Δ-[Co(en)3]Δ-[Cr(ox)3] where only the anion is paramagnetic. Single crystal studies with the former compound reveal that magnetic interactions along the c-axis chains, where the Cr–Cr distances are shortest, are weaker than those involving off-axis interactions. Magnetic measurements were also made on [Cr(en)2(ox)][Cr(en)(ox)2]·2H2O. The structure of this compound has been previously reported.

Iron Nitrosyl “Natural” Porphyrinates: Does the Porphyrin Matter?

The synthesis and spectroscopic characterization of three five-coordinate nitrosyliron(II) complexes, [Fe(Porph)(NO)], are reported. These three nitrosyl derivatives, where Porph represents protoporphyrin IX dimethyl ester, mesoporphyrin IX dimethyl ester, or deuteroporphyrin IX dimethyl ester, display notable differences in their properties relative to the symmetrical synthetic porphyrins such as OEP and TPP. The N–O stretching frequencies are in the range of 1651–1660 cm–1, frequencies that are lower than those of synthetic porphyrin derivatives. Mössbauer spectra obtained in both zero and applied magnetic field show that the quadrupole splitting values are slightly larger than those of known synthetic porphyrins. The electronic structures of these naturally occurring porphyrin derivatives are thus seen to be consistently different from those of the synthetic derivatives, the presumed consequence of the asymmetric peripheral substituent pattern. The molecular structure of [Fe(PPIX-DME)(NO)] has been determined by X-ray crystallography. Although disorder of the axial nitrosyl ligand limits the structural quality, this derivative appears to show the same subtle structural features as previously characterized five-coordinate nitrosyls.

Intermolecular Dynamics in Crystalline Iron Octaethylporphyrin (FeOEP)

The new technique of nuclear resonance vibrational spectroscopy (NRVS) has increased the range and quality of dynamical data from Fe-containing molecules that when combined with Raman and infrared spectroscopies impose stricter constraints on normal mode simulations, especially at lower frequencies. Going beyond the usual single molecule approximation, a classical normal-mode analysis that includes intermolecular coupling and the full crystalline symmetry is found to produce a better fit with fewer free parameters for the heme compound iron octaethylporphyrin (FeOEP), using NRVS data from polycrystalline material. Off-diagonal force constants were completely unnecessary, indicating that their role in previous single molecule fits was just to emulate intermolecular coupling. Sound velocities deduced from the calculated phonon dispersion curves are compared to NRVS measurements to further constrain the intermolecular force constants. The NRVS data by themselves are insufficient to rigorously determine all unknown force constants for molecules of this size, but the improved crystal model fit indicates the necessity of including intermolecular interactions for normal-mode analyses.

Mössbauer Studies of Fe(II)-nitrosyl Porphyrin Model Compounds

We have studied the temperature-dependent Mossbauer spectra of polycrystalline samples of several five-coordinate Fe(II)-NO model compounds: octaethylporphyrin (OEP), tetraphenylporphyrin (TPP), tetramesitylporphyrin (TMP), and tetratolylporphyrin (TTP). The OEP model shows a single symmetric quadrupole doublet, while for all the other species, the quadrupole double is asymmetric, with the lower-velocity line significantly broadened. X-ray structure determinations of Fe(TPP)NO show significant disorder of the Fe-N-O plane, and we believe the broadened Mossbauer spectra also reflect this disorder. Data on OEP and TPP at 4.2K in fields of 4T and 9T are fit well by a paramagnetic S=l/2 model. While the temperature variation of the quadrupole splitting and isomer shift values for these species is weak, the compounds exhibit an unusually rapid drop in absorption area with temperature, consistent with Debye temperatures of 81K(OEP), 64K(TMP), 55K(TPP) and 47K(TTP).