Electronic structures of highly deformed iron(III) porphyrin complexes

Inactivation of the heme degrading enzyme IsdI by an active site substitution that diminishes heme ruffling

IsdG and IsdI are paralogous heme degrading enzymes from the bacterium Staphylococcus aureus. Heme bound by these enzymes is extensively ruffled such that the meso-carbons at the sites of oxidation are distorted toward bound oxygen. In contrast, the canonical heme oxygenase family degrades heme that is bound with minimal distortion. Trp-66 is a conserved heme pocket residue in IsdI implicated in heme ruffling. IsdI variants with Trp-66 replaced with residues having less bulky aromatic and alkyl side chains were characterized with respect to catalytic activity, heme ruffling, and electrochemical properties. The heme degradation activity of the W66Y and W66F variants was approximately half that of the wild-type enzyme, whereas the W66L and W66A variants were inactive. A crystal structure and NMR spectroscopic analysis of the W66Y variant reveals that heme binds to this enzyme with less heme ruffling than observed for wild-type IsdI. The reduction potential of this variant (-96 ± 7 mV versus standard hydrogen electrode) is similar to that of wild-type IsdI (-89 ± 7 mV), so we attribute the diminished activity of this variant to the diminished heme ruffling observed for heme bound to this enzyme and conclude that Trp-66 is required for optimal catalytic activity.

Formation and characterization of five- and six-coordinate iron(III) corrolazine complexes

Electronic structures of five- and six-coordinate iron(III) corrolazine complexes are determined by means of 1H NMR, 13C NMR, EPR, and Mössbauer spectroscopy as well as SQUID magnetometry. A series of five-coordinate complexes, [FeIII(TBP8Cz)(L)]* where the axial ligands(L) are cyanide(CN-), imidazole(HIm), 1-methylimidazole(1-MeIm), 4-(N,N-dimethylamino)pyridine(DMAP), pyridine(Py), 4-cyanopyridine(4-CNPy), and tert-butylisocyanide(tBuNC), are obtained by the addition of 1 to 2 equiv. of the ligands to the dichloromethane solutions of FeIII(TBP8Cz) at 298 K: TBP8Cz is a trianion of 2,3,7,8,12,13,17,18-octakis(4-tert-butylphenyl)corrolazine. These complexes commonly show the S = 3/2 at 298 K. By contrast, formation of the six-coordinate complexes depends on the nature of the axial ligands. While the addition of 3 equiv. of CN- has completely converted FeIII(TBP8Cz) to (Bu4N)2[FeIII(TBP8Cz)(CN)2] at 298 K, the conversion to the bis-adduct is only attained below ca. 200 K in the case of HIm, 1-MeIm, and DMAP even in the presence of 50 equiv. of the ligands. If the axial ligand is Py, 4-CNPy, or tBuNC, the formation of [FeIII(TBP8Cz)(L)2] is confirmed only at an extremely low temperature (15 K). Close inspection of the 1H NMR and EPR spectra has revealed that all the bis-adducts adopt the (dxy)2(dxz, dyz)3 ground state. While FeIII(TBP8Cz) forms paramagnetic bis- and mono-adduct in toluene solution at 298 K in the presence of excess amount of CN- and tBuNC, respectively, the corresponding porphyrazine complex, [FeIII(TBP8Pz)]Cl , forms diamagnetic bis-CN and bis-tBuNC under the same conditions: TBP8Pz is a dianion of 2,3,7,8,12,13,17,18-octakis(4-tert-butylphenyl)-porphyrazine. Thus, the iron(III) ion of porphyrazine complex is more easily reduced than that of the corresponding corrolazine complex.

Revealing substituent effects on the electronic structure and planarity of Ni-porphyrins

Using density functional theory, we have studied the effects on structural and electronic consequences (including HOMO-LUMO energy gaps, vertical ionization potentials (IP_v), and vertical electron affinities (EA_v)) of the following two factors: (a) meso- and β-substituents acting as inductive donors (CH₃), inductive acceptors that are electron-donating through resonance (Br), inductive electron acceptors (CF₃), and resonance enabled acceptors (NO₂); and (b) complete replacement of pyrrole nitrogens with P-atoms. The principal results of the study are: (1) For the bare Ni-porphyrin, the solvents were found not to affect the HOMO-LUMO gaps but to change the IP_v and EA_v noticeably. (2) In the series CH₃ → Br → CF₃ → NO₂ the HOMO-LUMO energy gaps, IP_v, and EA_v increase for both meso- and β-substituents. The ruffling distortion of the porphyrin core is retained, and becomes stronger for the two acceptor groups. In general, effects of meso-substituents on the ruffling distortion of the porphyrin core is more pronounced. (3) Most significantly, complete replacement of pyrrole nitrogens in the NiP with phosphorus atoms produces the species, NiP(P)₄, with the structural and electronic features drastically different from the original NiP. This implies that NiP(P)₄ can possess interesting and unusual novel properties, including aromaticity and reactivity, leading to its various beneficial potential applications. Furthermore, NiP(P)₄ high stability both in the gas phase and different solvents was shown, implying the feasibility of its synthesis.

Electronic structure of five- and six-coordinate iron(III) tetraazaporphyrin complexes: pyrrole-Cαchemical shift as a useful probe

Electronic structure of a series of five-coordinate Fe ( OArTAzP ) X ( OAr = octaaryltetraazaporphyrin , X = Cl-, Br-, I-; Ar = 4-tert-butylphenyl) have been examined on the basis of1H NMR,13C NMR, and EPR spectroscopy as well as SQUID magnetometry. These complexes adopt the intermediate-spin state as in the case of analogous complexes reported by Fitzgerald et al. (Inorg. Chem. 1992; 31: 2006-2013) and Stuzhin et al. (Inorg. Chim. Acta 1995; 236: 131-139). The13C NMR studies using13C -enriched complexes at the pyrrole α positions have revealed that the pyrrole- Cαsignals appear at extraordinary upfield positions, i.e. -130 to -250 ppm at 273 K, due to the dz2-a2 uand dπ-3 eginteractions. The Curie plots of the pyrrole- Cαsignals have further revealed that the iodide complex adopts a much purer intermediate-spin state than the bromide and chloride complexes. In contrast to the case of Fe ( OArTAzP ) X , six-coordinate [ Fe ( OArTAzP )( CN )2]-showed the pyrrole- Cαsignal at 47 ppm at 273 K, which indicates that the complex adopts the low-spin state with the ( dxy)2( dxz, dyz)3electron configuration. Thus, the13C NMR chemical shift of the pyrrole- Cαsignal turns out to be quite a good probe to elucidate the spin state and electron configuration of iron(III) tetraazaporphyrins, where the1H NMR spectroscopy is less useful because of the absence of the hydrogen atoms as well as the alkyl or aryl groups directly attached to the meso positions.

Low-spin organoiron(III) N-confused pyriporphyrin

Oxidation of ( PyP H) Fe II Br , an iron(II) complex of 6,11,16,21-tetraaryl-3-aza-m-benziporphyrin ( N -confused pyriporphyrin, ( PyP H) H ) has been followed, in the presence of pyridine, by 1 H and 2 H NMR spectroscopy. One-electron oxidation with dioxygen, accompanied by deprotonation of a C (22) H fragment and formation of a Fe - C (22) bond, produced a low-spin, six-coordinate iron(III) complex [( PyP ) Fe III( py )2]+ as confirmed by combination of 1 H NMR, EPR and structural data. The characteristic patterns of 1 H NMR pyrrole and meso-aryl resonances resemble features assigned to the less common, low-spin ground electronic state (( d xz d yz )4( d xy )1) of iron(III) regular porphyrins. A conformational rearrangement process has been detected which involves two structures differentiated by macrocyclic ruffling. The structure of { H [( PyP ) Fe III( py )2]2}( Fe III Br 4)3· CH 2 Cl 2 has been determined by X-ray crystallography. The cationic complex involves a six-coordinate iron atom bound to the N -confused pyriporphyrin through its three nitrogens ( Fe - N (23) = 1.924(7), Fe - N (24) = 1.979(7), Fe - N (25) = 1.9343(7) Å) and the pirydyl trigonal C (22) atom ( Fe (1)- C (22) = 1.972(10) Å). The porphyrin is strongly ruffled, defining two deep grooves along C meso - C meso axes at right angles to each other. Two axial pyridine ligands are located in the prearranged equatorial ligand grooves. The iron lies in the N 3 C plane of the macrocycle defined by coordinating nitrogen and carbon atoms. In the solid, pairs of molecules are positioned along the line defined by Fe (1)- C (22) and Fe (2)- C (91) bonds. The structure demonstrates the head-to-head arrangement of two [( PyP ) Fe III( py )2]+ subunits revealing the adjacency of the two perimeter nitrogen atoms (the N (3)⋯ N (72) distance = 2.587(10) Å) linked by the N ⋯ H ⋯ N hydrogen bond.

Electronic properties of the highly ruffled heme bound to the heme degrading enzyme IsdI

IsdI, a heme-degrading protein from Staphylococcus aureus, binds heme in a manner that distorts the normally planar heme prosthetic group to an extent greater than that observed so far for any other heme-binding protein. To understand better the relationship between this distinct structural characteristic and the functional properties of IsdI, spectroscopic, electrochemical, and crystallographic results are reported that provide evidence that this heme ruffling is essential to the catalytic activity of the protein and eliminates the need for the water cluster in the distal heme pocket that is essential for the activity of classical heme oxygenases. The lack of heme orientational disorder in (1)H-NMR spectra of the protein argues that the catalytic formation of β- and δ-biliverdin in nearly equal yield results from the ability of the protein to attack opposite sides of the heme ring rather than from binding of the heme substrate in two alternative orientations.

A vanadium porphyrin with temperature-dependent phase transformation: synthesis, crystal structures, supramolecular motifs and properties

A vanadium porphyrin, V(O)TMeOPP (1; TMeOPP=5, 10, 15, 20-tetrakis(4-methoxyphenyl)-21 H, 23H-porphyrin), has been synthesized by solvothermal reactions and characterized by single-crystal X-ray diffractions at room temperature and low temperature to reveal two different structures 1R and 1L, respectively. Both 1R and 1L crystallized in the orthorhombic system, but their space groups were different: Pbca and Pca2(1) for 1R and 1L, respectively. The cell parameters of a, b, and c were different and the cell volume of 1R was larger than that of 1L by circa 200 Å(3). 1R and 1L were characteristic of an isolated motif with a five-coordinate vanadium(IV) ion and a saddle-distorted nonplanar porphyrin macrocycle. Molecules of 1R were interconnected through hydrogen-bonding interactions to yield a 3D framework; whilst for the low-temperature phase 1L, there were more hydrogen-bonding interactions that link the molecules to construct a more-complex 3D supramolecular network. In a solution of acetone, the title compound exhibited purple and green colors at room temperature and low temperature, respectively, which is unprecedented for vanadium porphyrins. The spectral data of UV/Vis, FT-IR, and MALDI-TOF-MS of 1R and 1L are reported together with the electrochemical data.

Structure:function relationships in molecular spin-crossover complexes

Spin-crossover compounds are becoming increasingly popular for device and sensor applications, and in soft materials, that make use of their switchable colour, paramagnetism and conductivity. The de novo design of new solid spin-crossover compounds with pre-defined switching properties is desirable for application purposes. This challenging problem of crystal engineering requires an understanding of how the temperature and cooperativity of a spin-transition are influenced by the structure of the bulk material. Towards that end, this critical review presents a survey of molecular spin-crossover compounds with good availability of crystallographic data. A picture is emerging that changes in molecular shape between the high- and low-spin states, and the ability of a lattice to accommodate such changes, can play an important role in determining the existence and the cooperativity of a thermal spin-transition in the solid state (198 references).

NMR and DFT investigation of heme ruffling: functional implications for cytochrome c

Out-of-plane (OOP) deformations of the heme cofactor are found in numerous heme-containing proteins and the type of deformation tends to be conserved within functionally related classes of heme proteins. We demonstrate correlations between the heme ruffling OOP deformation and the (13)C and (1)H nuclear magnetic resonance (NMR) hyperfine shifts of heme aided by density functional theory (DFT) calculations. The degree of ruffling in the heme cofactor of Hydrogenobacter thermophilus cytochrome c(552) has been modified by a single amino acid mutation in the second coordination sphere of the cofactor. The (13)C and (1)H resonances of the cofactor have been assigned using one- and two-dimensional NMR spectroscopy aided by selective (13)C-enrichment of the heme. DFT has been used to predict the NMR hyperfine shifts and electron paramagnetic resonance (EPR) g-tensor at several points along the ruffling deformation coordinate. The DFT-predicted NMR and EPR parameters agree with the experimental observations, confirming that an accurate theoretical model of the electronic structure and its response to ruffling has been established. As the degree of ruffling increases, the heme methyl (1)H resonances move upfield while the heme methyl and meso (13)C resonances move downfield. These changes are a consequence of altered overlap of the Fe 3d and porphyrin pi orbitals, which destabilizes all three occupied Fe 3d-based molecular orbitals and decreases the positive and negative spin density on the beta-pyrrole and meso carbons, respectively. Consequently, the heme ruffling deformation decreases the electronic coupling of the cofactor with external redox partners and lowers the reduction potential of heme.

Electronic structure of highly ruffled low-spin iron(III) porphyrinates with electron withdrawing heptafluoropropyl groups at the meso positions

Bis(pyridine)[meso-tetrakis(heptafluoropropyl)porphyrinato]iron(III), [Fe(THFPrP)Py(2)](+), was reported to be the low-spin complex that adopts the purest (d(xz), d(yz))(4)(d(xy))(1) ground state where the energy gap between the iron d(xy) and d(π)(d(xz), d(yz)) orbitals is larger than the corresponding energy gaps of any other complexes reported previously (Moore, K. T.; Fletcher, J. T.; Therien, M. J. J. Am. Chem. Soc. 1999, 121, 5196-5209). Although the highly ruffled porphyrin core expected for this complex contributes to the stabilization of the (d(xz), d(yz))(4)(d(xy))(1) ground state, the strongly electron withdrawing C(3)F(7) groups at the meso positions should stabilize the (d(xy))(2)(d(xz), d(yz))(3) ground state. Thus, we have reexamined the electronic structure of [Fe(THFPrP)Py(2)](+) by means of (1)H NMR, (19)F NMR, and electron paramagnetic resonance (EPR) spectroscopy. The CD(2)Cl(2) solution of [Fe(THFPrP)Py(2)](+) shows the pyrrole-H signal at -10.25 ppm (298 K) in (1)H NMR, the CF(2)(α) signal at -74.6 ppm (298 K) in (19)F NMR, and the large g(max) type signal at g = 3.16 (4.2 K) in the EPR. Thus, contrary to the previous report, the complex is unambiguously shown to adopt the (d(xy))(2)(d(xz), d(yz))(3) ground state. Comparison of the spectroscopic data of a series of [Fe(THFPrP)L(2)](+) with those of the corresponding meso-tetrapropylporphyrin complexes [Fe(TPrP)L(2)](+) with various axial ligands (L) has shown that the meso-C(3)F(7) groups stabilize the (d(xy))(2)(d(xz), d(yz))(3) ground state. Therefore, it is clear that the less common (d(xz), d(yz))(4)(d(xy))(1) ground state can be stabilized by the three major factors: (i) axial ligand with low-lying π* orbitals, (ii) ruffled porphyrin ring, and (iii) electron donating substituent at the meso position.

Mössbauer, NMR, geometric, and electronic properties in S = 3/2 iron porphyrins

Iron porphyrins with the intermediate spin S = 3/2 or admixed with S = 5/2 or 1/2 are models for a number of heme protein systems, including cytochromes c'. The (57)Fe Mossbauer quadrupole splittings and (1)H and (13)C NMR chemical shifts have been found to be useful probes of their electronic states. We present the results of the first successful quantum chemical calculations of the Mössbauer and NMR properties in various S = 3/2 iron porphyrin complexes, covering four-, five-, and six-coordinate states and three commonly seen porphyrin conformations: planar, ruffled, and saddled. Several interesting correlations among these useful experimental spectroscopic probes and geometric and electronic properties were discovered. These results should facilitate future investigations of related heme proteins and model systems.

Axial ligand coordination in sterically strained vanadyl porphyrins: synthesis, structure, and properties

A hitherto unknown family of six-coordinate vanadyl porphyrins of the sterically crowded, nonplanar 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetranitroporphyrin incorporating axial ligand L [where L is pyridine, tetrahydrofuran (THF), or methanol (MeOH)] has been isolated as VO(tn-OEP)(L) in the solid phase for the first time and also structurally characterized. The presence of four electron-withdrawing, bulky nitro groups at the meso positions of vanadyl octaethylporphyrins severely distorts the porphyrin macrocycles and significantly enhances the affinity for the axial ligands, where even weak sigma-donating ligands, such as MeOH, bind strongly enough to be isolable in the solid phase and that too under the offset effects of the macrocyclic distortions. Thus, the axial ligand affinity is influenced by both the electronic and conformational effect, which cannot be separated completely in this series. The solid-state magnetic measurements and their typical electron paramagnetic resonance (EPR) spectrum show the presence of a single, unpaired electron, consistent with V(IV) formulation. The VO stretching frequency for VO(tn-OEP) occurs as a sharp, strong peak at 1008 cm(-1), which is consistent with five-coordinate vanadyl porphyrins, while VO(tn-OEP)(L) displays a strong band at lower wavenumbers. The downshift in nu(VO) upon axial coordination increases with increasing donor strength of the axial ligands; for pyridine, the downshift is 30 cm(-1), while for THF and MeOH, the downshifts are nearly 18 cm(-1). X-ray structure determinations authenticate axial coordination in a purely saddle-distorted porphyrin macrocycle for all of the complexes reported here in which V-Np distances are significantly shorter, while the porphyrin cores have been expanded on axial ligand coordination. As a result, vanadium atoms are more inplane than in a five-coordinate species. The binding of L does not change the spin or metal oxidation states (V(IV), d(1)-system) of the complexes; therefore, the changes observed are truly the reflections of axial ligand coordination. Electrochemical data obtained from cyclic voltammetric studies reveal that the complexes are much easier to reduce (by approximately 1200 mV) but more difficult to oxidize (by approximately 500 mV) as compared to nearly planar VO(OEP). The complexes undergo two one-electron oxidations due to pi-cation radical and dication formation and three one-electron reductions. The first two reductions are because of pi-anion radical and dianion formation, while the third quasi-reversible reduction is assigned to a metal-centered process (V(IV) --> V(III)). These results can be useful for identifying the interaction of the vanadyl porphyrins with the biological targets in their reported involvement in potent insulinomimetic activity and in anti-HIV agents.

Ruffling of metalloporphyrins bound to IsdG and IsdI, two heme-degrading enzymes in Staphylococcus aureus

IsdG and IsdI are paralogous proteins that are intracellular components of a complex heme uptake system in Staphylococcus aureus. IsdG and IsdI were shown previously to reductively degrade hemin. Crystal structures of the apoproteins show that these proteins belong to a newly identified heme degradation family distinct from canonical eukaryotic and prokaryotic heme oxygenases. Here we report the crystal structures of an inactive N7A variant of IsdG in complex with Fe(3+)-protoporphyrin IX (IsdG-hemin) and of IsdI in complex with cobalt protoporphyrin IX (IsdI-CoPPIX) to 1.8 A or better resolution. These structures show that the metalloporphyrins are buried into similar deep clefts such that the propionic acids form salt bridges to two Arg residues. His(77) (IsdG) or His(76) (IsdI), a critical residue required for activity, is coordinated to the Fe(3+) or Co(3+) atoms, respectively. The bound porphyrin rings form extensive steric interactions in the binding cleft such that the rings are highly distorted from the plane. This distortion is best described as ruffled and places the beta- and delta-meso carbons proximal to the distal oxygen-binding site. In the IsdG-hemin structure, Fe(3+) is pentacoordinate, and the distal side is occluded by the side chain of Ile(55). However, in the structure of IsdI-CoPPIX, the distal side of the CoPPIX accommodates a chloride ion in a cavity formed through a conformational change in Ile(55). The chloride ion participates in a hydrogen bond to the side chain amide of Asn(6). Together the structures suggest a reaction mechanism in which a reactive peroxide intermediate proceeds with nucleophilic oxidation at the beta- or delta-meso carbon of the hemin.