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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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Crystal structure of SgcJ, an NTF2-like superfamily protein involved in biosynthesis of the nine-membered enediyne antitumor antibiotic C-1027

Comparative analysis of the enediyne biosynthetic gene clusters revealed sets of conserved genes serving as outstanding candidates for the enediyne core. Here we report the crystal structures of SgcJ and its homologue NCS-Orf16, together with gene inactivation and site-directed mutagenesis studies, to gain insight into enediyne core biosynthesis. Gene inactivation in vivo establishes that SgcJ is required for C-1027 production in Streptomyces globisporus. SgcJ and NCS-Orf16 share a common structure with the nuclear transport factor 2-like superfamily of proteins, featuring a putative substrate binding or catalytic active site. Site-directed mutagenesis of the conserved residues lining this site allowed us to propose that SgcJ and its homologues may play a catalytic role in transforming the linear polyene intermediate, along with other enediyne polyketide synthase-associated enzymes, into an enzyme-sequestered enediyne core intermediate. In conclusion, these findings will help formulate hypotheses and design experiments to ascertain the function of SgcJ and its homologues in nine-membered enediyne core biosynthesis.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2023

Triepoxide formation by a flavin-dependent monooxygenase in monensin biosynthesis

Monensin A is a prototypical natural polyether polyketide antibiotic. It acts by binding a metal cation and facilitating its transport across the cell membrane. Biosynthesis of monensin A involves construction of a polyene polyketide backbone, subsequent epoxidation of the alkenes, and, lastly, formation of cyclic ethers via epoxide-opening cyclization. MonCI, a flavin-dependent monooxygenase, is thought to transform all three alkenes in the intermediate polyketide premonensin A into epoxides. Our crystallographic study has revealed that MonCI’s exquisite stereocontrol is due to the preorganization of the active site residues which allows only one specific face of the alkene to approach the reactive C(4a)-hydroperoxyflavin moiety. Furthermore, MonCI has an unusually large substrate-binding cavity that can accommodate premonensin A in an extended or folded conformation which allows any of the three alkenes to be placed next to C(4a)-hydroperoxyflavin. Importantly, MonCI, with its ability to perform multiple epoxidations on the same substrate in a stereospecific manner, demonstrates the extraordinary versatility of the flavin-dependent monooxygenase family of enzymes.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2017

Strategies to enhance the excitation energy-transfer efficiency in a light-harvesting system using the intra-molecular charge transfer character of carotenoids

Fucoxanthin is a carotenoid that is mainly found in light-harvesting complexes from brown algae and diatoms. Due to the presence of a carbonyl group attached to polyene chains in polar environments, excitation produces an excited intra-molecular charge transfer. This intra-molecular charge transfer state plays a key role in the highly efficient (~95%) energy-transfer from fucoxanthin to chlorophyll a in the light-harvesting complexes from brown algae. In purple bacterial light-harvesting systems the efficiency of excitation energy-transfer from carotenoids to bacteriochlorophylls depends on the extent of conjugation of the carotenoids. In this study we were successful, for the first time, in incorporating fucoxanthin into a light-harvesting complex 1 from the purple photosynthetic bacterium, Rhodospirillum rubrum G9+ (a carotenoidless strain). Femtosecond pump-probe spectroscopy was applied to this reconstituted light-harvesting complex in order to determine the efficiency of excitation energy-transfer from fucoxanthin to bacteriochlorophyll a when they are bound to the light-harvesting 1 apo-proteins.

solar (fuels), photosynthesis (natural and artific↗
DOE OSTI2017

Induced polarization restricts the conformational distribution of a light-harvesting molecular triad in the ground state

The light-harvesting molecular triad consisting of carotenoid polyene (C), diaryl-porphyrin (P) and pyrrole-fullerene (C60) is a donor–acceptor molecule capable of absorbing incident light in the visible range. Its ability to convert solar energy to electrical excitation and charge separation energy suggests a great potential in real-world applications. The ensemble of its conformations under ambient conditions varies widely according to its electronic state. In previous work, we applied a non-polarizable model to study the conformational distribution of the molecular triad in the ground and charge separated states. However, due to the lack of polarization, which imparts subtle changes in the charge distribution on atoms, molecular simulations fail to produce accurate average dipole moments. We developed the first polarizable model for a molecular triad to investigate the structural and dynamic properties of a molecular triad in the ground state in an explicit organic solvent, tetrahydrofuran (THF). We performed first-principles electronic structure calculations of the individual components in the triad as well as THF and then fit the partial atomic charges to the electrostatic potential using the i-RESP methodology. We validated these force field parameters by comparing the thermodynamic and dynamic properties obtained from molecular dynamics simulations with those from experiments. We enhanced the sampling of the triad conformations with replica exchange molecular dynamics simulations. We characterized the effects of induced polarization on the structural stability of the triad by analyzing the free energy landscapes constructed with polarizable force fields. Furthermore, by using principal component analysis, we found that the molecular triad conformations adopted a small range of torsional angles with induced polarization. The triad conformation solvated in polar solvent with a polarizable force field qualitatively agrees with that obtained from nuclear magnetic resonance spectroscopy.

Starovoytov, Oleg N.↗
DOE OSTI2014

SS{sub p}G: A strongly orthogonal geminal method with relaxed strong orthogonality

Strong orthogonality is an important constraint placed on geminal wavefunctions in order to make variational minimization tractable. However, strong orthogonality prevents certain, possibly important, excited configurations from contributing to the ground state description of chemical systems. The presented method lifts strong orthogonality constraint from geminal wavefunction by computing a perturbative-like correction to each geminal independently from the corrections to all other geminals. The method is applied to the Singlet-type Strongly orthogonal Geminals variant of the geminal wavefunction. Comparisons of this new SS{sub p}G method are made to the non-orthogonal AP1roG and the unconstrained Geminal Mean-Field Configuration Interaction method using small atomic and molecular systems. The correction is also compared to Density Matrix Renormalization Group calculations performed on long polyene chains in order to assess its scalability and applicability to large strongly correlated systems. The results of these comparisons demonstrate that although the perturbative correction is small, it may be a necessary first step in the systematic improvement of any strongly orthogonal geminal method.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Ab initio molecular orbital-configuration interaction based quantum master equation (MOQME) approach to the dynamic first hyperpolarizabilities of asymmetric π-conjugated systems

In this study, we apply the ab initio molecular orbital - configuration interaction based quantum master equation (MOQME) approach to the calculation and analysis of the dynamic first hyperpolarizabilities (β) of asymmetric π-conjugated molecules. In this approach, we construct the excited state models by the ab initio configuration interaction singles method. Then, time evolutions of system reduced density matrix ρ(t) and system polarization p(t) are calculated by the QME approach. Dynamic β in the second harmonic generation is calculated based on the nonperturbative definition of nonlinear optical susceptibility, using the frequency domain system polarization p(ω). Spatial contributions of electrons to β are analyzed based on the dynamic hyperpolarizability density map, which visualizes the second-order response of charge density oscillating with a frequency of 2ω. We apply the present method to the calculation of the dynamic β of a series of donor/acceptor substituted polyene oligomers, and then discuss the applicability of the MOQME method to the calculation and analysis of dynamic NLO properties of molecular systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Preparation of highly luminescent and color tunable carbon nanodots under visible light excitation for in vitro and in vivo bio-imaging

Carbon nanodots (CDs) have generated enormous excitement because of their superiority in water solubility, chemical inertness, low toxicity, ease of functionalization and resistance to photobleaching. Here we report a facile thermal pyrolysis route to prepare CDs with high quantum yield (QY) using citric acid as the carbon source and ethylene diamine derivatives (EDAs) including triethylenetetramine (TETA), tetraethylenepentamine (TEPA) and polyene polyamine (PEPA) as the passivation agents. We find that the CDs prepared from EDAs, such as TETA, TEPA and PEPA, show relatively high photoluminescence (PL) QY (11.4, 10.6, and 9.8%, respectively) at 1 ex of 465 nm. The cytotoxicity of the CDs has been investigated through in vitro and in vivo bio-imaging studies. The results indicate that these CDs possess low toxicity and good biocompatibility. As a result, the unique properties such as the high PL QY at large excitation wave length and the low toxicity of the resulting CDs make them promising fluorescent nanoprobes for applications in optical bio-imaging and biosensing.

36 MATERIALS SCIENCE↗
NASA NTRS1971

Polymerization of perfluorobutadiene at near-ambient conditions

Peroxide catalyst under mild conditions initiates homopolymerization of perfluoro butadiene to new linear perfluoro polyenes and vulcanizable fluoro elastomers. Resulting polyperfluoro butadiene serves as hard elastomer for good chemical resistance, as intermediate in graft polymerizations, and as crosslink for high molecular weight materials.

Toy, M. S.↗
NASA NTRS1975

Synthesis and evaluation of polymers for use in early warning fire alarm devices

Conjugated polyacetylene polymers and one condensation polyene, all containing a high degree of conjugated unsaturation, were synthesized. These polymers were characterized by chemical analysis and by thermogravimetric analysis, as well as for their film-forming capability and gas/polymer interactions. It was found that those that had a high degree of conjugated unsaturation and had resonance - stabilizing groups were very thermally stable to 200 C, e.g., poly(dicyanoacetylene), poly(ethynylferrocene) and poly(phenylacetylene); while those with labile moieties, such as poly(p-formamidophenylacetylene), among others, suffered some degradation when heated in air. When subjected to gas/polymer interaction effects, the greatest change in electrical conductance was observed when ammonia was used as the gas and poly(p-nitrophenylacetylene) was the detector. Other polymers showed similar behavior. For example, poly(ethynylcarborane), considered to be an electron acceptor also showed a change in electrical conductance when exposed to ammonia, while poly(ethynylpyridine) and poly(ethylidenepyridazine) responded to carbon monoxide. However, for "fire gases" (gases from smoldering cotton), poly(ethynylferrocene) was the most responsive. Thus, the concept of polymers with different electronegativities forming charge-transfer complexes with different gases was found to be operable.

Byrd, N. R.↗
NASA NTRS1975

Semiconducting polymers for gas detection

Conjugated polyenes, and polyesters containing phthalocyanine in their backbone, were synthesized. These polymers were characterized by chemical analysis, thermogravimetric analysis, spectral analysis, and X-ray diffraction studies for crystallinity, as well as for their film-forming capability and gas/polymer interactions. Most of the polymers were relatively insensitive to water vapor up to 50 percent relative humidity, but the polyester/phthalocyanine (iron) polymer was relatively insensitive up to 100 percent RH. On the other hand, poly(p-dimethylaminophenylacetylene) was too conductive at 100 percent RH. Of the gases tested, the only ones that gave any evidence of interacting with the polymers were SO2, NOx, HCN and NH3. Poly(imidazole)/thiophene responded to each of these gases at all relative humidities, while the other polymers gave varying response, depending upon the RH. Thus, since most of these gases were electron-accepting, the electron-donating character of poly(imidazole)/thiophene substantiates the concept of electronegativity being the operating principle for interaction effects. Of the six polymers prepared, poly(imidazole)/thiophene first showed a very good response to smoldering cotton, but it later became nonresponsive; presumably due to oxidation effects.

Byrd, N. R.↗
NASA NTRS1984

Structure and functions of fungal cell surfaces

A review with 24 references on the biochemistry, molecular structure, and function of cell surfaces of fungi, especially dermatophytes: the chemistry and structure of the cell wall, the effect of polyene antibiotics on the morphology and function of cytoplasmic membranes, and the chemical structure and function of pigments produced by various fungi are discussed.

Nozawa, Y.↗
NASA NTRS1984

Optical properties and photoexcitation of a novel liquid form of soluble polyacetylene

Optical absorption, fluorescence emission, Raman scattering, and time-resolved emission experiments are reported on a new liquid form of soluble polyacetylene. This liquid consists of separate polyene chains, grown on activated sites of a polybutadiene chain. The depolarization ratios as well as the time-resolved spectrum of the secondary emission were measured. The relevant spectral features are discussed and analyzed in terms of the lack of interchain interactions which characterize the polymer in solution.

Tubino, R.↗
NASA NTRS1987

Nonlinear susceptibilities of finite conjugated organic polymers

Tight-binding calculations of the length dependence of the third-order molecular hyperpolarizability for polyenes and polyynes are reported. The pi-electron wave functions were determined by exploiting the limited translational symmetry of the molecules. Perturbation theory was used to calculate the longitudinal component of the electronic nonresonant hyperpolarizability. This is the first two-'band' calculation of third-order hyperpolarizabilities on finite pi-electron systems of varying length. In contrast to the results of the one-'band' models, the hyperpolarizability densities increase rapidly and then, after about 10-15 repeating units, approach an asymptotic value.

Beratan, David N.↗
NASA NTRS1999

In Site Analysis of a High Temperature Cure Reaction in Real Time Using Modulated Fiber-Optic FT-Raman Spectroscopy

The vibrational spectrum of a high temperature (330 C) polymerization reaction was successfully monitored in real time using a modulated fiber-optic FT-Raman spectrometer. A phenylethynyl terminated monomer was cured, and spectral evidence for two different reaction products was acquired. The products are a conjugated polyene chain and a cyclized trimer. This is the first report describing the use of FT-Raman spectroscopy to monitor a high temperature (greater than 250 C) reaction in real time.

Cooper, John↗
NASA NTRS1999

In Situ Analysis of a High-Temperature Cure Reaction in Real Time Using Modulated Fiber-Optic FT-Raman Spectroscopy

The vibrational spectrum of a high-temperature (330 C) polymerization reaction was successfully monitored in real time with the use of a modulated fiber-optic Fourier transform (FT)-Raman spectrometer. A phenylethynyl-terminated monomer was cured, and spectral evidence for two different reaction products was acquired. The products are a conjugated polyene chain and a cyclized trimer. This is the first report describing the use of FT-Raman spectroscopy to monitor a high temperature (greater than 250 C) reaction in real time.

Aust, Jeffrey F.↗
NASA NTRS1993

Deriving the intermediate spectra and photocycle kinetics from time-resolved difference spectra of bacteriorhodopsin. The simpler case of the recombinant D96N protein

The bacteriorhodopsin photocycle contains more than five spectrally distinct intermediates, and the complexity of their interconversions has precluded a rigorous solution of the kinetics. A representation of the photocycle of mutated D96N bacteriorhodopsin near neutral pH was given earlier (Varo, G., and J. K. Lanyi. 1991. Biochemistry. 30:5008-5015) as BRhv-->K<==>L<==>M1-->M2--> BR. Here we have reduced a set of time-resolved difference spectra for this simpler system to three base spectra, each assumed to consist of an unknown mixture of the pure K, L, and M difference spectra represented by a 3 x 3 matrix of concentration values between 0 and 1. After generating all allowed sets of spectra for K, L, and M (i.e., M1 + M2) at a 1:50 resolution of the matrix elements, invalid solutions were eliminated progressively in a search based on what is expected, empirically and from the theory of polyene excited states, for rhodopsin spectra. Significantly, the average matrix values changed little after the first and simplest of the search criteria that disallowed negative absorptions and more than one maximum for the M intermediate. We conclude from the statistics that during the search the solutions strongly converged into a narrow region of the multidimensional space of the concentration matrix. The data at three temperatures between 5 and 25 degrees C yielded a single set of spectra for K, L, and M; their fits are consistent with the earlier derived photocycle model for the D96N protein.

NASA Program Exobiology↗