Publications

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

The processes and functional constituents of biological photosynthetic systems can be mimicked to produce a variety of functional nanostructures and nanodevices. The photosynthetic nanostructures produced are analogs of the naturally occurring photosynthetic systems and are composed of biomimetic compounds (e.g., porphyrins). For example, photocatalytic nanotubes can be made by ionic self-assembly of two oppositely charged porphyrins tectons [1]. These nanotubes mimic the light-harvesting and photosynthetic functions of biological systems like the chlorosomal rods and reaction centers of green sulfur bacteria. In addition, metal-composite nanodevices can be made by using the photocatalytic activity of the nanotubes to reduce aqueous metal salts to metal atoms, which are subsequently deposited onto tube surfaces [2]. In another approach, spatial localization of photocatalytic porphyrins within templating surfactant assemblies leads to controlled growth of novel dendritic metal nanostructures [3].

Functional organic nanostructures such as well-formed tubes or fibers that can easily be fabricated into electronic and photonic devices are needed in many applications. Especially desirable from a national security standpoint are nanostructures that have enhanced sensitivity for the detection of chemicals and biological (CB) agents and other environmental stimuli. We recently discovered the first class of highly responsive and adaptive porphyrin-based nanostructures that may satisfy these requirements. These novel porphyrin nanostructures, which are formed by ionic self-assembly of two oppositely charged porphyrins, may function as conductors, semiconductors, or photoconductors, and they have additional properties that make them suitable for device fabrication (e.g., as ultrasensitive colorimetric CB microsensors). Preliminary studies with porphyrin nanotubes have shown that these nanostructures have novel optical and electronic properties, including strong resonant light scattering, quenched fluorescence, and electrical conductivity. In addition, they are photochemically active and capable of light-harvesting and photosynthesis; they may also have nonlinear optical properties. Remarkably, the nanotubes and potentially other porphyrin nanostructure are mechanically responsive and adaptive (e.g., the rigidity of the micrometers-long nanotubes is altered by light, ultrasound, or chemicals) and they self-heal upon removal the environmental stimulus. Given the tremendous degree of structural variation possible in the porphyrin subunits, additional types of nanostructures and greater control over their morphology can be anticipated. Molecular modification also provides a means of controlling their electronic, photonic, and other functional properties. In this work, we have greatly broadened the range of ionic porphyrin nanostructures that can be made, and determined the optical and responsivity properties of the nanotubes and other porphyrin nanostructures. We have also explored means for controlling their morphology, size, and placement on surfaces. The research proposed will lay the groundwork for the use of these remarkable porphyrin nanostructures in micro- and nanoscale devices, by providing a more detailed understanding of their molecular structure and the factors that control their structural, photophysical, and chemical properties.

We have developed a new nanotagging technology for detecting and imaging the self-organization of proteins and other components of membranes at nanometer resolution for the purpose of investigating cell signaling and other membrane-mediated biological processes. We used protein-, lipid-, or drug-bound porphyrin photocatalysts to grow in-situ nanometer-sized metal particles, which reveal the location of the porphyrin-labeled molecules by electron microscopy. We initially used photocatalytic nanotagging to image assembled multi-component proteins and to monitor the distribution of lipids and porphyrin labels in liposomes. For example, by exchanging the heme molecules in hemoproteins with a photocatalytic tin porphyrin, a nanoparticle was grown at each heme site of the protein. The result obtained from electron microscopy for a tagged multi-subunit protein such as hemoglobin is a symmetric constellation of a specific number of nanoparticle tags, four in the case of the hemoglobin tetramer. Methods for covalently linking photocatalytic porphyrin labels to lipids and proteins were also developed to detect and image the self-organization of lipids, protein-protein supercomplexes, and membrane-protein complexes. Procedures for making photocatalytic porphyrin-drug, porphyrin-lipid, and porphyrin-protein hybrids for non-porphyrin-binding proteins and membrane components were pursued and the first porphyrin-labeled lipids was investigated in liposomal membrane models. Our photocatalytic nanotagging technique may ultimately allow membrane self-organization and cell signaling processes to be imaged in living cells. Fluorescence and plasmonic spectra of the tagged proteins might also provide additional information about protein association and membrane organization. In addition, a porphyrin-aspirin or other NSAID hybrid may be used to grow metal nanotags for the pharmacologically important COX enzymes in membranes so that the distribution of the protein can be imaged at the nanometer scale.

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}{sub 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.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Photocatalytic porphyrins are used to reduce metal complexes from aqueous solution and, further, to control the deposition of metals onto porphyrin nanotubes and surfactant assembly templates to produce metal composite nanostructures and nanodevices. For example, surfactant templates lead to spherical platinum dendrites and foam-like nanomaterials composed of dendritic platinum nanosheets. Porphyrin nanotubes are reported for the first time, and photocatalytic porphyrin nanotubes are shown to reduce metal complexes and deposit the metal selectively onto the inner or outer surface of the tubes, leading to nanotube-metal composite structures that are capable of hydrogen evolution and other nanodevices.

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. Copyright © 2004 American Chemical Society.