103,890 research outputs found
Structure of a DNA duplex containing a site-specific Dewar isomer: Structural influence of the 3 -T center dot G base pair of the Dewar product.
No full textIn contrast to the pyrimidine (6-4)pyrimidone photoproduct [(6-4) adduct], its Dewar valence isomer (Dewar product) is low mutagenic and produces a broad range of mutations with a 42% replicating error frequency. In order to determine the origin of the mutagenic property of the Dewar product, we used experimental NMR restraints and molecular dynamics to determine the solution structure of a Dewar-lesion DNA decamer duplex, which contains a mismatched base pair between the 3'-T residue and an opposed G residue. The 3'-T of the Dewar lesion forms stable hydrogen bonds with the opposite G residue. The helical bending and unwinding angles of the DW/GA duplex, however, are much higher than those of the DW/AA duplex. The stable hydrogen bonding of the G15 residue does not increase the thermal stability of the overall helix. It also does not restore the distorted backbone conformation of the DNA helix that is caused by the forming of a Dewar lesion. These structural features implicate that no thermal stability, or conformational benefits of G over A opposite the 3'-T of the Dewar lesion, facilitate the preferential incorporation of an A. This is in accordance with the A rule during translesion replication and leads to the low frequent 3'-T-->C mutation at this site
The Dewar photoproduct of thymidylyl(3 -> 5 )-thymidine (Dewar product) exhibits mutagenic behavior in accordance with the "A rule"
No full textIn contrast to the highly mutagenic pyrimidine(6-4)pyrimidone photoproduct, its Dewar valence isomer (Dewar product) has low mutagenic potential and produces a broad range of mutations [LeClerc, J. E., Borden. A. & Lawrence, C. W. (1991) Proc. Natl. Acad. Sci. USA 88. 9685-9689]. To determine the origin of the mutagenic property of the Dewar product, we used experimental NMR restraints and molecular dynamics to determine the solution structure of a Dewar-lesion DNA decamer duplex. This DNA decamer duplex (DW/GA duplex) contains a mismatched base pair between the 3' T residue of the Dewar lesion (T6) and an opposed G residue (G15). The 3' T (T6) of the Dewar lesion formed stable hydrogen bonds with the opposing G15 residue. However, the helical bending and unwinding angles of the DW/GA duplex were much larger than those of a second duplex that contains the Dewar lesion and opposing A15 and A16 residues (DW/AA duplex). The DW/GA duplex showed poorer stacking interactions at the two bases of the Dewar product and at the adjacent A7.T14 base pair than did the DW/AA duplex. These structural features imply that no thermal stability or conformational benefit is obtained by incorporating a G instead of an A opposite the 3' T of the Dewar lesion. These properties may thus facilitate the preferential incorporation of an A in accordance with the A rule during translesion replication and lead to the low frequency of 3' T-->C mutations observed at this site
Revisiting the rearrangement of Dewar thiophenes
No full textIndexación: Scopus.The mechanism for the walk rearrangement in Dewar thiophenes has been clarified theoretically by studying the evolution of chemical bonds along the intrinsic reaction coordinates. Substituent effects on the overall mechanism are assessed by using combinations of the ring (R = H, CF3) and traveling (X = S, S = O, and CH2) groups. The origins of fluxionality in the S-oxide of perfluorotetramethyl Dewar thiophene are uncovered in this work. Dewar rearrangements are chemical processes that occur with a high degree of synchronicity. These changes are directly related to the activation energy.https://www.mdpi.com/1420-3049/25/2/28
Control Dewar Subcooler Heat Exchanger Calculations
No full textThe calculations done to size the control dewar subcooler were done to obtain a sufficient subcooler size based on some conservative assumptions. The final subcooler design proposed in the design report will work even better because (1) It has more tubing length, and (2) will have already subcooled liquid at the inlet due to the transfer line design. The subcooler design described in the 'Design Report of the 2 Tesla Superconducting Solenoid for the Fermilab D0 Detector Upgrade' is the final design proposed. A short description of this design follows. The subcooler is constructed of 0.50-inch OD copper tubing with 1.0-inch diameter fins. It has ten and one half spirals at a 11.375-inch centerline diameter to provide 31 feet of tubing length. The liquid helium supply for the solenoid flows through the subcooler and then is expanded through a J-T valve. The subcooler spirals are immersed in the return two phase helium process stream. The return stream is directed over the finned tubing by an annulus created by a 10-inch pipe inside a 12-inch pipe. The transfer line from the refrigerator to the control dewar is constructed such that the liquid helium supply tube is in the refrigerator return stream, thereby subcooling the liquid up to the point where the u-tubes connect the transfer line to the control dewar. The subcooler within the control dewar will remove the heat picked up in the helium supply u-tube/bayonets. The attached subcooler/heat exchanger calculations were done neglecting any subcooling in the transfer line. All heat picked up in the transfer line from the refrigerator storage dewar to the control dewar is absorbed by the supply stream. The subcooler was sized such that the two phase supply fluid is subcooled at 1.7 atm pressure and when expanded through a JT valve to 1.45 atm pressure it is at a saturated liquid state. The calculations apply during steady state operation and at a flow rate of 16 g/s. The analysis of the heat exchanger was broken into two parts relating to the heat transfer mode taking place. The first part is considered the condensing part in which the helium supply stream is changed from two phase fluid to one phase liquid. The second part is the subcooling part where the liquid temperature is lowered, i.e.. subcooled. A summary of the calculations and results appears on the next page. The raw calculations follow the summary
LN2 Dewar 42 Delivery Overfill Inlet Solution
No full textPressure vessels must be protected against overpressure scenarios. A scenario of particular concern is that from a high pressure LN2 pump, now standard on LN2 delivery trailers. A safety mechanism must be in place to prevent the overfilling, and subsequent overpressure from occurring because these pumps have a higher mass flow output than reasonably sized relief valves provide. The original solution to the problem was to close a valve on the fill line when a certain liquid level in the dewar is reached. The valve remains closed until the level drops below that threshold. The trigger level was about 13,000 gallons for the 20,000 gallon capacity dewar. The solution was in place from 1989 until present, 2004
Synthesis and aplication of Dewar benzenes
No full textDewar benzenes are valence isomers of benzenes, which can be prepared by their thermic or photochemical rearrangement.1,2,3 They posses interesting physical and chemical properties due to the presence of the strained bicyclic framework. I studied synthesis of para-substituted phenyl Dewar benzenes and consequently kinetics of their thermal rearrangement for new information about mechanism of this unclear reaction. I also showed synthesis of new substituted aromatic and polyaromatic compounds, in which the key intermediates were Dewar benzenes (Scheme 1). Their chemical and physical properties are also going to be discussed. Scheme 1 R R R Cl3Al R + Ar COOMe RR R R COOMe Ar DMSO Me Me Me Me C6H4-X COOMe Fe n CO2MeMe Me Me Me Me Me Me R R R R S Me MeO2C EtO2C R R R R 1. van Tamelen, E. E.;Pappas, S. P. J. Am. Chem. Soc. 1962, 84, 3789. 2. Koster, J. B.; Timmermans, G. J.; van Bekkum, H. Synthesis, 1971, 139. 3. Schäfer, W.; Hellmann, H. Angew. Chem. Int. Ed. 1967, 6, 518
The generation and trapping of a novel Dewar furan
No full text2,4,7-Trimethyl-7-aza-3-oxatricyclo[3.3.0.02,4]oct-1·(5)-ene-6,8-dione (7), a derivative of the rarely observed Dewar furan ring-system, has been prepared by Zn–TiCl4 debromination of dibromide (6) and trapped in in situ with furan.Ian G. Pitt, Richard A. Russell and Ronald N. Warrene
EXPERIMENTAL EVALUATION OF DEWAR VOLUME AND COLD FINGER SIZE IN A STIRLING CRYOCOOLER LIQUID AIR ENERGY STORAGE (LAES) SYSTEM
No full textThis paper uses an experimental approach to evaluate two design characteristics for a liquid air energy storage (LAES) and generation system as part of the verification and validation of system component design for a microgrid power system. The LAES subsystem evaluated utilized a Stirling engine–based cryocooler that employs a cold finger placed into Dewar, which allows the pumping of heat out of a Dewar. As the heat is pumped out, the air temperature in the Dewar cools to below the condensation point and the air in the Dewar liquifies and is stored in the Dewar. Using a design of experiments, the cold finger surface area and Dewar volume were evaluated to determine the criticality and significance of changing their dimensions on the total liquid air production mass and average liquid air production rate during the experiments. This analysis found that changing the surface area of the cryocooler cold finger was a statistically significant design characteristic that affected total liquid air production and average production rate while changing the volume of the Dewar was not statistically significant. Additional responses relative to the time when the first gram of liquid air was produced and the minimum cold tip temperature that the cryocooler was able to achieve provided additional insight into design characteristics that can be used to inform the engineer when making design tradeoffs for specific microgrid operational environments.Approved for public release. Distribution is unlimited.Lieutenant Colonel, United States Arm
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