228 research outputs found
Extreme 13C depletion of CCl2F2 in firn air samples from NEEM, Greenland
A series of 12 high volume air samples collected from the S2 firn core during the North Greenland Eemian Ice Drilling (NEEM) 2009 campaign have been measured for mixing ratio and stable carbon isotope composition of the chlorofluorocarbon CFC-12 (CCl2F2). While the mixing ratio measurements compare favorably to other firn air studies, the isotope results show extreme 13C depletion at the deepest measurable depth (65 m), to values lower than d13C = -80‰ vs. VPDB (the international stable carbon isotope scale), compared to present day surface tropospheric measurements near -40‰. Firn air modeling was used to interpret these measurements. Reconstructed atmospheric time series indicate even larger depletions (to -120‰) near 1950 AD, with subsequent rapid enrichment of the atmospheric reservoir of the compound to the present day value. Mass-balance calculations show that this change is likely to have been caused by a large change in the isotopic composition of anthropogenic CFC-12 emissions, probably due to technological advances in the CFC production process over the last 80 yr, though direct evidence is lacking
Reconstruction of Northern Hemisphere 1950–2010 atmospheric non-methane hydrocarbons
The short-chain non-methane hydrocarbons (NMHC) are mostly emitted into the
atmosphere by anthropogenic processes. Recent studies have pointed out a
tight linkage between the atmospheric mole fractions of the NMHC ethane and
the atmospheric growth rate of methane. Consequently, atmospheric NMHC are
valuable indicators for tracking changes in anthropogenic emissions,
photochemical ozone production, and greenhouse gases. This study investigates
the 1950–2010 Northern Hemisphere atmospheric C<sub>2</sub>–C<sub>5</sub> NMHC ethane,
propane, <i>i</i>-butane, <i>n</i>-butane, <i>i</i>-pentane, and <i>n</i>-pentane by (a)
reconstructing atmospheric mole fractions of these trace gases using firn air
extracted from three boreholes in 2008 and 2009 at the North Greenland Eemian
Ice Drilling (NEEM) site and applying state-of-the-art models of trace gas
transport in firn, and by (b) considering eight years of ambient NMHC
monitoring data from five Arctic sites within the NOAA Global Monitoring
Division (GMD) Cooperative Air Sampling Network. Results indicate that these
NMHC increased by ~40–120% after 1950, peaked around 1980 (with
the exception of ethane, which peaked approximately 10 yr earlier), and have
since dramatically decreased to be now back close to 1950 levels. The earlier
peak time of ethane vs. the C<sub>3</sub>–C<sub>5</sub> NMHC suggests that different
processes and emissions mitigation measures contributed to the decline in
these NMHC. The 60 yr record also illustrates notable increases in the
ratios of the isomeric <i>iso-/n</i>-butane and <i>iso-/n</i>-pentane
ratios. Comparison of the reconstructed NMHC histories with 1950–2000
volatile organic compounds (VOC) emissions data and with other recently
published ethane trend analyses from ambient air Pacific transect data showed
(a) better agreement with North America and Western Europe emissions than
with total Northern Hemisphere emissions data, and (b) better agreement with
other Greenland firn air data NMHC history reconstructions than with the
Pacific region trends. These analyses emphasize that for NMHC, having
atmospheric lifetimes on the order of < 2 months, the Greenland firn
air records are primarily a representation of Western Europe and North
America emission histories
Modern NMR spectroscopy of proteins and peptides in solution and its relevance to drug design
The knowledge of the three-dimensional (3D) structures and conformational dynamics of proteins and peptides is important for the understanding of biochemical and genetic data derived for these molecules. This understanding can ultimately be of help in drug design. We describe here the role of Nuclear Magnetic Resonance (NMR) spectroscopy in this process for three distinct situations: for small proteins, where relatively simple NMR methods can be used for full 3D structure determination; for larger proteins that require multinuclear multidimensional NMR but for which full 3D structures can still be obtained; and for small peptides that are studied in interaction with macromolecules (receptors) using specialized NMR techniques. A fourth situation, pertaining to large systems where only partial structural information can be obtained from NMR data, is briefly discussed. Molecules of interest to the biomedical field (C5a and stromelysin) are discussed as examples.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43356/1/11091_2005_Article_BF02174537.pd
Improved 13C-Resolved HSQC-NOESY Spectra in H2O, Using Pulsed Field Gradients
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30546/1/0000179.pd
An Auxiliary RF Channel with Convenient Phase Control for NMR Spectrometers
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30598/1/0000235.pd
Improvements in HSMQC-Type Double- and Triple-Resonance NMR Experiments by Using Full-Sweep (Semi-)Constant-Time Shift Labeling
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31522/3/0000444.pd
FLASH-PHOTOLYSIS NMR - CIDNP TIME-DEPENDENCE IN CYCLIC PHOTOCHEMICAL-REACTIONS
A microsecond NMR flash photolysis technique has been used to study the tune dependence of CIDNP spectra observed during the cyclic photochemical reaction of N-acetyltryptophan with 3-N-carboxy methyllumiflavin. The rapid fall in the flavin enhancements is attributed to cancellation of escape- and recombination-type polarizations. This is caused by the dispropornonation of flavin radicals. © 1981
A Three-Dimensional Method for the Separation of Zero-Quantum Coherence and NOE in NOESY Spectra
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30931/1/0000601.pd
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