Symmetry states of metalloporphyrin π-cation radicals, models for peroxidase compound I

Alternant Bond Distances in Octaethylporphyrin π-Cation Radicals

The possible appearance and nature of alternant bond distance patterns in the inner 16-membered ring of several octaethylporphyrin [Formula: see text]-cation radicals has been investigated. This study was made possible by recognizing an unexpected solvent system, namely dichloromethane/chloroform, even though the [Formula: see text]-cation species have extremely limited solubility in chloroform. A total of six [M(OEP[Formula: see text]][Formula: see text] derivatives were studied by single-crystal X-ray structure determinations. Two new zinc derivatives display, quantitatively, the same alternant pattern observed previously. A new nickel complex shows a smaller but now probably significant alternant pattern. However, a copper derivative, independently analyzed twice, shows no evidence for an alternant pattern. The importance of ring–ring interactions on the energy of the top two orbitals is shown by two distinct magnesium derivatives. The derivative with strongly overlapped rings displays an alternant bond distance pattern, whereas the other, with a modestly overlapped ring pair, does not. This suggests the importance of strong ring–ring interactions in leading to a pseudo-Jahn-Teller state; this hypothesis is also supported by other prior results.

Spectroscopic features of cytochrome P450 reaction intermediates

Cytochromes P450 constitute a broad class of heme monooxygenase enzymes with more than 11,500 isozymes which have been identified in organisms from all biological kingdoms [1]. These enzymes are responsible for catalyzing dozens chemical oxidative transformations such as hydroxylation, epoxidation, N-demethylation, etc., with very broad range of substrates [2,3]. Historically these enzymes received their name from 'pigment 450' due to the unusual position of the Soret band in UV-vis absorption spectra of the reduced CO-saturated state [4,5]. Despite detailed biochemical characterization of many isozymes, as well as later discoveries of other 'P450-like heme enzymes' such as nitric oxide synthase and chloroperoxidase, the phenomenological term 'cytochrome P450' is still commonly used as indicating an essential spectroscopic feature of the functionally active protein which is now known to be due to the presence of a thiolate ligand to the heme iron [6]. Heme proteins with an imidazole ligand such as myoglobin and hemoglobin as well as an inactive form of P450 are characterized by Soret maxima at 420nm [7]. This historical perspective highlights the importance of spectroscopic methods for biochemical studies in general, and especially for heme enzymes, where the presence of the heme iron and porphyrin macrocycle provides rich variety of specific spectroscopic markers available for monitoring chemical transformations and transitions between active intermediates of catalytic cycle.

Theoretical Analysis of the Structural and Electronic Properties of Metalloporphyrin π-Cation Radicals

A method for analyzing the A(1u)/A(2u) contents of metalloporphyrin pi-cation radicals is developed and applied to a series of unsubstituted planar metalloporphines (MPs) (M=Cr, Mn, Fe, Co, Ni, Cu, and Zn). The structures and electronic properties of the MPs and their cation radicals were calculated by density functional theory (DFT) and subsequently analyzed. It was found that the MPs with small core sizes have a tendency to form A(1u)-type radicals, while the MPs with large core size have a preference for an A(2u)-type. Neither of these pure-state species, however, is stable under the D(4)(h) symmetry, and both radical cation types are subject to pseudo-Jahn-Teller (pJT) distortion. The pJT distortion leads to structures with lower symmetry and states that have mixed character with respect to the A(1u) and A(2u) components. The degree of mixing could be estimated by employing orbital projection technique or a complementary spin density decomposition. Both techniques produce very similar results, pointing out that the frontier orbital, which becomes empty upon electron removal, plays a critical role in determining electronic properties.

Resonance Raman spectroscopy of oxoiron(IV) porphyrin π-cation radical and oxoiron(IV) hemes in peroxidase intermediates

The catalytic cycle intermediates of heme peroxidases, known as compounds I and II, have been of long standing interest as models for intermediates of heme proteins, such as the terminal oxidases and cytochrome P450 enzymes, and for non-heme iron enzymes as well. Reports of resonance Raman signals for compound I intermediates of the oxo-iron(IV) porphyrin pi-cation radical type have been sometimes contradictory due to complications arising from photolability, causing compound I signals to appear similar to those of compound II or other forms. However, studies of synthetic systems indicated that protein based compound I intermediates of the oxoiron(IV) porphyrin pi-cation radical type should exhibit vibrational signatures that are different from the non-radical forms. The compound I intermediates of horseradish peroxidase (HRP), and chloroperoxidase (CPO) from Caldariomyces fumago do in fact exhibit unique and characteristic vibrational spectra. The nature of the putative oxoiron(IV) bond in peroxidase intermediates has been under discussion in the recent literature, with suggestions that the Fe(IV)O unit might be better described as Fe(IV)-OH. The generally low Fe(IV)O stretching frequencies observed for proteins have been difficult to mimic in synthetic ferryl porphyrins via electron donation from trans axial ligands alone. Resonance Raman studies of iron-oxygen vibrations within protein species that are sensitive to pH, deuteration, and solvent oxygen exchange, indicate that hydrogen bonding to the oxoiron(IV) group within the protein environment contributes to substantial lowering of Fe(IV)O frequencies relative to those of synthetic model compounds.

Resonance Raman enhancement of FeIVO stretch in high-valent iron porphyrins: An insight from TD-DFT calculations

Density functional theory (DFT) has been applied to explain the origin of resonance Raman enhancement associated with the Fe(IV)=O stretch observed in iron(IV)oxo porphyrins. To accomplish this electronic excitations of the Im-(Por)Fe(IV)=O model were computed in the 1.5-4.0 eV spectral range using time-dependent DFT (TD-DFT). All electronic transitions having dominant pi-->pi* character were analyzed and assigned in terms of one-electron excitations. It was found that the most intense Soret band has a multi-component character, but the pi (a(2u))-->pi*(d(xz),d(yz)) and pi (a(1u))-->pi*(d(xz),d(yz)) electronic excitations are primarily responsible for observed resonance enhancement of the Fe(IV)=O stretch.

Effect of the axial cysteine ligand on the electronic structure and reactivity of high-valent iron(IV) oxo-porphyrins (Compound I): A theoretical study

The effect of axial ligands on the reactivity of high-valent iron(IV) oxo-porphyrins (Compound I) was investigated using the B3LYP hybrid density functional method. We studied alkane hydroxylation using four models: Compound I with thiolate, imidazole, phenolate, and chloride anions as axial ligands. The first three ligands were employed as models for cysteinate, histidine, and tyrosinate, respectively. Our calculations show that anionic ligands and neutral ligands favor different electronic states for stationary points in the reaction coordinate, and the calculated energy barrier and energy of several reaction intermediates show similar values. A remarkable effect of axial ligands was found in the final product release step. Our calculations show that the thiolate ligand weakens a bond between heme and an alcohol. In contrast, the imidazole ligand significantly increases the interaction between heme and an alcohol, which causes the catalytic cycle to be less efficient.

Electronic structure of compound I in human isoforms of cytochrome P450 from QM/MM modeling

Human cytochromes P450 play a vital role in drug metabolism. The key step in substrate oxidation involves hydrogen atom abstraction or C=C bond addition by the oxygen atom of the Compound I intermediate. The latter has three unpaired electrons, two on the Fe-O center and one shared between the porphyrin ring and the proximal cysteinyl sulfur atom. Changes in its electronic structure have been suggested to affect reactivity. The electronic and geometric structure of Compound I in three important human subfamilies of cytochrome P450 (P450, 2C, 2B, and 3A) that are major contributors to drug metabolism is characterized here using combined quantum mechanical/molecular mechanical (QM/MM) calculations at the B3LYP:CHARMM27 level. Compound I is remarkably similar in all isoforms, with the third unpaired electron located mainly on the porphyrin ring, and this prediction is not very sensitive to details of the QM/MM methodology, such as the DFT functional, the basis set, or the size of the QM region. The presence of substrate also has no effect. The main source of variability in spin density on the cysteinyl sulfur (from 26 to 50%) is the details of the system setup, such as the starting protein geometry used for QM/MM minimization. This conformational effect is larger than the differences between human isoforms, which are therefore not distinguishable on electronic grounds, so it is unlikely that observed large differences in substrate selectivity can be explained to a large extent in these terms.

Toward Identification of the Compound I Reactive Intermediate in Cytochrome P450 Chemistry: A QM/MM Study of Its EPR and Mössbauer Parameters

Quantum mechanical/molecular mechanical (QM/MM) methods have been used in conjunction with density functional theory (DFT) and correlated ab initio methods to predict the electron paramagnetic resonance (EPR) and Mossbauer (MB) properties of Compound I in P450(cam). For calibration purposes, a small Fe(IV)-oxo complex [Fe(O)(NH(3))(4)(H(2)O)](2+) was studied. The (3)A(2) and (5)A(1) states (in C(4)(v)() symmetry) are found to be within 0.1-0.2 eV. The large zero-field splitting (ZFS) of the (FeO)(2+) unit in the (3)A(2) state arises from spin-orbit coupling with the low-lying quintet and singlet states. The intrinsic g-anisotropy is very small. The spectroscopic properties of the model complex [Fe(O)(TMC)(CH(3)CN)](2+) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) are well reproduced by theory. In the model complexes [Fe(O)(TMP)(X)](+) (TMP = tetramesitylporphyrin, X = nothing or H(2)O) the computations again account for the observed spectroscopic properties and predict that the coupling of the (5)A(1) state of the (FeO)(2+) unit to the porphyrin radical leads to a low-lying sextet/quartet manifold approximately 12 kcal/mol above the quartet ground state. The calculations on cytochrome P450(cam), with and without the simulation of the protein environment by point charges, predict a small antiferromagnetic coupling (J approximately -13 to -16 cm(-)(1); H(HDvV) = - 2JS(A)S(B)) and a large ZFS > 15 cm(-)(1) (with E/D approximately 1/3) which will compete with the exchange coupling. This leads to three Kramers doublets of mixed multiplicity which are all populated at room temperature and may therefore contribute to the observed reactivity. The MB and ligand hyperfine couplings ((14)N, (1)H) are fairly sensitive to the protein environment which controls the spin density distribution between the porphyrin ring and the axial cysteinate ligand.

Resonance Raman spectroscopic studies of hydroperoxo-myoglobin at cryogenic temperatures

In agreement with previous reports (Gasyna, Z. FEBS Lett. 1979, 106, 213-218 and Leibl, W.; Nitschke, W.; Huettermann, J. Biochim. Biophys. Acta 1986, 870, 20-30) radiolytically reduced samples of oxygenated myoglobin at cryogenic temperatures have been shown by optical absorption and EPR studies to produce directly the peroxo-bound myoglobin at 77 K. Annealing to temperatures near 185 K induces proton transfer, resulting in the formation of the hydroperoxo heme derivative. Resonance Raman studies of the annealed samples has permitted, for the first time, the direct observation of the key nu(Fe-O) stretching mode of the physiologically important Fe-OOH fragment of this ubiquitous intermediate. The assignment of this mode to a feature appearing at 617 cm(-1) is strongly supported by documentation of a 25 cm(-1) shift to lower energy upon substitution with (18)O(2) and by a 5 cm(-1) shift to lower energy for samples prepared in solutions of deuterated solvent.

High-valent transition metal centers versus noninnocent ligands in metallocorroles: insights from electrochemistry and implications for high-valent heme protein intermediates

For relatively electron-rich corrole ligands, the halfwave potentials for oxidation of Cu(III), Sn(IV)Ph, Fe(IV)Ph, and Fe(IV)-O-Fe(IV) complexes are significantly lower than those of Sn(IV)Cl, Fe(IV)Cl, Mn(IV)Cl, and Cr(V)(O) complexes, suggesting that the corrole ligand is relatively electron-rich or 'innocent' in the former group of complexes and that it is relatively electron-deficient or 'noninnocent' in the latter group. Both the formal charge of the central metal ion and the nature of the axial ligand, if any, appear to be key determinants of the electronic character of the corrole ligand in metallocorrole complexes, a theme that has interesting resonances with recent findings on high-valent heme protein intermediates. However, for very strongly electron-deficient ligands such as meso-tris(pentafluorophenyl)corrole (TPFPC) and beta-octabromo-meso-tris(pentafluorophenyl)corrole (Br(8)TPFPC), which cannot sustain significant radical character, the various metal complexes all exhibit comparable halfwave potentials for oxidation and the ligand may be considered to be relatively innocent.

Symmetry-breaking phenomena in metalloporphyrin pi-cation radicals

Density functional theory (DFT) calculations of the energetics, molecular structures, and spin density profiles of metalloporphyrin pi-cation radicals suggest that the common practice of describing these radicals in terms of a universal A(1u)/A(2u) dichotomy is often not justified, confirming a possibility first foreseen by Prendergast and Spiro (ref 15) over a decade ago on the basis of vibrational spectroscopy and semiempirical calculations. Because of near-degeneracy of the a(1u) and a(2u) HOMOs of many metalloporphyrins, the cation radicals derived from these compounds undergo a pseudo-Jahn-Teller (pJT) distortion and are, therefore, best described as (2)A(u) with reference to the C(4h) point group, rather than as (2)A(1u) (D(4h) or (2)A(2u) (D(4h)). We find that the porphyrin cation radicals undergo a pJT distortion if the energy difference between the (2)A(1u) and (2)A(2u) pi-cation radicals, optimized under D(4h) symmetry constraints, is less than 0.15 eV. According to this criterion, metallo-porphine and metallo-OEP pi-cation radicals should always be pJT-distorted and metallo-meso-tetrahalogenoporphyrin radicals should not. For [Zn(TPP(*))](+), the (2)A(1u)/(2)A(2u) energy difference is almost exactly at the threshold of 0.15 eV, consistent with the experimental observation of both symmetry-broken and undistorted structures for this species. The (2)A(1u)/(2)A(2u) energy difference (when the molecular geometries are optimized under a D(4h) symmetry constraint) also appears to govern whether the real pJT-distorted cation radical is more A(1u)- or A(2u)-like in terms of its spin density profile. Because many metalloporphyrin pi-cation radicals exist as cofacial dimers in the crystalline phase, we examined the symmetries and structures of the model compounds [[Zn(P)](2)](+,2+) by means of DFT geometry optimizations. The results showed that dimerization has relatively little impact on the bond length alternation in the individual rings. A final interesting result, consistent with experiment, is that the bond length alternation in the delocalized mixed-valence dimer [[Zn(P)](2)](+) is about half that found for [[Zn(P)](2)](2+).

"True" iron(V) and iron(VI) porphyrins: a first theoretical exploration

We present here a first theoretical characterization of iron(V) (S = (3)/(2)) and iron(VI) (S = 0) porphyrin intermediates. The Fe(V) calculations exhibit exceptionally narrow convergence radii and we believe that for this reason they have long eluded researchers working on high-valent iron intermediates. The Fe(V)-N(nitrido) bond distance in the DFT(PW91/TZP) optimized geometry of Fe(V)(P)(N) is 1.722 A, comparable to and slightly longer than the Fe(IV)-O bond distance of 1.684 A in Fe(IV)(P)(O) and the Fe(IV)-N(imido) bond distance of 1.698 A in Fe(IV)(P)(NH). In contrast, the Fe(VI)-N(nitrido) bond distances in [Fe(VI)(P)(N)](+) (S = 0) and Fe(VI)(P)(N)(F) (S = 0) are dramatically shorter, 1.508 and 1.533 A, respectively, consistent with the formal triple bond character of the Fe(VI)-N(nitrido) bond. The nitrido ligand appears to be uniquely capable of stabilizing a "true" Fe(V) center, in the sense defined in the paper. All three unpaired electrons in Fe(V)(P)(N) are completely localized on the Fe(V)-N(nitrido) axis, with the Fe and N gross atomic spin populations being 1.579 and 1.550, respectively. In contrast, an axial ligand set consisting of an oxide and a fluoride do not stabilize an Fe(V) ground state but favor an electronic structure best described as an Fe(IV)-oxo porphyrin pi-cation radical.

Resonance Raman spectroscopy and density functional theoretical calculations of manganese corroles. A parallelism between high-valent metallocorroles and metalloporphyrins, relevant to horseradish peroxidase and chloroperoxidase compound I and II intermediates

Soret-excited resonance Raman (RR) spectra are reported for the Mn(III) and Mn(IV)Cl derivatives of meso-tris(p-(trifluoromethyl)phenyl)corrole, H(3)T(p-CF(3)-P)Cor, and the Mn(III) derivative of beta-octabromo-meso-tris(p-(trifluoromethyl)phenyl)corrole, H(3)Br(8)T(p-CF(3)-P)Cor. Three high-frequency bands in the RR spectrum of Mn(III)[T(p-CF(3)-P)Cor] at 1465, 1524 and 1615 cm(-1) appear to upshift to 1486, 1528 and 1620 cm(-1) for Mn(IV)[T(p-CF(3)-P)Cor]Cl. This suggests that the electronic character of the corrole ligand is significantly different for these two compounds, which is consistent with electrochemical evidence for partial radical character of the corrole ligand for Mn(IV)[T(p-CF(3)-P)Cor]Cl but not for Mn(III)[T(p-CF(3)-P)Cor]. The observed upshifts are also consistent with DFT calculations showing a shortening of some of the relevant bonds in the Mn(IV)Cl derivative relative to the Mn(III) derivative. The results raise the possibility of an extensive parallelism between the electronic structures of high-valent metallocorroles and metalloporphyrins. Three high-frequency bands in the RR spectrum of Mn(III)[T(p-CF(3)-P)Cor] at 1331, 1465 and 1545 cm(-1) appear to downshift to 1320, 1457 and 1537 cm(-1) for Mn(III)[Br(8)T(p-CF(3)-P)Cor]. This is consistent with the suspected longer carbon-carbon bond lengths in the brominated corrole macrocycle.