Search

The effects of ruffling on the axial ligation properties of a series of nickel(II) tetra(alkyl)porphyrins have been investigated with UV-visible absorption spectroscopy, resonance Raman spectroscopy, X-ray crystallography, classical molecular mechanics calculations, and normal-coordinate structural decomposition analysis. For the modestly nonplanar porphyrins, porphyrin ruffling is found to cause a decrease in binding affinity for pyrrolidine and piperidine, mainly caused by a decrease in the binding constant for addition of the first axial ligand; ligand binding is completely inhibited for the more nonplanar porphyrins. The lowered affinity, resulting from the large energies required to expand the core and flatten the porphyrin to accommodate the large high-spin nickel(II) ion, has implications for nickel porphyrin-based molecular devices and the function of heme proteins and methyl-coenzyme M reductase.

Resonance Raman and UV-visible absorption spectra show that nickel(II) microperoxidase-11 (NiMP-11) is four-coordinate in aqueous solution in the pH range from 1.0 to 13.0. In aqueous solutions of NiMP-11 in the absence of cetyltrimethylammonium bromide (CTAB), NiMP-11 is aggregated. In CTAB micellar solutions, where aggregation of NiMP-11 does not occur, the Raman spectra of NiMP-11 are similar to that of nickel(II) cytochrome c (NiCyt-c). The presence of the peptide segment shifts the equilibrium heavily in favor of the nonplanar form, just as does the entire protein component in the case of NiCyt-c. This further elucidates the structural mechanism by which the protein segment ruffles the heme, most likely modulating the redox potential as indicated for the cytochromes c3 [Ma, J.-G., et al. (1998) Biochemistry 37, 12431-12442]. Furthermore, the hydrophobic environment that is provided by the CTAB micelle is found to be crucial to the native folding of the pentapeptide and formation of two hydrogen bonds in the peptide backbone. These two H-bonds act to contract the peptide segment exerting the force on the macrocycle that causes the ruffling and makes the redox potential more negative than if the heme were to remain planar. The structure of the heme and pentapeptide may also be associated with redox-linked triggering of the formation and release of cytochrome-protein complexes.

Resonance Raman (RR) spectroscopy, molecular mechanics (MM) calculations, and normal-coordinate structural decomposition (NSD) have been used to investigate the conformational differences in the hemes in ferricytochromes c3. NSD analyses of heme structures obtained from X-ray crystallography and MM calculations of heme-peptide fragments of the cytochromes c3 indicate that the nonplanarity of the hemes is largely controlled by a fingerprint peptide segment consisting of two heme-linked cysteines, the amino acids between the cysteines, and the proximal histidine ligand. Additional interactions between the heme and the distal histidine ligand and between the heme propionates and the protein also influence the heme conformation, but to a lesser extent than the fingerprint peptide segment. In addition, factors that influence the folding pattern of the fingerprint peptide segment may have an effect on the heme conformation. Large heme structural differences between the baculatum cytochromes c3 and the other proteins are uncovered by the NSD procedure [Jentzen, W., Ma, J.-G., and Shelnutt, J. A. (1998) Biophys. J. 74, 753-763]. These heme differences are mainly associated with the deletion of two residues in the covalently linked segment of hemes 4 for the baculatum proteins. Furthermore, some of these structural differences are reflected in the RR spectra. For example, the frequencies of the structure-sensitive lines (nu4, nu3, and nu2) in the high-frequency region of the RR spectra are lower for the Desulfomicrobium baculatum cytochromes c3 (Norway 4 and 9974) than for the Desulfovibrio (D.) gigas, D. vulgaris, and D. desulfuricans strains, consistent with a more ruffled heme. Spectral decompositions of the nu3 and nu10 lines allow the assignment of the sublines to individual hemes and show that ruffling, not saddling, is the dominant factor influencing the frequencies of the structure-sensitive Raman lines. The distinctive spectra of the baculatum strains investigated are a consequence of hemes 2 and 4 being more ruffled than is typical of the other proteins.

Axial ligation of nickel(II) 5,10,15,20-tetraphenylporphyrin (NiTPP) with pyrrolidine or piperidine has been investigated using X-ray crystallography, UV-visible spectroscopy, resonance Raman spectroscopy, and molecular mechanics (MM) calculations. Distinct v{sub 4} Raman lines are found for the 4-, 5-, and 6-coordinate species of NiTPP. The equilibrium constants for addition of the first and second pyrrolidine axial ligands are 1.1 and 3.8 M{sup {minus}1}, respectively. The differences in the calculated energies of the conformers having different ligand rotational angles are small so they may coexist in solution. Because of the similarity in macrocyclic structural parameters of these conformers and the free rotation of the axial ligands, narrow and symmetric v{sub 2} and v{sub 8} Raman lines are observed. Nonetheless, the normal-coordinate structural-decomposition analysis of the nonplanar distortions of the calculated structures and the crystal structure of the bis(piperidine) complex reveals a relationship between the orientations of axial ligand(s) and the macrocyclic distortions. For the 5-coordinate complex with the plane of the axial ligand bisecting the Ni-N{sub pyrrole} bonds, a primarily ruffled deformation results. With the ligand plane eclipsing the Ni-N{sub pyrrole} bonds, a mainly saddled deformation occurs. With the addition of the second axial ligand, the small doming of the 5-coordinate complexes disappears,more » and ruffling or saddling deformations change depending on the relative orientation of the two axial ligands. The crystal structure of the NiTPP bis(piperidine) complex shows a macrocycle distortion composed of wav(x) and wav(y) symmetric deformations, but no ruffling, saddling, or doming. The difference in the calculated and observed distortions results partly from the phenyl group orientation imposed by crystal packing forces. MM calculations predict three stable conformers (ruf, sad, and planar) for 4-coordinate NiTPP, and resonance Raman evidence for these conformers was given previously.« less

The influence of central metals of different sizes on the macrocyclic structure of a series of 5,15-disubstituted metalloporphyrins has been investigated with X-ray crystallography, molecular mecha...

Electron transfer photosensitized by a tin lipoporphyrin [Sn(IV) octakis((methoxycarbonyl-methyl)-meso-tetrakis-(((eicosanyloxy)carbonyl) phenyl)-porphyrin (SnLipoP)] is investigated under various solution conditions using a donor—SnLipoP—methylviologen (MV2+) ternary system, where the donor is triethanolamine (TEA) or ethylenediaminetetraacetic acid (EDTA). The photoreaction of SnLipoP is compared with the photoreactions sensitized by common Sn porphyrins like tin protoporphyrin IX (SnPP) and octaethylporphyrin (SnOEP). A constant photoreaction rate is observed in a water/organic solvent (hexane, benzene) two-phase system in which the porphyrin (SnLipoP, SnOEP) is in the organic solvent and MV2+ is in the aqueous phase. The rate is monitored by the change in the UV—visible absorption spectra produced by aqueous methylviologen radical MV·+. In contrast with the two-phase system, macroscopically homogeneous solutions (aqueous SnPP and micellar solutions of SnLipoP, SnPP and SnOEP) give pseudo-logarithmic rates. These electron-transfer processes are completely consistent with reductive primary electron transfer to the tin porphyrin and optical shielding effects. Differences in the rates for SnLipoP and the other Sn porphyrins are explained by structural differences in the porphyrins. In particular, the structure of the porphyrin influences the phase in which the porphyrin resides, its location relative to interfacial regions, and the way it interacts with itself and other system components.

The coexistence in solution of at least two conformers of (meso-tetraphenylporphinato)nickel(II) [Ni(TPP)] is inferred from solution and single-crystal resonance Raman spectra obtained at different temperatures (170 − 297 K) and excitation wavelengths (413.1 and 457.9 nm). The shapes of the structure-sensitive Raman lines ν8 and ν2 are clearly asymmetric and change with temperature. These broad lines can be decomposed into at least two sublines, a low-frequency (LF) and a high-frequency (HF) component. In contrast, the corresponding single-crystal Raman lines of the nonplanar structure of Ni(TPP) in the crystal are narrow and symmetric. For the line ν2, the broad LF subline results from nonplanar conformers and the narrow HF subline arises from a more planar conformer. This assignment is consistent with the observation that the LF subline of ν2 is more enhanced upon changing the excitation wavelength from 413.1 to 457.9 nm. The selective resonance enhancement is caused by the red shifts of the UV−visible ...

The ligand orientations in the nonplanar porphyrin complexes 1a–e and 2a–e have been investigated using molecular mechanics calculations and proton NMR spectroscopy. The minimum energy structures calculated for complexes 1a–e show that the planes of the axial pyridine or imidazole ligands are orientated approximately parallel to the CoIII–Nporphyrin bonds, with the ligand ring planes being perpendicular to each other. For complexes 2a–e, the planes of the axial ligands in the calculated minimum energy structures are orientated along the porphyrin meso carbon axis and the ligand ring planes are perpendicular to each other. Thus, for both series of complexes the planes of the axial ligands are orientated parallel to cavities formed by these very nonplanar porphyrins. Proton NMR studies suggest that structures similar to those obtained from the molecular mechanics calculations are retained in solution. In some complexes, hindered rotation of the axial ligands is also observed. Complexes 1a–e and 2a–e are unusual examples of the porphyrin conformation influencing the orientations of axial ligands and, as such, may be useful as models for studying ligand orientation effects in relation to biological systems.

The ring currents in the sterically congested and highly nonplanar porphyrin complexes 1a and 2a have been investigated using a double-dipole model of the porphyrin ring current effect. The equivalent dipoles needed to simulate the ring current in the saddle-shaped complex 1a were indistinguishable from those previously determined for the planar or nearly planar complex 3a. A 5% decrease in the equivalent dipoles was required to reproduce the ring current shifts in the ruffled complex 2a. The extremely large nonplanar conformational distortions seen for the porphyrin macrocycles in complexes 1a and 2a thus seem to cause little decrease in the porphyrin ring currents measured by this empirical model.

The influence of substituents with increasing steric demands on the structure of nickel(II) 5,15-disubstituted porphyrins has been investigated with X-ray crystallography, UV−visible absorption spectroscopy, resonance Raman spectroscopy, molecular energy optimization calculations, and INDO/s molecular orbital calculations. Nickel 5,15-diphenylporphyrin is predicted by molecular mechanics calculations to be a mixture of planar and nonplanar conformers. All of the nickel dialkylporphyrins (where the alkyl group is propyl, isopropyl, and tert-butyl) are calculated to be in a predominantly gabled (gab) conformation resulting from an αα orientation of the substituents with respect to the macrocycle. This nonplanar gab distortion is made up of a linear combination of distortions along the lowest-frequency out-of-plane macrocycle normal coordinates of A2u (doming) and B1u (ruffling) symmetry types. A higher energy stable αβ conformer is also predicted for dialkylporphyrins, and its nonplanar structure can be rep...

The ring currents in the sterically congested and highly nonplanar porphyrin complexes 1a and 2a have been investigated using a double-dipole model of the porphyrin ring current effect. The equivalent dipoles needed to simulate the ring current in the saddle-shaped complex 1a were indistinguishable from those previously determined for the planar or nearly planar complex 3a. A 5% decrease in the equivalent dipoles was required to reproduce the ring current shifts in the ruffled complex 2a. The extremely large nonplanar conformational distortions seen for the porphyrin macrocycles in complexes 1a and 2a thus seem to cause little decrease in the porphyrin ring currents measured by this empirical model.

The ligand orientations in the nonplanar porphyrin complexes 1a–e and 2a–e have been investigated using molecular mechanics calculations and proton NMR spectroscopy. The minimum energy structures calculated for complexes 1a–e show that the planes of the axial pyridine or imidazole ligands are orientated approximately parallel to the CoIII–Nporphyrin bonds, with the ligand ring planes being perpendicular to each other. For complexes 2a–e, the planes of the axial ligands in the calculated minimum energy structures are orientated along the porphyrin meso carbon axis and the ligand ring planes are perpendicular to each other. Thus, for both series of complexes the planes of the axial ligands are orientated parallel to cavities formed by these very nonplanar porphyrins. Proton NMR studies suggest that structures similar to those obtained from the molecular mechanics calculations are retained in solution. In some complexes, hindered rotation of the axial ligands is also observed. Complexes 1a–e and 2a–e are unusual examples of the porphyrin conformation influencing the orientations of axial ligands and, as such, may be useful as models for studying ligand orientation effects in relation to biological systems.

Nonplanar distortions of tetrapyrroles are prevalent in the hemes of hemoproteins, the pigments of photosynthetic proteins, and cofactor F430 of methylreductase. The nonplanarity of these porphyrin cofactors is currently believed to influence factors in the biological activity of the proteins, in part, because the porphyrin deformations are often conserved within functional classes of proteins. The occurrence, classification, and study of nonplanar porphyrins in proteins and synthetic nonplanar porphyrin analogs are reviewed.

The new perhalogenated porphyrin 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(trifluoromethyl)porphinato-nickel(II) exhibits several striking features, including an extremely ruffled macrocycle with a very short Ni-N distance, an unusually red-shifted optical spectrum, and, surprisingly, hindered rotation of the meso-trifluoromethyl substituents ({Delta}G{sub 278}{sup +} = 47 kJ/mol).

The chiral nonplanar porphyrin zinc(II) 3,7,8,12,13,17,18-heptabromo-2-(2-methoxyphenyl)-5,10,15,20-tetraphenylporphyrin has been synthesized and its properties investigated.

Axial ligation of nickel(II) 5,10,15,20-tetraphenylporphyrin (NiTPP) with pyrrolidine or piperidine has been investigated using X-ray crystallography, UV-visible spectroscopy, resonance Raman spectroscopy, and molecular mechanics (MM) calculations. By varying the pyrrolidine concentration in dichloromethane, distinct nu(4) Raman lines are found for the four-, five-, and six-coordinate species of NiTPP. The equilibrium constants for addition of the first and second pyrrolidine axial ligands are 1.1 and 3.8 M(-)(1), respectively. The axial ligands and their orientations influence the type and magnitude of the calculated nonplanar distortion. The differences in the calculated energies of the conformers having different ligand rotational angles are small so they may coexist in solution. Because of the similarity in macrocyclic structural parameters of these conformers and the free rotation of the axial ligands, narrow and symmetric nu(2) and nu(8) Raman lines are observed. Nonetheless, the normal-coordinate structural-decomposition analysis of the nonplanar distortions of the calculated structures and the crystal structure of the bis(piperidine) complex reveals a relationship between the orientations of axial ligand(s) and the macrocyclic distortions. For the five-coordinate complex with the plane of the axial ligand bisecting the Ni-N(pyrrole) bonds, a primarily ruffled deformation results. With the ligand plane eclipsing the Ni-N(pyrrole) bonds, a mainly saddled deformation occurs. With the addition of the second axial ligand, the small doming of the five-coordinate complexes disappears, and ruffling or saddling deformations change depending on the relative orientation of the two axial ligands. The crystal structure of the NiTPP bis(piperidine) complex shows a macrocycle distortion composed of wav(x) and wav(y) symmetric deformations, but no ruffling, saddling, or doming. The difference in the calculated and observed distortions results partly from the phenyl group orientation imposed by crystal packing forces. MM calculations predict three stable conformers (ruf, sad, and planar) for four-coordinate NiTPP, and resonance Raman evidence for these conformers was given previously.

The out-of-plane and in-plane distortions of a series of nickel(II) meso-substituted porphyrins with 0, 1, 2, or 4 tert-butyl groups [nickel(II) porphine (NiP), nickel(II) mono-tert-butylporphyrin (NiMtBuP), nickel(II) di-tert-butylporphyrin (NiDtBuP), and nickel(II) tetra-tert-butylporphyrin (NiTtBuP)] are investigated using molecular mechanics (MM) calculations, X-ray crystallography, UV-visible absorption spectroscopy, and resonance Raman spectroscopy. MM calculations are used to predict the stable conformations for this series of porphyrins. The out-of-plane distortions are then analyzed in terms of displacements along the normal coordinates of the porphyrin macrocycle using a new normal-coordinate structural decomposition method. As expected, the distortions are found to occur primarily along the lowest-frequency normal coordinate of each symmetry type and the distortions could be adequately simulated using only the lowest-frequency normal coordinates as a basis (the minimal basis). However, the distortions could be simulated significantly more accurately by extending the minimal basis by including the second-lowest-frequency normal coordinate of all symmetries. Using the extended basis is most important for the in-plane distortions. Detailed analysis of the types of distortion revealed that both the out-of-plane and the in-plane distortions depend on the perturbation symmetry of the peripheral substituents. The symmetry primarily depends on the pattern of substitution (number and positions of substituents) and the orientations of substituents. Often the perturbation symmetry can be predicted for a given porphyrin simply from the possible orientations of the substituents. Then, the main type(s) of symmetric deformation occurring for each possible molecular symmetry can be readily predicted from a D(4)(h)() correlation table. The stable conformers predicted by MM for the series of tert-butyl-substituted porphyrins confirm this simple but informative approach. Experimental verification of the calculated contributions of the symmetric deformations is provided by normal-coordinate structural decomposition of the available X-ray crystal structures of NiP, NiMtBuP, and NiDtBuP. The solid-state results are also supported by the resonance Raman and UV-visible absorption spectroscopic characterization of the porphyrins in solutions. The X-ray crystal structure of NiMtBuP is reported here for the first time.

The influence of the protein on the nonplanarity of the macrocycle for nickel(II)-reconstituted cytochrome c (NiCyt-c) has been investigated with pH-dependent resonance Raman and UV-visible absorption spectroscopy and molecular mechanics calculations. The spectra reveal that NiCyt-c near neutral pH has axially coordinated Ni, but below pH 3 and above pH 12, four-coordinate species predominate. The shape of the structure-sensitive Raman line nu10 of NiCyt-c is asymmetric and broad and it changes with pH. This broad line can be decomposed well into at least two sublines, a low-frequency line that results from a nonplanar conformer and a high-frequency line that arises from a nearly planar conformer. Upon lowering the pH from 3.0 to 1.0, the amount of the nonplanar conformer decreases relative to that of the planar conformer. The decreased nonplanarity can be accounted for in terms of the disruption of a hydrogen-bonding network in the peptide backbone upon lowering the pH. Molecular mechanics (MM) calculations on iron(III) and nickel(II) microperoxidase 5 (MP-5) as well as some model heme derivatives have been carried out in order to locate the part of the protein that causes the heme distortion observed in the X-ray crystal structures of cytochromes c. The energy-optimized structures of MP-5 and the model compounds were analyzed using the normal-coordinate structural decomposition method to specify and quantify the out-of-plane macrocyclic distortions. MM calculations for MP-5 show that two hydrogen bonds formed between the amide groups in the peptide backbone are important in maintaining the ruffled deformation of the macrocycle. All evidence presented supports the hypothesis that the nonplanar distortion of the porphyrin of cytochromes c is largely maintained by a relatively small protein segment including the cysteines, the amino acids between the cysteines, and the adjacent histidine ligand. Hydrogen bonding within the backbone of this segment is important in maintaining the conformation of the peptide that induces the porphyrin distortion.

The chiral nonplanar porphyrin zinc(II) 3,7,8,12,13,17,18-heptabromo-2-(2-methoxyphenyl)-5,10,15,20-tetraphenylporphyrin has been synthesized and its properties investigated.

The new perhalogenated porphyrin 2,3,7,8,12,13,17,18- octabromo-5,10,15,20-tetrakis(trifluoromethyl)porphinatonickel(II) exhibits several striking features, including an extremely ruffled macrocycle with a very short Ni–N distance, an unusually red-shifted optical spectrum, and, surprisingly, hindered rotation of the meso-trifluoromethyl substituents (ΔG‡278 = 47 kJ mol–1).

Reference EPFL-ARTICLE-203018doi:10.1021/ja972947fView record in Web of Science Record created on 2014-11-07, modified on 2017-11-27