866 research outputs found
Ultrafast absorption kinetics of NADH in folded and unfolded conformations
The non-radiative energy transfer is shown to occur on a ~3ps time scale for NADH in the folded form in H2O. Addition of methanol thermodynamically favours the open form, for which energy transfer does not occur
A Transport Analysis of the BEEM Spectroscopy of Au/Si Schottky Barriers
A systematic transport study of the ballistic electron emission microscopy
(BEEM) of Au/Si(100) and Au/Si(111) Schottky barriers for different thicknesses
of the metal layer and different temperatures is presented. It is shown that
the existing experimental data are compatible with a recently predicted
bandstructure-induced non-forward electron propagation through the Au(111)
layer.Comment: 5 pages, Latex-APS, 1 postscript figure,
http://www.icmm.csic.es/Pandres/pedro.htm. Phys. Stat. Sol. (b) (to appear),
HCIS-10 Conf, Berlin 199
Imaging the Effects of Oxygen Saturation Changes in Voluntary Apnea and Hyperventilation on Susceptibility-Weighted Imaging
BACKGROUND AND PURPOSE: Cerebrovascular oxygenation changes during respiratory challenges have clinically important implications for brain function, including cerebral autoregulation and the rate of brain metabolism. SWI is sensitive to venous oxygenation level by exploitation of the magnetic susceptibility of deoxygenated blood. We assessed cerebral venous blood oxygenation changes during simple voluntary breath-holding (apnea) and hyperventilation by use of SWI at 3T.
MATERIALS AND METHODS: We performed SWI scans (3T; acquisition time of 1 minute, 28 seconds; centered on the anterior commissure and the posterior commissure) on 10 healthy male volunteers during baseline breathing as well as during simple voluntary hyperventilation and apnea challenges. The hyperventilation and apnea tasks were separated by a 5-minute resting period. SWI venograms were generated, and the signal changes on SWI before and after the respiratory stress tasks were compared by means of a paired Student t test.
RESULTS: Changes in venous vasculature visibility caused by the respiratory challenges were directly visualized on the SWI venograms. The venogram segmentation results showed that voluntary apnea decreased the mean venous blood voxel number by 1.6% (P < .0001), and hyperventilation increased the mean venous blood voxel number by 2.7% (P < .0001). These results can be explained by blood CO2 changes secondary to the respiratory challenges, which can alter cerebrovascular tone and cerebral blood flow and ultimately affect venous oxygen levels.
CONCLUSIONS: These results highlight the sensitivity of SWI to simple and noninvasive respiratory challenges and its potential utility in assessing cerebral hemodynamics and vasomotor responses
Non-Fickian Diffusion Affects the Relation between the Salinity and Hydrate Capacity Profiles in Marine Sediments
On-site measurements of water salinity (which can be directly evaluated from
the electrical conductivity) in deep-sea sediments is technically the primary
source of indirect information on the capacity of the marine deposits of
methane hydrates. We show the relation between the salinity (chlorinity)
profile and the hydrate volume in pores to be significantly affected by
non-Fickian contributions to the diffusion flux---the thermal diffusion and the
gravitational segregation---which have been previously ignored in the
literature on the subject and the analysis of surveys data. We provide amended
relations and utilize them for an analysis of field measurements for a real
hydrate deposit.Comment: 7 pages, 2 figures, 1 table, submitted to Compte Rendus Mecaniqu
Anisotropic Colossal Magnetoresistance Effects in Fe_{1-x}Cu_xCr_2S_4
A detailed study of the electronic transport and magnetic properties of
FeCuCrS () on single crystals is presented. The
resistivity is investigated for K in magnetic fields up to
14 Tesla and under hydrostatic pressure up to 16 kbar. In addition
magnetization and ferromagnetic resonance (FMR) measurements were performed.
FMR and magnetization data reveal a pronounced magnetic anisotropy, which
develops below the Curie temperature, , and increases strongly
towards lower temperatures. Increasing the Cu concentration reduces this
effect. At temperatures below 35 K the magnetoresistance, , exhibits a strong dependence on the direction of the
magnetic field, probably due to an enhanced anisotropy. Applying the field
along the hard axis leads to a change of sign and a strong increase of the
absolute value of the magnetoresistance. On the other hand the
magnetoresistance remains positive down to lower temperatures, exhibiting a
smeared out maximum with the magnetic field applied along the easy axis. The
results are discussed in the ionic picture using a triple-exchange model for
electron hopping as well as a half-metal utilizing a band picture.Comment: some typos correcte
Macroscopic coherence of a single exciton state in a polydiacetylene organic quantum wire
We show that a single exciton state in an individual ordered conjugated
polymer chain exhibits macroscopic quantum spatial coherence reaching tens of
microns, limited by the chain length. The spatial coherence of the k=0 exciton
state is demonstrated by selecting two spatially separated emitting regions of
the chain and observing their interference.Comment: 12 pages with 2 figure
Tempo and mode of performance evolution across multiple independent origins of adhesive toe pads in lizards
Understanding macroevolutionary dynamics of trait evolution is an important endeavor in evolutionary biology. Ecological opportunity can liberate a trait as it diversifies through trait space, while genetic and selective constraints can limit diversification. While many studies have examined the dynamics of morphological traits, diverse morphological traits may yield the same or similar performance and as performance is often more proximately the target of selection, examining only morphology may give an incomplete understanding of evolutionary dynamics. Here, we ask whether convergent evolution of padâbearing lizards has followed similar evolutionary dynamics, or whether independent origins are accompanied by unique constraints and selective pressures over macroevolutionary time. We hypothesized that geckos and anoles each have unique evolutionary tempos and modes. Using performance data from 59 species, we modified Brownian motion (BM) and OrnsteinâUhlenbeck (OU) models to account for repeated origins estimated using Bayesian ancestral state reconstructions. We discovered that adhesive performance in geckos evolved in a fashion consistent with Brownian motion with a trend, whereas anoles evolved in bounded performance space consistent with more constrained evolution (an OrnsteinâUhlenbeck model). Our results suggest that convergent phenotypes can have quite distinctive evolutionary patterns, likely as a result of idiosyncratic constraints or ecological opportunities
Evaluation of Brain Iron Content Based on Magnetic Resonance Imaging (MRI): Comparison among Phase Value, R2* and Magnitude Signal Intensity
Background and Purpose: Several magnetic resonance imaging (MRI) techniques are being exploited to measure brain iron levels increasingly as iron deposition has been implicated in some neurodegenerative diseases. However, there remains no unified evaluation of these methods as postmortem measurement isnât commonly available as the reference standard. The purpose of this study was to make a comparison among these methods and try to find a new index of brain iron. Methods: We measured both phase values and R2 * in twenty-four adults, and performed correlation analysis among the two methods and the previously published iron concentrations. We also proposed a new method using magnitude signal intensity and compared it with R2 * and brain iron. Results: We found phase value correlated with R2 * in substantia nigra (r = 20.723, p,0.001) and putamen (r = 20.514, p = 0.010), while no correlations in red nucleus (r = 20.236, p = 0.268) and globus pallidus (r = 20.111, p = 0.605). And the new magnitude method had significant correlations in red nucleus (r = 20.593, p = 0.002), substantia nigra (r = 20.521, p = 0.009), globus pallidus (r = 20.750, p,0.001) and putamen (r = 20.547, p = 0.006) with R2*. A strong inverse correlation was also found between the new magnitude method and previously published iron concentrations in seven brain regions (r = 20.982, P,0.001). Conclusions: Our study indicates that phase value may not be used for assessing the iron content in some brain region
Imaging the Effects of Oxygen Saturation Changes in Voluntary Apnea and Hyperventilation on Susceptibility-Weighted Imaging
ABSTRACT BACKGROUND AND PURPOSE: Cerebrovascular oxygenation changes during respiratory challenges have clinically important implications for brain function, including cerebral autoregulation and the rate of brain metabolism. SWI is sensitive to venous oxygenation level by exploitation of the magnetic susceptibility of deoxygenated blood. We assessed cerebral venous blood oxygenation changes during simple voluntary breath-holding (apnea) and hyperventilation by use of SWI at 3T
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