Protonation of Planar and Nonplanar Porphyrins: A Calorimetric and Computational Study.

Herein, we report the first calorimetric study of the protonation of planar and nonplanar free-base porphyrins: H2OETPP (strongly saddled by its substituents), H2T(tBu)P (strongly ruffled by its substituents), and the nominally planar porphyrins (npPs) H2OEP, H2TPP, H2T(nPe)P, and H2T(iPr)P. The observed enthalpies of protonation in solution (ΔHprotsoln) for formation of the dications in 1,1,2,2-tetrachloroethane with 2% trifluoroacetic acid are -45 ± 1 kcal mol-1 for the npPs, -52.0 kcal mol-1 for H2T(tBu)P, and -70.9 kcal mol-1 for H2OETPP. The corresponding enthalpies of protonation (ΔHDFT) obtained from DFT calculations (-27 ± 5, -42, and -63 kcal mol-1, respectively) reproduce this trend. The much more negative enthalpy of protonation seen for H2OETPP is consistent with this molecule being pre-deformed into the saddle structure favored by porphyrin dications. Except for OETPP, the calculated enthalpies of the first protonations (ΔH1) are significantly more positive than the enthalpies of the second protonations (ΔH2). In addition, the structural strain energies for the first protonations (ΔEst(1)) are also significantly more positive than ΔEst(2). According to the calculations, the monocations thus have higher proton affinities than the corresponding free-base porphyrins due to a structural strain effect, which is consistent with the generally elusive nature of the porphyrin monocation. The recent observations of monocations for free-base porphyrins with a high degree of saddling can be rationalized in terms of ΔH1 and ΔH2 being similar; so, the monocation is no longer an unstable intermediate.

Metalloporphines: Dimers and Trimers.

Procedures for the purification and subsequent crystallization of the slightly soluble four-coordinate metallporphines, the simplest possible porphyrin derivatives, are described. Crystals of the porphine derivatives of cobalt(II), copper(II), platinum(II), and two polymorphs of zinc(II) were obtained. Analysis of the crystal and molecular structures shows that all except the platinum(II) derivative form an unusual trimeric species in the solid state. The isomorphous cobalt(II), copper(II), and one zinc(II) polymorph pack in the unit cell to form dimers as well as the trimers. Interplanar spacings between porphine rings are similar in both the dimers and trimers and range between 3.24 and 3.37 Å. Porphine rings are strongly overlapped with lateral shifts between ring centers in both the dimers and trimers with values between 1.52 and 1.70 Å or in Category S as originally defined by Scheidt and Lee. Periodic trends in the M-Np bond distances parallel those observed previously for tetraphenyl- and octaethylporphyrin derivatives.

A thermodynamic perspective of the metastability of holey sheets: the role of curvature.

Producing nanostructures with high surface area that are stable is important to accomplish sustained use of catalytic materials in practical settings. Avoiding the processes of ripening and sintering that typically hinder stability has long been recognized as a significant challenge and much research is focused on addressing these issues. In this article, we investigate a Pt nanostructure-a holey nanosheet-that exhibits high surface area and stability. The findings from lattice gas simulations produce a stability diagram that relates a critical hole diameter to sheet thickness. The stability is now addressed from a thermodynamic point of view, and, in particular, the crucial role of curvature is considered. We find that the stability of certain sized holes is due to the near zero mean curvature of the surface of the holes and of the surrounding flat sheet. Molecular dynamics simulations of Pt (using an embedded atom potential) are reported for small nanoclusters and model holes in sheets to illustrate the strong effects of curvature on thermodynamic properties such as the lowering of melting and surface melting temperatures.

Nickel(II) chelatase variants directly evolved from murine ferrochelatase: porphyrin distortion and kinetic mechanism.

The heme biosynthetic pathway culminates with the ferrochelatase-catalyzed ferrous iron chelation into protoporphyrin IX to form protoheme. The catalytic mechanism of ferrochelatase has been proposed to involve the stabilization of a nonplanar porphyrin to present the pyrrole nitrogens to the metal ion substrate. Previously, we hypothesized that the ferrochelatase-induced nonplanar distortions of the porphyrin substrate impose selectivity for the divalent metal ion incorporated into the porphyrin ring and facilitate the release of the metalated porphyrin through its reduced affinity for the enzyme. Using resonance Raman spectroscopy, the structural properties of porphyrins bound to the active site of directly evolved Ni(2+)-chelatase variants are now examined with regard to the mode and extent of porphyrin deformation and related to the catalytic properties of the enzymes. The Ni(2+)-chelatase variants (S143T, F323L, and S143T/F323L), which were directly evolved to exhibit an enhanced Ni(2+)-chelatase activity over that of the parent wild-type ferrochelatase, induced a weaker saddling deformation of the porphyrin substrate. Steady-state kinetic parameters of the evolved variants for Ni(2+)- and Fe(2+)-chelatase activities increased compared to those of wild-type ferrochelatase. In particular, the reduced porphyrin saddling deformation correlated with increased catalytic efficiency toward the metal ion substrate (Ni(2+) or Fe(2+)). The results lead us to propose that the decrease in the induced protoporphyrin IX saddling mode is associated with a less stringent metal ion preference by ferrochelatase and a slower porphyrin chelation step.

Templated growth of platinum nanowheels using the inhomogeneous reaction environment of bicelles.

Novel platinum nanowheels were synthesized by the reduction of aqueous platinum complex with ascorbic acid in the presence of disk-like bicelles. The platinum nanowheels possess thickened centers and flared edges that are connected by dendritic platinum nanosheets. This structural complexity can be attributed to the inhomogeneous micro-environment of the templating bicelles consisting of a central bi-layer region and a high curvature rim. The formation mechanism of the nanowheels was investigated by imaging nanostructures at different stages of the reaction. The templating bicelles were also imaged by TEM with the aid of negative staining. The variation of reaction parameters including platinum concentration, temperature, and total concentration of surfactants (CTAB + FC7) led to other types of platinum nanostructures, such as circular dendritic nanosheets with a tunable diameter and rectangular dendritic nanosheets. Interestingly, under irradiation by a TEM electron beam, the dendritic nanosheet portion of the nanowheels transforms into a metastable holey sheet. In addition, the platinum nanowheels have an electrochemical active surface area comparable to that of ETEK platinum black and thus are expected to have potential applications in catalysis.

Chelatases: distort to select?

Chelatases catalyze the insertion of a specific metal ion into porphyrins, a key step in the synthesis of metalated tetrapyrroles that are essential for many cellular processes. Despite apparent common structural features among chelatases, no general reaction mechanism accounting for metal ion specificity has been established. We propose that chelatase-induced distortion of the porphyrin substrate not only enhances the reaction rate by decreasing the activation energy of the reaction but also modulates which divalent metal ion is incorporated into the porphyrin ring. We evaluate the recently recognized interaction between ferrochelatase and frataxin as a way to regulate iron delivery to ferrochelatase, and thus iron and heme metabolism. We postulate that the ferrochelatase-frataxin interaction controls the type of metal ion that is delivered to ferrochelatase.

Donor-acceptor biomorphs from the ionic self-assembly of porphyrins.

Microscale four-leaf clover-shaped structures are formed by self-assembly of anionic and cationic porphyrins. Depending on the metal complexed in the porphyrin macrocycle (Zn or Sn), the porphyrin cores are either electron donors or electron acceptors. All four combinations of these two metals in cationic tetra(N-ethanol-4-pyridinium)porphyrin and anionic tetra(sulfonatophenyl)porphyrin result in related cloverlike structures with similar crystalline packing indicated by X-ray diffraction patterns. The clover morphology transforms as the ionic strength and temperature of the self-assembly reaction are increased, but the structures maintain 4-fold symmetry. The ability to alter the electronic and photophysical properties of these solids (e.g., by altering the metals in the porphyrins) and to vary cooperative interactions between the porphyrin subunits raises the possibility of producing binary solids with tunable functionality. For example, we show that the clovers derived from anionic Zn porphyrins (electron donors) and cationic Sn porphyrins (electron acceptors) are photoconductors, but when the metals are reversed in the two porphyrins, the resulting clovers are insulators.

Molecular organization in self-assembled binary porphyrin nanotubes revealed by resonance Raman spectroscopy.

Porphyrin nanotubes were formed by the ionic self-assembly of tetrakis(4-sulfonatophenyl) porphyrin diacid (H(4)TPPS(4)(2-)) and Sn(IV) tetra(4-pyridyl) porphyrin (Sn(OH(-))(X)TPyP(4+/5+) [X = OH(-) or H(2)O]) at pH 2.0. As reported previously, the tubes are hollow as revealed by transmission electron microscopy, approximately 60 nm in diameter, and can be up to several micrometres long. The absorption spectrum of the porphyrin nanotubes presents monomer-like Soret bands, as well as two additional red-shifted bands characteristic of porphyrin J-aggregates (offset face-to-face stacks). To elucidate the origin of the J-aggregate bands and the internal interactions of the porphyrins, the resonance Raman spectra have been obtained for the porphyrin nanotubes with excitations near resonance with the Soret J-aggregate band and the monomer-like bands. The resonance Raman data reveal that the Sn porphyrins are not electronically coupled to the J-aggregates within the tubes, which are formed exclusively by H(4)TPPS(4)(2-). This suggests that the internal structure of the nanotubes has H(4)TPPS(4)(2-) in aggregates that are similar to the widely studied H(4)TPPS(4)(2-) self-aggregates and that are segregated from the Sn porphyrins. Possible internal structures of the nanotubes and mechanisms for their formation are discussed.

Cobalt-porphyrin catalyzed electrochemical reduction of carbon dioxide in water. 2. Mechanism from first principles.

We apply first principles computational techniques to analyze the two-electron, multistep, electrochemical reduction of CO(2) to CO in water using cobalt porphyrin as a catalyst. Density functional theory calculations with hybrid functionals and dielectric continuum solvation are used to determine the steps at which electrons are added. This information is corroborated with ab initio molecular dynamics simulations in an explicit aqueous environment which reveal the critical role of water in stabilizing a key intermediate formed by CO(2) bound to cobalt. By use of potential of mean force calculations, the intermediate is found to spontaneously accept a proton to form a carboxylate acid group at pH < 9.0, and the subsequent cleavage of a C-OH bond to form CO is exothermic and associated with a small free energy barrier. These predictions suggest that the proposed reaction mechanism is viable if electron transfer to the catalyst is sufficiently fast. The variation in cobalt ion charge and spin states during bond breaking, DFT+U treatment of cobalt 3d orbitals, and the need for computing electrochemical potentials are emphasized.

Evolution of dendritic platinum nanosheets into ripening-resistant holey sheets.

Under electron-beam irradiation, dendritic platinum nanosheets structurally evolve into metastable "holey" nanosheets. Monte Carlo simulations of this structural transformation agree well with electron microscope images detailing the ripening process. The experiments and simulations show that nanoscale holes of a critical size are persistent and give holey sheets their morphological stability and sustained high surface area. Platinum nanostructures composed of these holey nanosheets exhibit improved durability in electrocatalytic reactions due to their remarkable ripening resistance.

Calcium-dependent heme structure in the reduced forms of the bacterial cytochrome c peroxidase from Paracoccus pantotrophus.

This work reports for the first time a resonance Raman study of the mixed-valence and fully reduced forms of Paracoccus pantotrophus bacterial cytochrome c peroxidase. The spectra of the active mixed-valence enzyme show changes in the structure of the ferric peroxidatic heme compared to the fully oxidized enzyme; these differences are observed upon reduction of the electron-transferring heme and upon full occupancy of the calcium site. For the mixed-valence form in the absence of Ca(2+), the peroxidatic heme is six-coordinate and low-spin on the basis of the frequencies of the structure-sensitive Raman lines: the enzyme is inactive. With added Ca(2+), the peroxidatic heme is five-coordinate high-spin and active. The calcium-dependent spectral differences indicate little change in the conformation of the ferrous electron-transferring heme, but substantial changes in the conformation of the ferric peroxidatic heme. Structural changes associated with Ca(2+) binding are indicated by spectral differences in the structure-sensitive marker lines, the out-of-plane low-frequency macrocyclic modes, and the vibrations associated with the heme substituents of that heme. The Ca(2+)-dependent appearance of a strong gamma 15 saddling-symmetry mode for the mixed-valence form is consistent with a strong saddling deformation in the active peroxidatic heme, a feature seen in the Raman spectra of other peroxidases. For the fully reduced form in the presence of Ca(2+), the resonance Raman spectra show that the peroxidatic heme remains high-spin.

Synthesis of platinum nanowheels using a bicellar template.

Disk-like surfactant bicelles provide a unique meso-structured reaction environment for templating the wet-chemical reduction of platinum(II) salt by ascorbic acid to produce platinum nanowheels. The Pt wheels are 496 +/-55 nm in diameter and possess thickened centers and radial dendritic nanosheets (about 2-nm in thickness) culminating in flared dendritic rims. The structural features of the platinum wheels arise from confined growth of platinum within the bilayer that is also limited at edges of the bicelles. The size of CTAB/FC7 bicelles is observed to evolve with the addition of Pt(II) complex and ascorbic acid. Synthetic control is demonstrated by varying the reaction parameters including metal salt concentration, temperature, and total surfactant concentration. This study opens up opportunities for the use of other inhomogeneous soft templates for synthesizing metals, metal alloys, and possibly semiconductors with complex nanostructures.

Light-driven synthesis of hollow platinum nanospheres.

Hollow platinum nanospheres that are porous and have uniform shell thickness are prepared by templating platinum growth on polystyrene beads with an adsorbed porphyrin photocatalyst irradiated by visible light.

Synthesis of peptide-nanotube platinum-nanoparticle composites.

Nanotubes prepared by the self-assembly of D-Phe-D-Phe molecules are investigated by electron microscopy and Monte Carlo simulations; the nanotubes appear to be porous and are capable of forming novel peptide-nanotube platinum-nanoparticle composites.

Substituent-Induced Perturbation Symmetries and Distortions of meso-tert-Butylporphyrins.

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.

Axial Coordination and Conformational Heterogeneity of Nickel(II) Tetraphenylporphyrin Complexes with Nitrogenous Bases.

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.

Comparative Analysis of the Conformations of Symmetrically and Asymmetrically Deca- and Undecasubstituted Porphyrins Bearing Meso-Alkyl or -Aryl Groups.

Conformational analysis of highly substituted porphyrins has potential implications for modeling the behavior of macrocycles in tetrapyrrole-containing protein complexes and during catalytic reactions. In order to study the influence of different substituent patterns on the conformation of the porphyrin macrocycle, a series of metal free and nickel(II) decasubstituted porphyrins bearing aryl or ethyl groups at opposite meso positions and alkyl groups at the pyrrole positions have been synthesized and characterized by X-ray crystallography. Crystal structures of the free-base porphyrins with 5,15-diaryl substituents showed negligible out-of-plane distortion but a large amount of in-plane distortion along the 5,15-axis accompanied by large bond angle changes similar to those previously seen for related porphyrins with 5,15-dialkyl substituents. Nickel(II) complexes of the 5,15-diaryl-substituted porphyrins show planar or modestly nonplanar conformations, suggesting that these complexes are not intrinsically nonplanar, whereas a complex with 5,15-diethyl substituents has a very ruffled conformation similar to those observed for related complexes with other metals. The nickel(II) complexes are also elongated along the 5,15-axis in a qualitatively similar but less dramatic fashion than are the free-base porphyrins. Spectrosopic studies ((1)H NMR, optical, and resonance Raman spectroscopy) suggest that conformations similar to those determined by X-ray crystallography are present in solution for the 5,15-dialkyl- and 5,15-diaryl-substituted porphyrins. Several asymmetric nickel(II) and metal-free deca- and undecasubstituted porphyrins containing both aryl and alkyl meso-substituents were also investigated. Metal-free 5,15-disubstituted porphyrins with one aryl and one alkyl group showed considerably elongated porphyrin cores, whereas nickel(II) complexes of porphyrins with 5,10- or 5,10,15-substitution patterns showed very nonplanar structures consisting mainly of ruffle and saddle type distortions.

Synthesis and Electrochemical Studies of a Series of Fluorinated Dodecaphenylporphyrins.

Dodecaphenylporphyrins with varying degrees of fluorination of the peripheral phenyl rings (F(x)()DPPs) were synthesized as model compounds for studying electronic effects in nonplanar porphyrins, and detailed electrochemical studies of the chloroiron(III) complexes of these compounds were undertaken. The series of porphyrins, represented as FeDPPCl and as FeF(x)()DPPCl where x = 4, 8 (two isomers), 12, 20, 28, or 36, could be reversibly oxidized by two successive one-electron transfer steps in dichloromethane to give pi-cation radicals and pi-dications, respectively. All of the compounds investigated could also be reduced by three electrons in benzonitrile or pyridine. In benzonitrile, three reversible reductions were observed for the unfluorinated compound FeDPPCl, whereas the FeF(x)()DPPCl complexes generally exhibited irreversible first and second reductions which were coupled to chemical reactions. The chemical reaction associated with the first reduction involved a loss of the chloride ion after generation of [Fe(II)F(x)()DPPCl](-). The second chemical reaction involved a conversion between the initially generated Fe(II) porphyrin pi-anion radical and the final Fe(I) porphyrin reduction product. In pyridine, three reversible one-electron reductions were observed with the second reduction affording stable Fe(II) porphyrin pi-anion radicals for all of the complexes investigated.

Hierarchical cooperative binary ionic porphyrin nanocomposites.

Cooperative binary ionic (CBI) solids comprise a versatile new class of opto-electronic and catalytic materials consisting of ionically self-assembled pairs of organic anions and cations. Herein, we report CBI nanocomposites formed by growing nanoparticles of one type of porphyrin CBI solid onto a second porphyrin CBI substructure with complementary functionality.

Binary ionic porphyrin nanosheets: electronic and light-harvesting properties regulated by crystal structure.

Crystalline solids self-assembled from anionic and cationic porphyrins provide a new class of multifunctional optoelectronic micro- and nanomaterials. A 1 : 1 combination of zinc(II) tetra(4-sulfonatophenyl)porphyrin (ZnTPPS) and tin(IV) tetra(N-methyl-4-pyridiniumyl)porphyrin (SnTNMePyP) gives porphyrin nanosheets with high aspect ratios and varying thickness. The room temperature preparation of the nanosheets has provided the first X-ray crystal structure of a cooperative binary ionic (CBI) solid. The unit cell contains one and one-half molecules of aquo-ZnTPPS(4-) (an electron donor) and three half molecules of dihydroxy-SnTNMePyP(4+) (an electron acceptor). Charge balance in the solid is reached without any non-porphyrinic ions, as previously determined for other CBI nanomaterials by non-crystallographic means. The crystal structure reveals a complicated molecular arrangement with slipped π-π stacking only occurring in isolated dimers of one of the symmetrically unique zinc porphyrins. Consistent with the crystal structure, UV-visible J-aggregate bands indicative of exciton delocalization and extended π-π stacking are not observed. XRD measurements show that the structure of the Zn/Sn nanosheets is distinct from that of Zn/Sn four-leaf clover-like CBI solids reported previously. In contrast with the Zn/Sn clovers that do exhibit J-aggregate bands and are photoconductive, the nanosheets are not photoconductive. Even so, the nanosheets act as light-harvesting structures in an artificial photosynthesis system capable of reducing water to hydrogen but not as efficiently as the Zn/Sn clovers.