Publications

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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.

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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 foam-like 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.

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.

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We have investigated the possibility of constructing nanoscale metallic vehicles powered by biological motors or flagella that are activated and powered by visible light. The vehicle's body is to be composed of the surfactant bilayer of a liposome coated with metallic nanoparticles or nanosheets grown together into a porous single crystal. The diameter of the rigid metal vesicles is from about 50 nm to microns. Illumination with visible light activates a photosynthetic system in the bilayer that can generate a pH gradient across the liposomal membrane. The proton gradient can fuel a molecular motor that is incorporated into the membrane. Some molecular motors require ATP to fuel active transport. The protein ATP synthase, when embedded in the membrane, will use the pH gradient across the membrane to produce ATP from ADP and inorganic phosphate. The nanoscale vehicle is thus composed of both natural biological components (ATPase, flagellum; actin-myosin, kinesin-microtubules) and biomimetic components (metal vehicle casing, photosynthetic membrane) as functional units. Only light and storable ADP, phosphate, water, and weak electron donor are required fuel components. These nano-vehicles are being constructed by self-assembly and photocatalytic and autocatalytic reactions. The nano-vehicles can potentially respond to chemical gradients and other factors such as light intensity and field gradients, in a manner similar to the way that magnetic bacteria navigate. The delivery package might include decision-making and guidance components, drugs or other biological and chemical agents, explosives, catalytic reactors, and structural materials. We expected in one year to be able only to assess the problems and major issues at each stage of construction of the vehicle and the likely success of fabricating viable nanovehicles with our biomimetic photocatalytic approach. Surprisingly, we have been able to demonstrate that metallized photosynthetic liposomes can indeed be made. We have completed the synthesis of metallized liposomes with photosynthetic function included and studied these structures by electron microscopy. Both platinum and palladium nanosheeting have been used to coat the micelles. The stability of the vehicles to mechanical stress and the solution environment is enhanced by the single-crystalline platinum or palladium coating on the vesicle. With analogous platinized micelles, it is possible to dry the vehicles and re-suspend them with full functionality. However, with the liposomes drying on a TEM grid may cause the platinized liposomes to collapse, although probably stay viable in solution. It remains to be shown whether a proton motive force across the metallized bilayer membrane can be generated and whether we will also be able to incorporate various functional capabilities including ATP synthesis and functional molecular motors. Future tasks to complete the nanovehicles would be the incorporation of ATP synthase into metallized liposomes and the incorporation of a molecular motor into metallized liposomes.

Our overall goal is to understand and develop a novel light-driven approach to the controlled growth of unique metal and semiconductor nanostructures and nanomaterials. In this photochemical process, bio-inspired porphyrin-based photocatalysts reduce metal salts in aqueous solutions at ambient temperatures to provide metal nucleation and growth centers. Photocatalyst molecules are pre-positioned at the nanoscale to control the location and morphology of the metal nanostructures grown. Self-assembly, chemical confinement, and molecular templating are some of the methods used for nanoscale positioning of the photocatalyst molecules. When exposed to light, the photocatalyst molecule repeatedly reduces metal ions from solution, leading to deposition and the synthesis of the new nanostructures and nanostructured materials. Studies of the photocatalytic growth process and the resulting nanostructures address a number of fundamental biological, chemical, and environmental issues and draw on the combined nanoscience characterization and multi-scale simulation capabilities of the new DOE Center for Integrated Nanotechnologies, the University of New Mexico, and Sandia National Laboratories. Our main goals are to elucidate the processes involved in the photocatalytic growth of metal nanomaterials and provide the scientific basis for controlled synthesis. The nanomaterials resulting from these studies have applications in nanoelectronics, photonics, sensors, catalysis, and micromechanical systems. The proposed nanoscience concentrates on three thematic research areas: (1) the creation of nanoscale structures for realizing novel phenomena and quantum control, (2) understanding nanoscale processes in the environment, and (3) the development and use of multi-scale, multi-phenomena theory and simulation. Our goals for FY03 have been to understand the role of photocatalysis in the synthesis of dendritic platinum nanostructures grown from aqueous surfactant solutions under ambient conditions. The research is expected to lead to highly nanoengineered materials for catalysis mediated by platinum, palladium, and potentially other catalytically important metals. The nanostructures made also have potential applications in nanoelectronics, nanophotonics, and nanomagnetic systems. We also expect to develop a fundamental understanding of the uses and limitations of biomimetic photocatalysis as a means of producing metal and semiconductor nanostructures and nanomaterials. The work has already led to a relationship with InfraSUR LLC, a small business that is developing our photocatalytic metal reduction processes for environmental remediation. This work also contributes to science education at a predominantly Hispanic and Native American university.

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Previous studies of 5,10,15,20-tetraarylporphyrins have shown that the barrier for meso aryl-porphyrin rotation (ΔG‡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 = 4H2+) 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) 1H NMR spectroscopy are used to examine the relationship between the arylporphyrin 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 = 4H2+ and saddle and/or ruffle conformations for M = Ni. VT NMR studies show that the effect of protonation in the TArOPPs is to increase ΔG‡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 = 4H2+. 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.

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This project set out to scientifically-tailor ''smart'' interfacial films and 3-D composite nanostructures to exhibit photochromic responses to specific, highly-localized chemical and/or mechanical stimuli, and to integrate them into optical microsystems. The project involved the design of functionalized chromophoric self-assembled materials that possessed intense and environmentally-sensitive optical properties (absorbance, fluorescence) enabling their use as detectors of specific stimuli and transducers when interfaced with optical probes. The conjugated polymer polydiacetylene (PDA) proved to be the most promising material in many respects, although it had some drawbacks concerning reversibility. Throughout his work we used multi-task scanning probes (AFM, NSOM), offering simultaneous optical and interfacial force capabilities, to actuate and characterize the PDA with localized and specific interactions for detailed characterization of physical mechanisms and parameters. In addition to forming high quality mono-, bi-, and tri-layers of PDA via Langmuir-Blodgett deposition, we were successful in using the diacetylene monomer precursor as a surfactant that directed the self-assembly of an ordered, mesostructured inorganic host matrix. Remarkably, the diacetylene was polymerized in the matrix, thus providing a PDA-silica composite. The inorganic matrix serves as a perm-selective barrier to chemical and biological agents and provides structural support for improved material durability in microsystems. Our original goal was to use the composite films as a direct interface with microscale devices as optical elements (e.g., intracavity mirrors, diffraction gratings), taking advantage of the very high sensitivity of device performance to real-time dielectric changes in the films. However, our optical physics colleagues (M. Crawford and S. Kemme) were unsuccessful in these efforts, mainly due to the poor optical quality of the composite films.

The structural changes in the heme macrocycle and substituents caused by binding of Ca2+ to the diheme cytochrome c peroxidase from Paracoccus pantotrophus were clarified by resonance Raman spectroscopy of the inactive fully oxidized form of the enzyme. The changes in the macrocycle vibrational modes are consistent with a Ca2+-dependent increase in the out-of-plane distortion of the low-potential heme, the proposed peroxidatic heme. Most of the increase in out-of-plane distortion occurs when the high-affinity site I is occupied, but a small further increase in distortion occurs when site II is also occupied by Ca2+ or Mg2+. This increase in the heme distortion explains the red shift in the Soret absorption band that occurs upon Ca2+ binding. Changes also occur in the low-frequency substituent modes of the heme, indicating that a structural change in the covalently attached fingerprint pentapeptide of the LP heme occurs upon Ca2+ binding to site I. These structural changes may lead to loss of the sixth ligand at the peroxidatic heme in the semireduced form of the enzyme and activation.

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Principal component analysis, cluster analysis, and various structural parameters have been used to differentiate the nonplanar deformations of cobalt(III) porphyrins. The results were compared with normal-coordinate structural decomposition analysis. Cobalt(III) porphyrins discussed in this paper do not undergo large wav, dom, and pro deformations and they were not considered in our analysis. The cis Cα-N-N-Cα, dihedral angle is the best structural measure of ruffling and it is the only structural parameter that does not overestimate the extent of ruffling due to the presence of saddling. The average distance between the Cβ carbons and the plane comprising the four nitrogens, the four meso carbons and the cobalt ion is the best structural measure of saddling. No structural parameters were found that could be used in principal component analysis to find PC's that quantified the nonplanar deformations in cobalt(III) porphyrins. Cluster analysis was able to separate the sad, ruf, and planar structures, however, the preparation and symmetry adaptation of all the structures was complicated and was no more informative than the use of some of the univarient structural parameters. The NSD deformations are related to the vibrational energies and motions of the macrocycle and are thus the preferred description, but the more easily obtained structural parameters are useful measures of the normal coordinate deformations whenever a full NSD analysis is not possible. © 2001 Plenum Publishing Corporation.

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The authors use scanning probe microscopy to actuate and characterize the nanoscale mechanochromism of polydiacetylene monolayer on atomically-flat silicon oxide substrates. They find explicit evidence that the irreversible blue-to-red transformation is caused by shear forces exerted normal to the polydiacetylene polymer backbone. The anisotropic probe-induced transformation is characterized by a significant change in the tilt orientation of the side chains with respect to the surface normal. They also describe a new technique, based on shear force microscopy, that allows them to image friction anisotropy of polydiacetylene monolayer independent of scan direction. Finally, they discuss preliminary molecular mechanics modeling and electronic structure calculations that allow them to understand the correlation of mechanochromism with bond-angle changes in the conjugated polymer backbone.

The authors have investigated the effects of heme rotational isomerism in sperm-whale carbonmonoxy myoglobin using computational techniques. Several molecular dynamics simulations have been performed for the two rotational isomers A and B, which are related by a 180{degree} 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 heroes in orientations A and B. The results suggest that the binding site prefers a particular macrocycle conformation, and a 180{degree} 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 ruf-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.

{sup 1}H NMR studies of the protonation of highly nonplanar porphyrins with strong acids reveal the presence of the previously elusive monocation, and show that its stability can be related to the amount of saddle distortion induced by protonation; the amount of saddle distortion for a porphyrin dication is also found to correlate well with the rate of intermolecular proton transfer.

A novel porphyrin-based receptor molecule for chiral amine ligands is described in which nonplanarity of the porphyrin macrocycle is used to orient the ligand and to enhance porphyrin-ligand interactions. The porphyrin macrocycle provides a versatile platform upon which to build elaborate superstructures, and this feature coupled with a rich and well-developed synthetic chemistry has led to the synthesis of many elegant models of heme protein active sites and numerous porphyrin-based receptor molecules. One design feature which is not usually considered in the design of porphyrin-based receptor molecules is nonplanarity of the porphyrin ring, although there are a few systems such as the pyridine sensitive Venus Flytrap and the chirality-memory molecule which illustrate that nonplanar porphyrin-based receptors can display unique and interesting behavior. Given the novel properties of these receptors and the continuing interest in the effects of nonplanarity on the properties of porphyrins the authors decided to investigate in more detail the potential applications of nonplanarity in the design of porphyrin-based receptors. Herein, they describe the design, synthesis, and characterization of a new kind of nonplanar porphyrin-based receptor molecule for chiral amines.

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).