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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Perturbation of the Monomer-Monomer Interfaces of the Benzoylformate Decarboxylase Tetramer

The X-ray structure of benzoylformate decarboxylase (BFDC) from Pseudomonas putida ATCC 12633 shows it to be a tetramer. This was believed to be typical of all thiamin diphosphate-dependent decarboxylases until recently when the structure of KdcA, a branched-chain 2-keto acid decarboxylase from Lactococcus lactis, showed it to be a homodimer. This lent credence to earlier unfolding experiments on pyruvate decarboxylase from Saccharomyces cerevisiae that indicated that it might be active as a dimer. To investigate this possibility in BFDC, we sought to shift the equilibrium toward dimer formation. Point mutations were made in the noncatalytic monomer–monomer interfaces, but these had a minimal effect on both tetramer formation and catalytic activity. Subsequently, the R141E/Y288A/A306F variant was shown by analytical ultracentrifugation to be partially dimeric. It was also found to be catalytically inactive. Further experiments revealed that just two mutations, R141E and A306F, were sufficient to markedly alter the dimer–tetramer equilibrium and to provide an ~450-fold decrease in k cat . Equilibrium denaturation studies suggested that the residual activity was possibly due to the presence of residual tetramer. The structures of the R141E and A306F variants, determined to <1.5 Å resolution, hinted that disruption of the monomer interfaces will be accompanied by movement of a loop containing Leu109 and Leu110. As these residues contribute to the hydrophobicity of the active site and the correct positioning of the substrate, it seems that tetramer formation may well be critical to the catalytic activity of BFDC.

Andrews, Forest H.↗
DOE OSTI2015

Manipulation of Guaiacyl and Syringyl Monomer Biosynthesis in an Arabidopsis Cinnamyl Alcohol Dehydrogenase Mutant Results in Atypical Lignin Biosynthesis and Modified Cell Wall Structure

Modifying lignin composition and structure is a key strategy to increase plant cell wall digestibility for biofuel production. Disruption of the genes encoding both cinnamyl alcohol dehydrogenases (CADs), including CADC and CADD, in Arabidopsis thaliana results in the atypical incorporation of hydroxycinnamaldehydes into lignin. Another strategy to change lignin composition is downregulation or overexpression of ferulate 5-hydroxylase (F5H), which results in lignins enriched in guaiacyl or syringyl units, respectively. In this work, we combined these approaches to generate plants enriched in coniferaldehyde-derived lignin units or lignins derived primarily from sinapaldehyde. The cadc cadd and ferulic acid hydroxylase1 (fah1) cadc cadd plants are similar in growth to wild-type plants even though their lignin compositions are drastically altered. In contrast, disruption of CAD in the F5H-overexpressing background results in dwarfism. The dwarfed phenotype observed in these plants does not appear to be related to collapsed xylem, a hallmark of many other lignin-deficient dwarf mutants. cadc cadd, fah1 cadc cadd, and cadd F5H-overexpressing plants have increased enzyme-catalyzed cell wall digestibility. Given that these CAD-deficient plants have similar total lignin contents and only differ in the amounts of hydroxycinnamaldehyde monomer incorporation, these results suggest that hydroxycinnamaldehyde content is a more important determinant of digestibility than lignin content.

09 BIOMASS FUELS↗
DOE OSTI2018

Silver Nanocube and Nanobar Growth via Anisotropic Monomer Addition and Particle Attachment Processes

Understanding the growth mechanism of noble metal nanocrystals during solution synthesis is of significant importance for shape and property control. However, much remains unknown about the growth pathways of metal nanoparticles due to lack of direct observation. Using an in-situ transmission electron microscopy technique, we directly observed Ag nanocube and nanobar growth in aqueous solution through both classical monomer-by-monomer addition and non-classical particle attachment processes. During the particle attachment process, Ag nanocubes and nanobars formed via both oriented and non-oriented attachment. Our calculations, along with dynamics of the observed attachment, showed that van der Waals force overcame hydrodynamic and friction forces and drove the particles toward each other. During classical growth, an anisotropic growth was also revealed, and the resulting unsymmetrical shape constituted an intermediate state for Ag nanocube growth. We hypothesized that the temporary symmetry breaking resulted from different growth rates on {001} surfaces due to a local surface concentration variation caused by the imbalance between the consumption of Ag+ near the surface and the diffusion of Ag+ from bulk to surface.

Xiao, Dongdong↗
DOE OSTI2014

Photoionization of cold gas phase coronene and its clusters: Autoionization resonances in monomer, dimer, and trimer and electronic structure of monomer cation

Polycyclic aromatic hydrocarbons (PAHs) are key species encountered in a large variety of environments such as the Interstellar Medium (ISM) and in combustion media. Their UV spectroscopy and photodynamics in neutral and cationic forms are important to investigate in order to learn about their structure, formation mechanisms, and reactivity. Here, we report an experimental photoelectron-photoion coincidence study of a prototypical PAH molecule, coronene, and its small clusters, in a molecular beam using the vacuum ultraviolet (VUV) photons provided by the SOLEIL synchrotron facility. Mass-selected high resolution threshold photoelectron (TPES) and total ion yield spectra were obtained and analyzed in detail. Intense series of autoionizing resonances have been characterized as originating from the monomer, dimer, and trimer neutral species, which may be used as spectral fingerprints for their detection in the ISM by VUV absorption spectroscopy. Finally, a full description of the electronic structure of the monomer cation was made and discussed in detail in relation to previous spectroscopic optical absorption data. Tentative vibrational assignments in the near-threshold TPES spectrum of the monomer have been made with the support of a theoretical approach based on density functional theory.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Flexible monomer-based covalent organic frameworks: design, structure and functions

The design and synthesis of flexible monomer-based covalent organic frameworks (COFs) have been considered a challenge. We describe the design and synthesis of a new flexible monomer-based COF, termed PPN-30 (PPN meaning porous polymer network), by the use of a flexible alkyl amine, 1,4-cyclohexanediamine, combined with 1,3,5-triformylbenzene. The structure and gas adsorption properties of PPN-30 were carefully investigated.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Electronic and Molecular Structures of the CeB 6 Monomer

The electronic and molecular structure of the CeB 6 molecular unit has been probed by anion PE spectroscopy and DFT calculations to gain insight into structural and electronic relaxation on edge and corner sites of this ionic material. While boron in bulk lanthanide hexaboride materials assumes octahedral B 6 3– units, the monomer assumes a less compact structure to delocalize the charge. Two competitive molecular structures were identified for the anion and neutral species, which include a boat-like structure and a planar or near-planar teardrop structure. Ce adopts different orbital occupancies in the two isomers; the boat-like structure has a 4f superconfiguration while the teardrop favors a 4f 6s occupancy. The B 6 ligand in these structures carries a charge of –4 and –3, respectively. The teardrop structure, which was calculated to be isoenergetic with the boat structure, was most consistent with the experimental spectrum. B 6 -local orbitals crowd the energy window between the Ce 4f and 6s (HOMO) orbitals. A low-lying transition from the B-based orbitals is observed slightly less than 1 eV above the ground state. Here, the results suggest that edge and corner conductivity involves stabilized, highly diffuse 6s orbitals or bands rather than the bulk-favored 5d band. High-spin and open-shell low-spin states were calculated to be very close in energy for both the anion and neutral, a characteristic that reflects how decoupled the 4f electron is from the B 6 2p- and Ce 6s-based molecular orbitals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Directing Boron–Phosphorus Bonds in Crystalline Solid: Oxidative Polymerization of P=B=P Monomers into 1D Chains

Over 20 years after the last inorganic ternary B–P compound was reported, Na 2 BP 2 , a new compound containing one-dimensional B–P polyanionic chains has been synthesized. Common high-temperature synthetic methods required for the direct reaction of boron and phosphorus negate the possible formation of metastable or low temperature phases. In this study, oxidative elimination was used to successfully condense 0D BP 2 3– anionic monomers found in a Na 3 BP 2 precursor into unique 1D BP 2 2– chains consisting of five-member B 2 P 3 rings connected by bridging P atoms in the crystal structure of Na 2 BP 2 . In conclusion, the synthesis was guided by in situ X-ray powder diffraction studies, which revealed the metastable nature of the products of oxidative elimination reactions. Na 2 BP 2 is predicted to be an electron balanced semiconductor which was confirmed by UV–vis spectroscopy with the experimentally determined band gap of 1.1 eV.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗