200 research outputs found
Glutamate 270 plays an essential role in K activation and domain closure of Thermus thermophilus isopropylmalate dehydrogenase
The mutant E270A of Thermus thermophilus 3-isopropylmalate dehydrogenase exhibits largely reduced (∼1%) catalytic activity and negligible activation by K+ compared to the wild-type enzyme. A 3–4 kcal/mol increase in the activation energy of the catalysed reaction upon this mutation could also be predicted by QM/MM calculations. In the X-ray structure of the E270A mutant a water molecule was observed to take the place of K+. SAXS and FRET experiments revealed the essential role of E270 in stabilisation of the active domain-closed conformation of the enzyme. In addition, E270 seems to position K+ into close proximity of the nicotinamide ring of NAD+ and the electron-withdrawing effect of K+ may help to polarise the aromatic ring in order to aid the hydride-transfer
Isovector unpolarized quark distribution in the nucleon in the large-N_c limit
We calculate the isovector (flavor-nonsinglet) unpolarized quark- and
antiquark distributions in the nucleon at a low normalization point in the
large-N_c limit. The nucleon is described as a soliton of the effective chiral
theory. The isovector distribution appears in the next-to-leading order of the
1/N_c-expansion. Numerical results for the quark- and antiquark distributions
compare well with the parametrizations of the data at a low normalization
point. This large-N_c approach gives a flavor asymmetry of the antiquark
distribution (violation of the Gottfried sum rule) in good agreement with the
measurements.Comment: 31 pages, LaTeX, 1 table, 4 figures included using eps
Covariant derivative expansion of Yang-Mills effective action at high temperatures
Integrating out fast varying quantum fluctuations about Yang--Mills fields
A_i and A_4, we arrive at the effective action for those fields at high
temperatures. Assuming that the fields A_i and A_4 are slowly varying but that
the amplitude of A_4 is arbitrary, we find a non-trivial effective gauge
invariant action both in the electric and magnetic sectors. Our results can be
used for studying correlation functions at high temperatures beyond the
dimensional reduction approximation, as well as for estimating quantum weights
of classical static configurations such as dyons.Comment: Minor changes. References added. Paper accepted for publication in
Phys.Rev.
Transversity distributions in the nucleon in the large-N_c limit
We compute the quark and antiquark transversity distributions in the nucleon
at a low normalization point of 600 MeV in the large- limit, where the
nucleon can be described as a soliton of an effective chiral theory (chiral
quark-soliton model). The flavor-nonsinglet distributions, and , appear in leading order
of the -expansion, while the flavor-singlet distributions, and , are non-zero only in
next-to-leading order. The transversity quark and antiquark distributions are
found to be significantly different from the longitudinally polarized
distributions and , respectively, in contrast to the prediction of the naive
non-relativistic quark model. We show that this affects the predictions for the
spin asymmetries in Drell-Yan pair production in transversely polarized pp and
ppbar collisions.Comment: 45 pages, 16 figure
Skewed and double distributions in pion and nucleon
We study the non-forward matrix elements of twist-2 QCD light-ray operators
and their representations in terms of skewed and double distributions,
considering the pion as well as the nucleon. We point out the importance of
explicitly including all twist-2 structures in the double distribution
representation, which naturally leads to a ``two-component'' structure of the
skewed distribution, with different contributions in the regions |X| > xi/2 and
|X| < \xi/2. We compute the skewed and double quark distributions in the pion
at a low normalization point in the effective chiral theory based on the
instanton vacuum. Also, we derive the crossing relations expressing the skewed
quark distribution in the pion through the distribution amplitude for two--pion
production. Measurement of the latter in two-pion production in gamma^* gamma
and gamma^* N reactions could provide direct information about the skewed as
well as the usual quark/antiquark-distribution in the pion.Comment: 37 pages, LaTeX. 5 figures included using eps
Theoretical study of the insulating oxides and nitrides: SiO2, GeO2, Al2O3, Si3N4, and Ge3N4
An extensive theoretical study is performed for wide bandgap crystalline
oxides and nitrides, namely, SiO_{2}, GeO_{2}, Al_{2}O_{3}, Si_{3}N_{4}, and
Ge_{3}N_{4}. Their important polymorphs are considered which are for SiO_{2}:
-quartz, - and -cristobalite and stishovite, for
GeO_{2}: -quartz, and rutile, for Al_{2}O_{3}: -phase, for
Si_{3}N_{4} and Ge_{3}N_{4}: - and -phases. This work
constitutes a comprehensive account of both electronic structure and the
elastic properties of these important insulating oxides and nitrides obtained
with high accuracy based on density functional theory within the local density
approximation. Two different norm-conserving \textit{ab initio}
pseudopotentials have been tested which agree in all respects with the only
exception arising for the elastic properties of rutile GeO_{2}. The agreement
with experimental values, when available, are seen to be highly satisfactory.
The uniformity and the well convergence of this approach enables an unbiased
assessment of important physical parameters within each material and among
different insulating oxide and nitrides. The computed static electric
susceptibilities are observed to display a strong correlation with their mass
densities. There is a marked discrepancy between the considered oxides and
nitrides with the latter having sudden increase of density of states away from
the respective band edges. This is expected to give rise to excessive carrier
scattering which can practically preclude bulk impact ionization process in
Si_{3}N_{4} and Ge_{3}N_{4}.Comment: Published version, 10 pages, 8 figure
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Effect of dietary vitamin D3 and 25-hydroxyvitamin D3 supplementation on plasma and milk 25-hydroxyvitamin D3 concentration in dairy cows
Milk enriched with vitamin D by supplementing dairy cow diets could provide a valuable dietary source of vitamin D, but information on the feasibility of this approach is limited. In the current study, the effects of supplementing dairy cows with either vitamin D3 or 25(OH) D3 over the transition/early lactation period on plasma and milk vitamin D concentrations were compared. Sixty dairy cows were randomly allocated to one of four dietary treatments from 14 days pre-calving to 56 days post-calving. Treatments were a control diet (Control) for both pre-calving and post-calving periods containing 0.625 mg/day vitamin D3; a pre-calving diet supplemented with 6 mg 25(OH) D3/day, but with a post-calving diet matching that of the control diet (25(OH) D3 pre-calving); the control diet pre-calving but with the post-calving diet supplemented with 2 mg vitamin D3/day (D3max), and the control diet pre-calving but with the post-calving diet supplemented with 1.5 mg 25(OH) D3/day (25(OH) D3 post-calving). No treatment effect on milk yield, composition or 25(OH) D3 concentration was observed. However there was an interaction of treatment and time for plasma 25(OH) D3 concentration; this increased within two weeks of supplementation for the 25(OH) D3 pre-calving treatment (peaking just after calving, 202 ng/ml), whereas that of the 25(OH) D3 post-calving group had a slower response following supplementation, continuing to increase at 56 days. There were correlations between plasma and milk 25(OH) D3 concentrations at days 4 and 14 of lactation, but not at later sampling times. The D3max treatment did not increase 25(OH) D3 concentration in plasma or milk. Overall, results from this study indicate that supplemental 25(OH) D3 is an effective means of enhancing dairy cow plasma 25(OH) D3 concentrations compared with vitamin D3 supplementation, but not necessarily milk concentrations
Genetic architecture of EEG power spectra in early life
We measured the electroencephalogram (EEG) in 209 5 year old monozygotic (MZ) and dizygotic (DZ) twin pairs to estimate the relative contribution of genetic and environmental factors to EEG power spectra in early life. Data from same-sex and from opposite-sex twin pairs were used to test for sex differences in genetic influences. Results showed high concordance for EEGs of MZ twins for absolute and relative power in δ, θ, α1, α2, β1 and β2 bands. A model with additive genetic and unique environmental influences explained individual differences in both absolute and relative power in almost all bands and all electrode positions. Heritability of EEG power spectra was high. For absolute power the highest heritabilities were observed in θ, α1, α2 and β1 power bands (mean heritability 81, 81, 78, and 73%, respectively). Somewhat lower heritabilities were found in δ and β2 bands (mean heritability 55 and 64%, respectively). For relative power heritabilities were 63, 76, 71, 72, 68, and 65 for δ, θ, α1, α2, β1, and β2, respectively. Virtually no sex differences in heritability were found. These findings indicate that the background EEG is one of the most heritable characteristics in early life
Hadronic matrix elements of gluon operators in the instanton vacuum
We propose a method to evaluate hadronic matrix elements of QCD gluon
operators in the instanton vacuum. We construct the ground state of the
interacting instanton ensemble for non-zero --angle using a
variational principle. A method to study the --dependence of
observables on the lattice is suggested. We then derive the effective fermion
action, which allows to calculate hadronic correlation functions in a
--expansion (Nambu--Jona-Lasinio type effective fermion theory). Gluon
operators are systematically represented as effective fermion operators.
Physical matrix elements are obtained after integrating the correlation
functions over fluctuations of the numbers of instantons. The influence of the
fermion determinant on the topological susceptibility is taken into account.
Our effective description gives matrix elements fully consistent with the trace
and anomalies. The approach allows to consistently evaluate the
nucleon matrix elements of various gluon and mixed quark--gluon operators in a
chiral soliton picture of the nucleon.Comment: 40 pp. The discussion of the value of g_A^0 and of ref.[36] has been
updated. A trivial error in formulas of section 5 has been corrected. This is
the final version to appear in Nucl. Phys.
Detector Description and Performance for the First Coincidence Observations between LIGO and GEO
For 17 days in August and September 2002, the LIGO and GEO interferometer
gravitational wave detectors were operated in coincidence to produce their
first data for scientific analysis. Although the detectors were still far from
their design sensitivity levels, the data can be used to place better upper
limits on the flux of gravitational waves incident on the earth than previous
direct measurements. This paper describes the instruments and the data in some
detail, as a companion to analysis papers based on the first data.Comment: 41 pages, 9 figures 17 Sept 03: author list amended, minor editorial
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