1,041 research outputs found
Stereoselective Fluorescence Quenching in the Electron Transfer Photooxidation of Nucleobase-Related Azetidines by Cyanoaromatics
[EN] Electron transfer involving nucleic acids and their derivatives is an important field in bioorganic chemistry, specifically in connection with its role in the photo-driven DNA damage and repair. Four-membered ring heterocyclic oxetanes and azetidines have been claimed to be the intermediates involved in the repair of DNA (6-4) photoproduct by photolyase. In this context, we examine here the redox properties of the two azetidine isomers obtained from photocycloaddition between 6-aza-1,3-dimethyluracil and cyclohexene. Steady-state and time-resolved fluorescence experiments using a series of photoreductants and photooxidants have been run to evaluate the efficiency of the electron transfer process. Analysis of the obtained quenching kinetics shows that the azetidine compounds can act as electron donors. Additionally, it appears that the cis isomer is more easily oxidized than its trans counterpart. This result is in agreement with electrochemical studies performed on both azetidine derivatives.Spanish Government (CTQ2015-70164-P, RIRAAF RETICS RD12/0013/0009, Red de Fotoquimica Biologica CTQ2015-71896-REDT, Severo Ochoa program/SEV-2012-0267 and SVP-2013-068057 for A. B. F.-R. grant) and Generalitat Valenciana (Prometeo II/2013/005) are gratefully acknowledged.Fraga-Timiraos, AB.; RodrĂguez Muñiz, GM.; Peiro-Penalba, V.; Miranda Alonso, MĂ.; Lhiaubet, VL. (2016). Stereoselective Fluorescence Quenching in the Electron Transfer Photooxidation of Nucleobase-Related Azetidines by Cyanoaromatics. Molecules. 21(12). https://doi.org/10.3390/molecules21121683S2112Arnold, A. R., Grodick, M. A., & Barton, J. K. (2016). DNA Charge Transport: from Chemical Principles to the Cell. Cell Chemical Biology, 23(1), 183-197. doi:10.1016/j.chembiol.2015.11.010Jia, C., Ma, B., Xin, N., & Guo, X. (2015). Carbon ElectrodeâMolecule Junctions: A Reliable Platform for Molecular Electronics. Accounts of Chemical Research, 48(9), 2565-2575. doi:10.1021/acs.accounts.5b00133Beratan, D. N., Liu, C., Migliore, A., Polizzi, N. F., Skourtis, S. S., Zhang, P., & Zhang, Y. (2014). Charge Transfer in Dynamical Biosystems, or The Treachery of (Static) Images. Accounts of Chemical Research, 48(2), 474-481. doi:10.1021/ar500271dKawai, K., & Majima, T. (2013). Hole Transfer Kinetics of DNA. Accounts of Chemical Research, 46(11), 2616-2625. doi:10.1021/ar400079sSancar, A. (2003). Structure and Function of DNA Photolyase and Cryptochrome Blue-Light Photoreceptors. Chemical Reviews, 103(6), 2203-2238. doi:10.1021/cr0204348Kanvah, S., Joseph, J., Schuster, G. B., Barnett, R. N., Cleveland, C. L., & Landman, U. (2010). Oxidation of DNA: Damage to Nucleobases. Accounts of Chemical Research, 43(2), 280-287. doi:10.1021/ar900175aKelley, S. O. (1999). Electron Transfer Between Bases in Double Helical DNA. Science, 283(5400), 375-381. doi:10.1126/science.283.5400.375Breeger, S., von Meltzer, M., Hennecke, U., & Carell, T. (2006). Investigation of the Pathways of Excess Electron Transfer in DNA with Flavin-Donor and Oxetane-Acceptor Modified DNA Hairpins. Chemistry - A European Journal, 12(25), 6469-6477. doi:10.1002/chem.200600074Boussicault, F., & Robert, M. (2008). Electron Transfer in DNA and in DNA-Related Biological Processes. Electrochemical Insights. Chemical Reviews, 108(7), 2622-2645. doi:10.1021/cr0680787The Nobel Prize in Chemistry 2015âAdvanced Informationhttp://www.nobelprize.org/nobel_prizes/chemistry/laureates/2015/advanced.htmlBrettel, K., & Byrdin, M. (2010). Reaction mechanisms of DNA photolyase. Current Opinion in Structural Biology, 20(6), 693-701. doi:10.1016/j.sbi.2010.07.003Dandliker, P. J. (1997). Oxidative Thymine Dimer Repair in the DNA Helix. Science, 275(5305), 1465-1468. doi:10.1126/science.275.5305.1465Vicic, D. A., Odom, D. T., NĂșñez, M. E., Gianolio, D. A., McLaughlin, L. W., & Barton, J. K. (2000). Oxidative Repair of a Thymine Dimer in DNA from a Distance by a Covalently Linked Organic Intercalator. Journal of the American Chemical Society, 122(36), 8603-8611. doi:10.1021/ja000280iHartman, T., & Cibulka, R. (2016). Photocatalytic Systems with Flavinium Salts: From Photolyase Models to Synthetic Tool for Cyclobutane Ring Opening. Organic Letters, 18(15), 3710-3713. doi:10.1021/acs.orglett.6b01743Scannell, M. P., Fenick, D. J., Yeh, S.-R., & Falvey, D. E. (1997). Model Studies of DNA Photorepair: Reduction Potentials of Thymine and Cytosine Cyclobutane Dimers Measured by Fluorescence Quenching. Journal of the American Chemical Society, 119(8), 1971-1977. doi:10.1021/ja963360oPĂ©rez-Ruiz, R., JimĂ©nez, M. C., & Miranda, M. A. (2014). Hetero-cycloreversions Mediated by Photoinduced Electron Transfer. Accounts of Chemical Research, 47(4), 1359-1368. doi:10.1021/ar4003224Boussicault, F., & Robert, M. (2006). Electrochemical Approach to the Repair of Oxetanes Mimicking DNA (6â4) Photoproducts. The Journal of Physical Chemistry B, 110(43), 21987-21993. doi:10.1021/jp062425zPrakash, G., & Falvey, D. E. (1995). Model studies of the (6-4) photoproduct DNA photolyase: Synthesis and photosensitized splitting of a thymine-5,6-oxetane. Journal of the American Chemical Society, 117(45), 11375-11376. doi:10.1021/ja00150a050Friedel, M. G., Cichon, M. K., & Carell, T. (2005). Model compounds for (6â4) photolyases: a comparative flavin induced cleavage study of oxetanes and thietanes. Organic & Biomolecular Chemistry, 3(10), 1937. doi:10.1039/b503205aFraga-Timiraos, A. B., Lhiaubet-Vallet, V., & Miranda, M. A. (2016). Repair of a Dimeric Azetidine Related to the Thymine-Cytosine (6-
4) Photoproduct by Electron Transfer Photoreduction. Angewandte Chemie International Edition, 55(20), 6037-6040. doi:10.1002/anie.201601475Andreu, I., Delgado, J., EspinoÌs, A., PeÌrez-Ruiz, R., JimeÌnez, M. C., & Miranda, M. A. (2008). Cycloreversion of Azetidines via Oxidative Electron Transfer. Steady-State and Time-Resolved Studies. Organic Letters, 10(22), 5207-5210. doi:10.1021/ol802181uPac, C., Ohtsuki, T., Shiota, Y., Yanagida, S., & Sakurai, H. (1986). Photochemical Reactions of Aromatic Compounds. XLII. Photosensitized Reactions of Some Selected Diarylcyclobutanes by Aromatic Nitriles and Chloranil. Implications of Charge-Transfer Contributions on Exciplex Reactivities. Bulletin of the Chemical Society of Japan, 59(4), 1133-1139. doi:10.1246/bcsj.59.1133Swenton, J. S., & Hyatt, J. A. (1974). Photosensitized cycloadditions to 1,3-dimethyl-6-azauracil and 1,3-dimethyl-6-azathymine. Imine linkage unusually reactive toward photocycloaddition. Journal of the American Chemical Society, 96(15), 4879-4885. doi:10.1021/ja00822a027Scannell, M. P., Prakash, G., & Falvey, D. E. (1997). Photoinduced Electron Transfer to Pyrimidines and 5,6-Dihydropyrimidine Derivatives: Reduction Potentials Determined by Fluorescence Quenching Kinetics. The Journal of Physical Chemistry A, 101(24), 4332-4337. doi:10.1021/jp970164aPavlishchuk, V. V., & Addison, A. W. (2000). Conversion constants for redox potentials measured versus different reference electrodes in acetonitrile solutions at 25°C. Inorganica Chimica Acta, 298(1), 97-102. doi:10.1016/s0020-1693(99)00407-
Minimizing the Ground Effect for Photophoretically Levitating Disks
Photophoretic levitation is a propulsion mechanism in which lightweight
objects can be lifted and controlled through their interactions with light.
Since photophoretic forces on macroscopic objects are usually maximized at low
pressures, they may be tested in vacuum chambers in close proximity to the
chamber floor and walls. We report here experimental evidence that the terrain
under levitating microflyers, including the chamber floor or the launchpad from
which microflyers lift off, can greatly increase the photophoretic lift forces
relative to their free-space (mid-air) values. To characterize this so-called
"ground effect" during vacuum chamber tests, we introduced a new miniature
launchpad composed of three J-shaped (candy-cane-like) wires that minimized a
microflyer's extraneous interactions with underlying surfaces. We compared our
new launchpads to previously used wire-mesh launchpads for simple levitating
mylar-based disks with diameters of 2, 4, and 8 cm. Importantly, wire-mesh
launchpads increased the photophoretic lift force by up to sixfold. A
significant ground effect was also associated with the bottom of the vacuum
chamber, particularly when the distance to the bottom surface was less than the
diameter of the levitating disk. We provide guidelines to minimize the ground
effect in vacuum chamber experiments, which are necessary to test photophoretic
microflyers intended for high-altitude exploration and surveillance on Earth or
on Mars.Comment: 7 pages, 7 figures, including the Supplemental Materia
A Chandra and Spitzer census of the young star cluster in the reflection nebula NGC 7129
The reflection nebula NGC 7129 has long been known to be a site of recent
star formation as evidenced, e.g., by the presence of deeply embedded
protostars and HH objects. However, studies of the stellar population produced
in the star formation process have remained rudimentary. At a presumed age of
~3 Myr, NGC7129 is in the critical range where disks around young stars
disappear. We make use of Chandra X-ray and Spitzer and 2MASS IR imaging
observations to identify the pre-main sequence stars in NGC7129. We define a
sample of Young Stellar Objects based on color-color diagrams composed from IR
photometry between 1.6 and 8 mu, from 2MASS and Spitzer, and based on X-ray
detected sources from a Chandra observation. This sample is composed of 26
Class II and 25 Class III candidates. The sample is estimated to be complete
down to ~ 0.5 solar masses. The most restricted and least biased sub-sample of
pre-main sequence stars is composed of lightly absorbed (A_V < 5 mag) stars in
the cluster core. This sample comprises 7 Class II and 14 Class III sources, it
has a disk fraction of 33^{+24}_{-19} %, and a median X-ray luminosity of log
(L_x) [erg/s] = 30.3. Despite the various uncertainties related to the sample
selection, absorption, mass distribution, distance and, consequently, the
computation of disk fraction and X-ray luminosities, the data yield consistent
results. In particular, we confirm the age of ~3 Myr for the NGC7129 cluster.
The derived disk fraction is similar to that of sigma Orionis, smaller than
that of Cha I (~2 Myr), and larger than that of Upper Sco (5 Myr). The X-ray
luminosity function is similar to that of NGC 2264 (2 Myr) but fainter than
that of the Orion Nebula Cluster (1 Myr).Comment: accepted for publication in Astronomy & Astrophysic
Air quality assessment for Portugal
According to the Air Quality Framework Directive, air pollutant concentration levels have to be assessed and reported annually by each European Union member state, taking into consideration European air quality standards. Plans and programmes should be implemented in zones and agglomerations where pollutant concentrations exceed the limit and target values. The main objective of this study is to perform a long-term air quality simulation for Portugal, using the CHIMERE chemistry-transport model, applied over Portugal, for the year 2001. The model performance was evaluated by comparing its results to air quality data from the regional monitoring networks and to data from a diffusive sampling experimental campaign. The results obtained show a modelling system able to reproduce the pollutant concentrations' temporal evolution and spatial distribution observed at the regional networks of air quality monitoring. As far as the fulfilment of the air quality targets is concerned, there are excessive values for nitrogen and sulfur dioxides, ozone also being a critical gaseous pollutant in what concerns hourly concentrations and AOT40 (Accumulated Over Threshold 40 ppb) values
Density Waves in Layered Systems with Fermionic Polar Molecules
A layered system of two-dimensional planes containing fermionic polar
molecules can potentially realize a number of exotic quantum many-body states.
Among the predictions, are density-wave instabilities driven by the anisotropic
part of the dipole-dipole interaction in a single layer. However, in typical
multilayer setups it is reasonable to expect that the onset and properties of a
density-wave are modified by adjacent layers. Here we show that this is indeed
the case. For multiple layers the critical strength for the density-wave
instability decreases with the number of layers. The effect depends on density
and is more pronounced in the low density regime. The lowest solution of the
instability corresponds to the density waves in the different layers being
in-phase, whereas higher solutions have one or several adjancet layers that are
out of phase. The parameter regime needed to explore this instability is within
reach of current experiments.Comment: 7 pages, 4 figures. Final version in EPJD, EuroQUAM special issue
"Cold Quantum Matter - Achievements and Prospects
Cohort profile: The Cohorts Consortium of Latin America and the Caribbean (CC-LAC)
Why was the cohort set up?
Latin America and the Caribbean (LAC) are characterized by much diversity in terms of socio-economic status, ecology, environment, access to health care,1,2 as well as the frequency of risk factors for and prevalence or incidence of non-communicable diseases;3â7 importantly, these differences are observed both between and within countries in LAC.8,9 LAC countries share a large burden of non-communicable (e.g. diabetes and hypertension) and cardiovascular (e.g. ischaemic heart disease) diseases, with these conditions standing as the leading causes of morbidity, disability and mortality in most of LAC.10â12 These epidemiological estimatesâe.g. morbidityâcannot inform about risk factors or risk prediction, which are relevant to identify prevention avenues. Cohort studies, on the other hand, could provide this evidence. Pooled analysis, using data from multiple cohort studies, have additional strengths such as increased statistical power and decreased statistical uncertainty.13 LAC cohort studies have been under-represented,14 or not included at all,15â17 in international efforts aimed at pooling data from multiple cohort studies. We therefore set out to pool data from LAC cohorts to address research questions that individual cohort studies would not be able to answer.
Drawing from previous successful regional enterprises (e.g. Asia Pacific Cohort Studies Collaboration),18,19 we established the Cohorts Consortium of Latin America and the Caribbean (CC-LAC). The main aim of the CC-LAC is to start a collaborative cohort data pooling in LAC to examine the association between cardio-metabolic risk factors (e.g. blood pressure, glucose and lipids) and non-fatal and fatal cardiovascular outcomes (e.g. stroke or myocardial infarction). In so doing, we aim to provide regional risk estimates to inform disease burden metrics, as well as other ambitious projects including a cardiovascular risk score to strengthen cardiovascular prevention in LAC.
Initial funding has been provided by a fellowship from the Wellcome Trust Centre for Global Health Research at Imperial College London (Strategic Award, Wellcome TrustâImperial College Centre for Global Health Research, 100693/Z/12/Z). Additional funding is being provided by an International Training Fellowship from the Wellcome Trust (214185/Z/18/Z). At the time of writing, the daily operations and pooled database are hosted at Imperial College London, though a mid-term goal is to transfer this expertise and operations to LAC. The collaboration relies fundamentally on a strong regional network of health researchers and practitioner
Observational evidence of dissipative photospheres in gamma-ray bursts
The emission from a gamma-ray burst (GRB) photosphere can give rise to a
variety of spectral shapes. The spectrum can retain the shape of a Planck
function or it can be broadened and have the shape of a Band function. This
fact is best illustrated by studying GRB090902B: The main gamma-ray spectral
component is initially close to a Planck function, which can only be explained
by emission from the jet photosphere. Later, the same component evolves into a
broader Band function. This burst thus provides observational evidence that the
photosphere can give rise to a non-thermal spectrum. We show that such a
broadening is most naturally explained by subphotospheric dissipation in the
jet. The broadening mainly depends on the strength and location of the
dissipation, on the magnetic field strength, and on the relation between the
energy densities of thermal photons and of the electrons. We suggest that the
evolution in spectral shape observed in GRB090902B is due to a decrease of the
bulk Lorentz factor of the flow, leading to the main dissipation becoming
subphotospheric. Such a change in the flow parameters can also explain the
correlation observed between the peak energy of the spectrum and low-energy
power law slope, alpha, a correlation commonly observed in GRBs. We conclude
that photospheric emission could indeed be a ubiquitous feature during the
prompt phase in GRBs and play a decisive role in creating the diverse spectral
shapes and spectral evolutions that are observed.Comment: Submitted to MNRAS, 14 pages, 7 figure
Mercury Levels in an Urban Pregnant Population in Durham County, North Carolina
The adverse effects of prenatal mercury exposure, most commonly resulting from maternal fish consumption, have been detected at very low exposure levels. The omega-3 fatty acids found in fish, however, have been shown to support fetal brain and vision development. Using data from a prospective, cohort study of pregnant women from an inland area in the US South, we sought to understand the fish consumption habits and associated mercury levels across subpopulations. Over 30% of women had at least 1 ÎŒg/L of mercury in their blood, and about 2% had blood mercury levels above the level of concern during pregnancy (â„3.5 ÎŒg/L). Mercury levels were higher among Asian/Pacific Islander, older, higher educated, and married women. Fish consumption from any source was reported by 2/3 of the women in our study, with older women more likely to consume fish. Despite eating more fish meals per week, lower income, lower educated women had lower blood mercury levels than higher income, higher educated women. This suggests the different demographic groups consume different types of fish. Encouraging increased fish consumption while minimizing mercury exposure requires careful crafting of a complex health message
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