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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS2000

A comparison of RNA with DNA in template-directed synthesis

Nonenzymatic template-directed copying of RNA sequences rich in cytidylic acid using nucleoside 5'-(2-methylimidazol-1-yl phosphates) as substrates is substantially more efficient than the copying of corresponding DNA sequences. However, many sequences cannot be copied, and the prospect of replication in this system is remote, even for RNA. Surprisingly, wobble-pairing leads to much more efficient incorporation of G opposite U on RNA templates than of G opposite T on DNA templates.

Non-NASA Center↗
NASA NTRS1999

Kinetics of template-directed pyrophosphate-linked dideoxyguanylate synthesis as a function of 2-MeImpdG and poly(C) concentration: insights into the mechanism

Aqueous solutions of deoxyguanosine 5'-monophosphate 2-methylimidazolide, 2-MeImpdG, yield primarily deoxyguanosine 5'-monophosphate, 5'dGMP, and pyrophosphate-linked dideoxyguanylate, dG5'ppdG, abbreviated G2p (see Chart 1). The initial rate of G2p formation, d[G2p]/dt in M h-1, determined at 23 degrees C, pH 7.8, 1.0 M NaCl and 0.2 M Mg2+ by timed high-performance liquid chromatography (HPLC) analysis, exhibits a second-order dependence on 2-MeImpdG concentration, [G]o, indicating a bimolecular mechanism of dimerization in the range 0.02 M < or = [G]o < or = 0.09 M. In the presence of polycytidylate, poly(C), G2p synthesis is accelerated and oligodeoxyguanylate products are formed by incorporation of 2-MeImpdG molecules. The kinetics of G2p formation as a function of both monomer and polymer concentration, expressed in C equivalents, were also determined under the above conditions and exhibited a complex behavior. Specifically, at a constant [poly(C)], values of d[G2p]/dt typically increased with [G]o with a parabolic upward curvature. At a constant [G]o, values of d[G2p]/dt increase with [poly(C)], but level off at the higher poly(C) concentrations. As [G]o increases this saturation occurs at a higher poly(C) concentration, a result opposite to expectation for a simple complexation of two reacting monomers with the catalyst prior to reaction. Nevertheless, these results are shown to be quantitatively consistent with a template-directed (TD) mechanism of dimerization where poly(C) acts as the template to bind 2-MeImpdG in a cooperative manner and lead, for the first time, to the formulation of principles that govern template-directed chemistry. Analysis of the kinetic data via a proposed TD cooperative model provides association constants for the affinity between polymer and monomer and the intrinsic reactivity of 2-MeImpdG toward pyrophosphate synthesis. To the best of our knowledge, poly(C)/2-MeImpdG is the first system that could serve as a textbook example of a TD reaction under conditions such that the template is fully saturated by monomers and under conditions that it is not.

NASA Discipline Exobiology↗
NASA NTRS1993

Kinetic dissection of individual steps in the poly(C)-directed oligoguanylate synthesis from guanosine 5'-monophosphate 2-methylimidazolide

A kinetic study of oligoguanylate synthesis on a polycytidylate template, poly(C), as a function of the concentration of the activated monomer, guanosine 5'-monophosphate 2-methylimidazolide, 2-MeImpG, is reported. Reactions were run with 0.005-0.045 M 2-MeImpG in the presence of 0.05 M poly(C) at 23 degrees C. The kinetic results are consistent with a reaction scheme (eq 1) that consists of a series of consecutive steps, each step representing the addition of one molecule of 2-MeImpG to the growing oligomer. This scheme allows the calculation of second-order rate constants for every step by analyzing the time-dependent growth of each oligomer. Computer simulations of the course of reaction based on the determined rate constants and eq 1 are in excellent agreement with the product distributions seen in the HPLC profiles. In accord with an earlier study (Fakhrai, H.; Inoue, T.; Orgel, L. E. Tetrahedron 1984, 40, 39), rate constants, ki, for the formation of the tetramer and longer oligomers up to the 16-mer were found to be independent of length and somewhat higher than k3 (formation of trimer), which in turn is much higher than k2 (formation of dimer). The ki (i > or = 4), k3, and k2 values are not true second-order rate constants but vary with monomer concentration. Mechanistic models for the dimerization (Scheme I) and elongation reactions (Scheme II) are proposed that are consistent with our results. These models take into account that the monomer associates with the template in a cooperative manner. Our kinetic analysis allowed the determination of rate constants for the elementary processes of covalent bond formation between two monomers (dimerization) and between an oligomer and a monomer (elongation) on the template. A major conclusion from our study is that bond formation between two monomer units or between a primer and a monomer is assisted by the presence of additional next-neighbor monomer units. This is consistent with recent findings with hairpin oligonucleotides (Wu, T.; Orgel, L. E. J. Am. Chem. Soc. 1992, 114, 317). Our study is the first of its kind that shows the feasibility of a thorough kinetic analysis of a template-directed oligomerization and provides a detailed mechanistic model of these reactions.

Non-NASA Center↗
NASA NTRS1995

Affinity of guanosine derivatives for polycytidylate revisited

Evidence is presented for complexation of guanosine 5'-monophosphate 2-methylimidazolide (2-MeImpG) with polycytidylate (poly(C)) at pH 8.0 and 23 degrees C in the presence of 1.0 M NaCl2 and 0.2 M MgCl2 in water. The association of 2-MeImpG with poly(C) was investigated using UV-vis spectroscopy as well as by monitoring the kinetics of the nucleophilic substitution reaction of the imidazole moiety by amines. The results of both methods are consistent with moderately strong poly(C) 2-MeImpG complexation and the spectrophotometric measurements allowed the construction of a binding isotherm with a concentration of 2-MeImpG equal to 5.55 +/- 0.15 mM at half occupancy. UV spectroscopy was employed to establish the binding of other guanosine derivatives on poly(C). These derivatives are guanosine 5'-monophosphate (5'GMP), guanosine 5'-monophosphate imidazolide (ImpG), and guanosine 5'-monophosphate morpholidate (morpG). Within experimental error these guanosine derivatives exhibit the same affinity for poly(C) as 2-MeImpG.

Non-NASA Center↗
NASA NTRS1993

Climbing Darwin's ladder

The work of Bartel and Szostak, in which RNA molecules were selected to enhance the ability to catalyze a reaction similar to a step in protein-catalyzed RNA replication, is discussed. An important aspect of this experiment was the ability to reach a high level of functional organization in ten evolutionary steps. Further steps necessary to obtain an RNA enzyme with RNA replicase activity include performing the reaction with mononucleoside 5'-triphosphates, generalizing the reaction to include a variety of sequences without loss of template-dependent specificity, and overcoming template self-structure that could prevent some regions from being copied efficiently.

NASA Discipline Exobiology↗
NASA NTRS1992

Nonenzymatic template-directed synthesis on hairpin oligonucleotides. 2. Templates containing cytidine and guanosine residues

We have prepared hairpin oligonucleotides in which a 5'-terminal single-stranded segment contains cytidylate (C) and guanylate (G) residues. When these hairpin substrates are incubated with a mixture of cytidine 5'-phosphoro(2-methly)imidazolide (2-MeImpC) and guanosine 5'-phosphoro(2-methyl)imidazolide (2-MeImpG), the 5'-terminal segment acts as a template to facilitate sequence-specific addition of G and C residues to the 3'-terminus of the hairpin. If an isolated G residue is present at the 3'-end of the template strand, it is copied regiospecifically in the presence of 2-MeImpC and 2-MeImpG to give a product containing an isolated C residue linked to its G neighbors by 3'-5'-internucleotide bonds. However, if only 2-MeImpC is present in the reaction mixture, very little reaction occurs. Thus, the presence of 2-MeImpG catalyzes the incorporation of C. If the template strand contains a short sequence of G residues, it is copied in the presence of a mixture of 2-MeImpC and 2-MeImpG. If only 2-MeImpC is present in the reaction mixture, efficient synthesis occurs to give a final product containing one fewer C residue than the number of G residues in the template.

NASA Program Exobiology↗
NASA NTRS1996

Rapid Inspection of Aerospace Structures - Is It Autonomous Yet?

The trend to increase the usage of aging aircraft added a great deal of urgency to the ongoing need for low-cost, rapid, simple-to-operate, reliable and efficient NDE methods for detection and characterization of flaws in aircraft structures. In many cases, the problem of inspection is complex due to the limitation of current technology and the need to disassemble aircraft structures and testing them in lab conditions. To overcome these limitations, reliable field inspection tools are being developed for rapid NDE of large and complex-shape structures, that can operate at harsh, hostal and remote conditions with minimum human interface. In recent years, to address the need for rapid inspection in field conditions, numerous portable scanners were developed using NDE methods, including ultrasonics, shearography, thermography. This paper is written with emphasis on ultrasonic NDE scanners, their evolution and the expected direction of growth.

Crawling System autonomous robotics↗
NASA NTRS2007

Post-Newtonian Black-Hole Inspiral Initial Data with Waves for Puncture Simulations

We present post-Newtonian-inspired initial data for non-spinning black-hole binaries, suitable for numerical evolution with punctures. We revisit the work of Tichey et al. and explicitly calculate remaining integral terms.Thereby we improve the accuracy in the far zone, by including realistic gravitational waves in the initial data. We investigate the behavior of this data both at the center of mass and in the far zone, demonstrating agreement of the transverse-traceless parts of the new metric with quadrupole-approximation waveforms. An advantage of these data is that they can be used for numerical evolutions to make a direct connection between the merger waveforms and the post-Newtonian inspiral waveforms.

Kelly, B. J.↗
NASA NTRS2012

Joint Analysis of X-Ray and Sunyaev-Zel'Dovich Observations of Galaxy Clusters Using an Analytic Model of the Intracluster Medium

We perform a joint analysis of X-ray and Sunyaev-Zel'dovich effect data using an analytic model that describes the gas properties of galaxy clusters. The joint analysis allows the measurement of the cluster gas mass fraction profile and Hubble constant independent of cosmological parameters. Weak cosmological priors are used to calculate the overdensity radius within which the gas mass fractions are reported. Such an analysis can provide direct constraints on the evolution of the cluster gas mass fraction with redshift. We validate the model and the joint analysis on high signal-to-noise data from the Chandra X-ray Observatory and the Sunyaev-Zel'dovich Array for two clusters, A2631 and A2204.

Galaxies↗
NASA NTRS2014

Cross-Scale Observational Signatures of Magnetic Reconnection

Magnetic reconnection is a significant mechanism for energy release across many astrophysical applications. In the solar atmosphere, reconnection is considered a primary contributor of flare evolution and coronal heating. Directly observing reconnection occurring in the solar atmosphere, however, is not trivial considering that the scale size of the diffusion region is magnitudes smaller than the observational capabilities of current instrumentation, and coronal magnetic field measurements are not currently sufficient to capture the process. Meanwhile, reconnection occurring in the Earth's magnetosphere transfers energy from the solar wind through a comparable process, although on vastly different scales. Magnetospheric measurements are made in situ rather than remotely; ergo, comparison of observations between the two regimes allows for potentially significant insight into reconnection as a stochastic and possibly turbulent process. We will present a set of observations from long-duration solar events and compare them to in situ measurements from the magnetosphere.

Savage, Sabrina↗
NASA NTRS2018

Lightweight, Durable, and Multifunctional Electrical Insulation Material Systems for High Voltage Applications

Newly developed multilayer structures of well-known polymer insulation materials significantly improved dielectric breakdown voltage, VB, or dielectric strength, K, if well-bonded, when compared to those of single material insulations or the commercial SOA systems, such as Teflon-Kapton-Teflon (TKT), at the same overall thickness. To date, the greatest improvement of the new structures from a few candidate materials, including various types of Kapton PIs and PFA or PET as bond layer (BL), was about 61% higher than that of the Kapton PI alone films, 40.1 vs. 24.9 kV, which was translated to 86.3% decrease in insulation thickness, thus significant volume and weight reduction of the final system. However, it was of interest to note that most improvements of the multilayer structures occurred at thicker overall thicknesses, above ~ 0.15 mm. Extensive analyses also showed that K of the multilayer structures increased with (i) decreasing individual layer thickness regardless of material type, (ii) increasing total accumulated thickness of PI or overall PI/BL ratio, and (iii) increasing number of interface or total number of layers, but only above the aforementioned overall thickness limit. Increases in VB of the multilayer structures were directly correlated with damage evolution and failure mode. With further material-design-process optimizations of the multilayer structures, it was expected to achieve other multifunctionalities, such as high partial discharge (PD) resistance, improved durability, EMI shielding, and high thermal dissipation in addition to high dielectric strength. These new structures can be used in various high voltage and high temperature applications, such as future hybrid or all electric aircraft wiring and power transmission as well as many other non-aerospace high power cables, electronic parts and components, printed circuit board, and so forth. The multilayer insulation system can be easily processed and manufactured with various conductor types via calendaring, compression-molding, stamping, laminating, vacuum-bagging and autoclaving, or 3D printing, even for complex 3-D components. Based on their unique structural configurations and potential capabilities, the new insulation system was identified as micro-multilayer multifunctional electrical insulation (MMEI). Patent application of the MMEI concept and current design configurations was filed for a 1-year provisional application (OAI-58834, Serial No.: 62/659,234), pending conversion to a U.S. utility application. This paper presents details of the MMEI structures, their dielectric performance analyses, potential mechanisms, and commercial scaleup feasibility assessment.

Electric aircrafts↗
NASA NTRS2018

Lightweight, Durable, and Multifunctional Electrical Insulation Material Systems for High Voltage Applications

Newly developed multilayer structures of well-known polymer insulation materials significantly improved dielectric breakdown voltage, VB, or dielectric strength, K, if well-bonded, when compared to those of single material insulations or the commercial SOA systems, such as Teflon-Kapton-Teflon (TKT), at the same overall thickness. To date, the greatest improvement of the new structures from a few candidate materials, including various types of Kapton PIs and PFA or PET as bond layer (BL), was about 61% higher than that of the Kapton PI alone films, 40.1 vs. 24.9 kV, which was translated to 86.3% decrease in insulation thickness, thus significant volume and weight reduction of the final system. However, it was of interest to note that most improvements of the multilayer structures occurred at thicker overall thicknesses, above ~ 0.15 mm. Extensive analyses also showed that K of the multilayer structures increased with (i) decreasing individual layer thickness regardless of material type, (ii) increasing total accumulated thickness of PI or overall PI/BL ratio, and (iii) increasing number of interface or total number of layers, but only above the aforementioned overall thickness limit. Increases in VB of the multilayer structures were directly correlated with damage evolution and failure mode. With further material-design-process optimizations of the multilayer structures, it was expected to achieve other multifunctionalities, such as high partial discharge (PD) resistance, improved durability, EMI shielding, and high thermal dissipation in addition to high dielectric strength. These new structures can be used in various high voltage and high temperature applications, such as future hybrid or all electric aircraft wiring and power transmission as well as many other non-aerospace high power cables, electronic parts and components, printed circuit board, and so forth. The multilayer insulation system can be easily processed and manufactured with various conductor types via calendaring, compression-molding, stamping, laminating, vacuum-bagging and autoclaving, or 3D printing, even for complex 3-D components. Based on their unique structural configurations and potential capabilities, the new insulation system was identified as micro-multilayer multifunctional electrical insulation (MMEI). Patent application of the MMEI concept and current design configurations was filed for a 1-year provisional application (OAI-58834, Serial No.: 62/659,234), pending conversion to a U.S. utility application. This paper presents details of the MMEI structures, their dielectric performance analyses, potential mechanisms, and commercial scaleup feasibility assessment.

Multifunctional↗
NASA NTRS1995

The Evolution of Plane Wakes Subjected to Irrotational Strains

Three direct numerical simulations of time-evolving turbulent plane wakes developing in the presence of various irrotational plane strains have been generated. A pseudospectral numerical method with up to 26 million modes is used to solve the equations in a reference frame moving with the irrotational strain. The initial condition for each simulation is taken from a previous turbulent self-similar plane wake direct numerical simulation at a velocity deficit Reynolds number, Re, of about 2000. Three different plane strains (of the same magnitude) are imposed. In the first two simulations the strain is in a plane normal to the streamwise wake direction (the two cases having strain of opposite sign); in the third the wake is compressed in the streamwise direction and stretched in the inhomogeneous cross-stream direction. The two flows that are stretched in the cross-stream direction experience an exponential increase of Re; flow visualization indicates many small-scale vortices with little or no organized large-scale structure. In the flow that is compressed in the cross-stream direction Re decays exponentially and the layer appears to be relaminarizing. The evolution of several turbulence statistics in each of these flows is examined.

Rogers, M. R.↗
NASA NTRS1985

Contamination of the fetch-limited directional wave spectrum by waves emanating from an embayment

The Surface Contour Radar (SCR) was used to map the evolution of the fetch-limited directional wave spectrum (DWS) off the eastern seaboard. Flight lines were displaced both north and south of the Delaware Bay following the passing of a weather front. The near shore DWS was found to be dominated by waves emanating from the Delaware Bay for distances of at least 85 km up the coastline and out to sea for 200 km from the mouth of the bay.

Walsh, E. J.↗
NASA NTRS1996

The Effect of Strain Rate on the Evolution of Plane Wakes Subjected to Irrotational Strains

Direct numerical simulations of time-evolving turbulent plane wakes developing in the presence of irrotational plane strain applied at three different strain rates have been generated. The strain geometry is such that the flow is compressed in the streamwise direction and expanded in the cross-stream direction with the spanwise direction being unstrained. This geometry is the temporally evolving analogue of a spatially evolving wake in an adverse pressure gradient. A pseudospectral numerical method with up to 16 million modes is used to solve the equations in a reference frame moving with the irrotational strain. The initial condition for each simulation is taken from a previous turbulent self-similar plane wake direct numerical simulation at a velocity deficit Reynolds number, Re, of about 2,000. Although the evolutions of many statistics are nearly collapsed when plotted against total strain, there are some differences owing to the different strain rate histories. The impact of strain-rate on the wake spreading rate, the peak velocity deficit, the Reynolds stress profiles, and the flow structure is examined.

Rogers, Michael M.↗
NASA NTRS1983

Contamination of the fetch-limited directional wave spectrum by waves emanating from an embayment

The Surface Contour Radar (SCR) has been used to map the evolution of the fetch-limited directional wave spectrum (DWS) off the eastern seaboard. Flight lines were displaced both north and south of the Delaware Bay following the passing of a weather front. The near shore DWS was found to be dominated by waves emanating from the Delaware Bay for distances of at least 85 km up to the coastline and out to sea for 200 km from the mouth of the bay.

Walsh, E. J.↗