Synthesis of platinum nanocages by using liposomes containing photocatalyst molecules.
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Journal Article↗
Biomedical subjects
John A Shelnutt
Publications and source records attributed to John A Shelnutt.
Interfacial synthesis of dendritic platinum nanoshells templated on benzene nanodroplets stabilized in water by a photocatalytic lipoporphyrin.
Nanoscale metal shells have many potential uses and in some applications offer significant advantages over nanoparticles. The synthesis of platinum nanoshells using stabilized nanodroplets of benzene in water as growth templates is described; the nanodroplets are stabilized by a surfactant-like tin(IV)-porphyrin complex localized at the benzene-water interface. The porphyrin also acts as a photocatalyst that reduces the platinum complex and deposits metal onto the nanodroplets to form dendritic metal nanoshells. Below the solubility limit of benzene in water, the lipoporphyrin-stabilized nanodroplets have a reproducible number, size distribution, and surface area, which allows the thickness of the platinum shell walls to be controlled by changing the amount of platinum complex. Nanoscale platinum shells with magnetic interiors can be made by dispersing Fe3O4 nanoparticles in the benzene nanodroplets.
Journal Article↗
The conserved active-site loop residues of ferrochelatase induce porphyrin conformational changes necessary for catalysis.
Binding of porphyrin to murine ferrochelatase, the terminal enzyme of the heme biosynthetic pathway, is investigated by employing a set of variants harboring mutations in a putative porphyrin-binding loop. Using resonance Raman (RR) spectroscopy, the structural properties of the ferrochelatase-bound porphyrins are examined, especially with respect to the porphyrin deformation occurring in the environment of the active site. This deformation is thought to be a key step in the enzymatic insertion of ferrous iron into the porphyrin ring to make heme. Our previous RR spectroscopic studies of binding of porphyrin to murine ferrochelatase led us to propose that the wild-type enzyme induces porphyrin distortion even in the absence of the metal ion substrate. Here, we broaden this view by presenting evidence that the degree of a specific nonplanar porphyrin deformation contributes to the catalytic efficiency of ferrochelatase and its variants. The results also suggest that the conserved Trp256 (murine ferrochelatase numbering) is partially responsible for the observed porphyrin deformation. Binding of porphyrin to the ferrochelatase variants causes a decrease in the intensity of RR out-of-plane vibrational mode gamma(15), a saddling-like mode that is strong in the wild-type enzyme. In particular, the variant with a catalytic efficiency 1 order of magnitude lower than that of the wild-type enzyme is estimated to produce less than 30% of the wild-type saddling deformation. These results suggest that specific conserved loop residues (especially Trp256) are directly involved in the saddling of the porphyrin substrate.
Amino Acid Sequence↗
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.
Animals↗
Energetics and structural consequences of axial ligand coordination in nonplanar nickel porphyrins.
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.
Crystallography, X-Ray↗
Self-metallization of photocatalytic porphyrin nanotubes.
Porphyrin nanotubes represent a new class of nanostructures for which the molecular building blocks can be altered to control their structural and functional properties. Nanotubes containing tin(IV) porphyrins are photocatalytically active and can reduce metal ions from aqueous solution. The metal is deposited selectively onto tube surfaces, producing novel composite nanostructures that have potential applications as nanodevices. Two examples presented here are nanotubes with a continuous gold wire in the core and a gold ball at the end and nanotubes coated with platinum nanoparticles mainly on their outer surfaces. The latter are capable of photocatalytic reduction of water to hydrogen.
Journal Article↗
Porphyrin nanotubes by ionic self-assembly.
Porphyrin nanotubes are made by ionic self-assembly of two oppositely charged synthetic porphyrin molecules. The diameter of the nanotubes can be altered by modifying the structure of one of the porphyrin tectons. The nanotubes are photocatalytically active, mechanically responsive to light, and are composed of J-aggregates that exhibit resonance light scattering.
Metalloporphyrins↗
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.
Hydrophobic and Hydrophilic Interactions↗
Controlled synthesis of 2-D and 3-D dendritic platinum nanostructures.
Seeding and autocatalytic reduction of platinum salts in aqueous surfactant solution using ascorbic acid as the reductant leads to remarkable dendritic metal nanostructures. In micellar surfactant solutions, spherical dendritic metal nanostructures are obtained, and the smallest of these nanodendrites resemble assemblies of joined nanoparticles and the nanodendrites are single crystals. With liposomes as the template, dendritic platinum sheets in the form of thin circular disks or solid foamlike nanomaterials can be made. Synthetic control over the morphology of these nanodendrites, nanosheets, and nanostructured foams is realized by using a tin-porphyrin photocatalyst to conveniently and effectively produce a large initial population of catalytic growth centers. The concentration of seed particles determines the ultimate average size and uniformity of these novel two- and three-dimensional platinum nanostructures.
Journal Article↗
Unusual aryl-porphyrin rotational barriers in peripherally crowded porphyrins.
Previous studies of 5,10,15,20-tetraarylporphyrins have shown that the barrier for meso aryl-porphyrin rotation (DeltaG++(ROT)) varies as a function of the core substituent M and is lower for a small metal (M = Ni) compared to a large metal (M = Zn) and for a dication (M = 4H(2+)) versus a free base porphyrin (M = 2H). This has been attributed to changes in the nonplanar distortion of the porphyrin ring and the deformability of the macrocycle caused by the core substituent. In the present work, X-ray crystallography, molecular mechanics (MM) calculations, and variable temperature (VT) (1)H NMR spectroscopy are used to examine the relationship between the aryl-porphyrin rotational barrier and the core substituent M in some novel 2,3,5,7,8,10,12,13,15,17,18,20-dodecaarylporphyrins (DArPs), and specifically in some 5,10,15,20-tetraaryl-2,3,7,8,12,13,17,18-octaphenylporphyrins (TArOPPs), where steric crowding of the peripheral groups always results in a very nonplanar macrocycle. X-ray structures of DArPs indicate differences in the nonplanar conformation of the macrocycle as a function of M, with saddle conformations being observed for M = Zn, 2H or M = 4H(2+) and saddle and/or ruffle conformations for M = Ni. VT NMR studies show that the effect of protonation in the TArOPPs is to increase DeltaG++(ROT), which is the opposite of the effect seen for the TArPs, and MM calculations also predict a strikingly high barrier for the TArOPPs when M = 4H(2+). These and other findings suggest that the aryl-porphyrin rotational barriers in the DArPs are closely linked to the deformability of the macrocycle along a nonplanar distortion mode which moves the substituent being rotated out of the porphyrin plane.
Crystallography, X-Ray↗
Functional nanocomposites prepared by self-assembly and polymerization of diacetylene surfactants and silicic acid.
Conjugated polymer/silica nanocomposites with hexagonal, cubic, or lamellar mesoscopic order were synthesized by self-assembly using polymerizable amphiphilic diacetylene molecules as both structure-directing agents and monomers. The self-assembly procedure is rapid and incorporates the organic monomers uniformly within a highly ordered, inorganic environment. By tailoring the size of the oligo(ethylene glycol) headgroup of the diacetylene-containing surfactant, we varied the resulting self-assembled mesophases of the composite material. The nanostructured inorganic host altered the diacetylene polymerization behavior, and the resulting nanocomposites show unique thermo-, mechano-, and solvatochromic properties. Polymerization of the incorporated surfactants resulted in polydiacetylene (PDA)/silica nanocomposites that were optically transparent and mechanically robust. Molecular modeling and quantum calculations and (13)C spin-lattice relaxation times (T(1)) of the PDA/silica nanocomposites indicated that the surfactant monomers can be uniformly organized into precise spatial arrangements prior to polymerization. Nanoindentation and gas transport experiments showed that these nanocomposite films have increased hardness and reduced permeability as compared to pure PDA. Our work demonstrates polymerizable surfactant/silica self-assembly to be an efficient, general approach to the formation of nanostructured conjugated polymers. The nanostructured inorganic framework serves to protect, stabilize, and orient the polymer, mediate its performance, and provide sufficient mechanical and chemical stability to enable integration of conjugated polymers into devices and microsystems.
Acetylene↗
Origin of the red shifts in the optical absorption bands of nonplanar tetraalkylporphyrins.
The view that the large red shifts seen in the UV-visible absorption bands of peripherally crowded nonplanar porphyrins are the result of nonplanar deformations of the macrocycle has recently been challenged by the suggestion that the red shifts arise from substituent-induced changes in the macrocycle bond lengths and bond angles, termed in-plane nuclear reorganization (IPNR). We have analyzed the contributions to the UV-visible band shifts in a series of nickel or zinc meso-tetraalkylporphyrins to establish the origins of the red shifts in these ruffled porphyrins. Structures were obtained using a molecular mechanics force field optimized for porphyrins, and the nonplanar deformations were quantified by using normal-coordinate structural decomposition (NSD). Transition energies were calculated by the INDO/S semiempirical method. These computational studies demonstrate conclusively that the large Soret band red shifts ( approximately 40 nm) seen for very nonplanar meso-tetra(tert-butyl)porphyrin compared to meso-tetra(methyl)porphyrin are primarily the result of nonplanar deformations and not IPNR. Strikingly, nonplanar deformations along the high-frequency 2B(1u) and 3B(1u) normal coordinates of the macrocycle are shown to contribute significantly to the observed red shifts, even though these deformations are an order of magnitude smaller than the observed ruffling (1B(1u)) deformation. Other structural and electronic influences on the UV-visible band shifts are discussed and problems with the recent studies are examined (e.g., the systematic underestimation of the 2B(1u) and 3B(1u) modes in artificially constrained porphyrin structures that leads to a mistaken attribution of the red shift to IPNR). The effect of nonplanar deformations on the UV-visible absorption bands is then probed experimentally with a series of novel bridled nickel chiroporphyrins. In these compounds, the substituent effect is essentially invariant and the amount of nonplanar deformation decreases as the length of the straps connecting adjacent meso-cyclopropyl substituents decreases (the opposite of the effect observed for conventional strapped porphyrins). Several spectroscopic markers for nonplanarity (UV-visible bands, resonance Raman lines, and (1)H NMR resonances) are found to correlate with time-averaged deformations obtained from an NSD analysis of molecular dynamics snapshot structures. These results suggest that UV-visible band shifts of tetrapyrroles in proteins are potentially useful indicators of changes in nonplanarity provided other structural and electronic factors can be eliminated.
Journal Article↗
Influence of electronic and structural effects on the oxidative behavior of nickel porphyrins.
With the aim of better understanding the electronic and structural factors which govern electron-transfer processes in porphyrins, the electrochemistry of 29 nickel(II) porphyrins has been examined in dichloromethane containing either 0.1 M tetra-n-butylammonium perchlorate (TBAP) or tetra-n-butylammonium hexafluorophosphate (TBAPF(6)) as supporting electrolyte. Half-wave potentials for the first oxidation and first reduction are only weakly dependent on the supporting electrolyte, but E(1/2) for the second oxidation varies considerably with the type of supporting electrolyte. E(1/2) values for the first reduction to give a porphyrin pi-anion radical are effected in large part by the electronic properties of the porphyrin macrocycle substituents, while half-wave potentials for the first oxidation to give a pi-cation radical are affected by the substituents as well as by nonplanar deformations of the porphyrin macrocycle. The potential difference between the first and second oxidations (Delta/Ox(2) - Ox(1)/) is highly variable among the 29 investigated compounds and ranges from 0 mV (two overlapped oxidations) to 460 mV depending on the macrocycle substituents and the anion of the supporting electrolyte. The magnitude of Delta/Ox(2) - Ox(1)/ is generally smaller for compounds with very electron-withdrawing substituents and when TBAP is used as the supporting electrolyte. This behavior is best explained in terms of differences in the binding strengths of anions from the supporting electrolyte (ClO(4)(-) or PF(6)(-)) to the doubly oxidized species. A closer analysis suggests two factors which are important in modulating Delta/Ox(2) - Ox(1)/ and thus the binding affinity of the anion to the porphyrin dication. One is the type of pi-cation radical (a proxy for the charge distribution in the dication), and the other is the conformation of the porphyrin macrocycle (either planar or nonplanar). These findings imply that the redox behavior of porphyrins can be selectively tuned to display separate or overlapped oxidation processes.
Binding Sites↗
Binding of protoporphyrin IX and metal derivatives to the active site of wild-type mouse ferrochelatase at low porphyrin-to-protein ratios.
Resonance Raman (RR) spectroscopy is used to examine porphyrin substrate, product, and inhibitor interactions with the active site of murine ferrochelatase (EC 4.99.1.1), the terminal enzyme in the biosynthesis of heme. The enzyme catalyzes in vivo Fe(2+) chelation into protoporphyrin IX to give heme. The RR spectra of native ferrochelatase show that the protein, as isolated, contains varying amounts of endogenously bound high- or low-spin ferric heme, always at much less than 1 equiv. RR data on the binding of free-base protoporphyrin IX and its metalated complexes (Fe(III), Fe(II), and Ni(II)) to active wild-type protein were obtained at varying ratios of porphyrin to protein. The binding of ferric heme, a known inhibitor of the enzyme, leads to the formation of a low-spin six-coordinate adduct. Ferrous heme, the enzyme's natural product, binds in the ferrous high-spin five-coordinate state. Ni(II) protoporphyrin, a metalloporphyrin that has a low tendency toward axial ligation, becomes distorted when bound to ferrochelatase. Similarly for free-base protoporphyrin, the natural substrate of ferrochelatase, the RR spectra of porphyrin-protein complexes reveal a saddling distortion of the porphyrin. These results corroborate and extend our previous findings that porphyrin distortion, a crucial step of the catalytic mechanism, occurs even in the absence of bound metal substrate. Moreover, RR data reveal the presence of an amino acid residue in the active site of ferrochelatase which is capable of specific axial ligation to metals.
Animals↗
Heme distortions in sperm-whale carbonmonoxy myoglobin: correlations between rotational strengths and heme distortions in MD-generated structures.
We have investigated the effects of heme rotational isomerism in sperm-whale carbonmonoxymyoglobin using computational techniques. Several molecular dynamics simulations have been performed for the two rotational isomers A and B, which are related by a 180 degrees rotation around the alpha-gamma axis of the heme, of sperm-whale carbonmonoxy myoglobin in water. Both neutron diffraction and NMR structures were used as starting structures. In the absence of an experimental structure, the structure of isomer B was generated by rotating the heme in the structure of isomer A. Distortions of the heme from planarity were characterized by normal coordinate structural decomposition and by the angle of twist of the pyrrole rings from the heme plane. The heme distortions of the neutron diffraction structure were conserved in the MD trajectories, but in the NMR-based trajectories, where the heme distortions are less well defined, they differ from the original heme deformations. The protein matrix induced similar distortions on the hemes in orientations A and B. Our results suggest that the binding site prefers a particular macrocycle conformation, and a 180 degrees rotation of the heme does not significantly alter the protein's preference for this conformation. The intrinsic rotational strengths of the two Soret transitions, separated according to their polarization in the heme plane, show strong correlations with the ruffling deformation and the average twist angle of the pyrrole rings. The total rotational strength, which includes contributions from the chromophores in the protein, shows a weaker correlation with heme distortions.
Animals↗