26,458 research outputs found
A model of the near-earth plasma environment and application to the ISEE-A and -B orbit
A model of the near-earth environment to obtain a best estimate of the average flux of protons and electrons in the energy range from 0.1 to 100 keV for the International Sun-Earth Explorer (ISEE)-A and -B spacecraft. The possible radiation damage to the thermal coating on these spinning spacecraft is also studied. Applications of the model to other high-altitude satellites can be obtained with the appropriate orbit averaging. This study is the first attempt to synthesize an overall quantitative environment of low-energy particles for high altitude spacecraft, using data from in situ measurements
Structure of cytochrome a3-Cua3 couple in cytochrome c oxidase as revealed by nitric oxide binding studies
The addition of NO to oxidized cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) causes the appearance of a high-spin heme electron paramagnetic resonance (EPR) signal due to cytochrome a3. This suggests that NO coordinates to Cu{a3}+2 and breaks the antiferromagnetic couple by forming a cytochrome a3+3-Cu{a3}+2-NO complex. The intensity of the high-spin cytochrome a3 signal depends on the method of preparation of the enzyme and maximally accounts for 58% of one heme. The effect of N3- on the cytochrome a3+3-Cu{a3}+2-NO complex is to reduce cytochrome a3 to the ferrous state, and this is followed by formation of a new complex that exhibits EPR signals characteristic of a triplet species. On the basis of optical and EPR results, a NO bridge between cytochrome a3+2 and Cu{a3}+2 is proposed-i.e., cytochrome a3+2-NO-Cu{a3}+2. The half-field transition observed at g = 4.34 in the EPR spectrum of this triplet species exhibits resolved copper hyperfine splittings with |A{}| = 0.020 cm-1, indicating that the Cu{a3}+2 in the cytochrome a3+2-NO-Cu{a3}+2 complex is similar to a type 2 copper site
A Proton Magnetic Resonance Study of the Association of Lysozyme with Monosaccharide Inhibitors
It has been shown that the acetamido methyl protons of N-acetyl-d-glucosamine undergo a chemical shift to higher fields in their proton magnetic resonance spectrum when the inhibitor is bound to lysozyme. The observed chemical shift in the presence of the enzyme is different for the agr- and ß-anomeric forms of 2-acetamido-2-deoxy-d-glucopyranose indicating either a difference in the affinity of the anomeric forms for lysozyme or different magnetic environments for the methyl protons in their enzyme-bound state. That the agr- and ß-anomeric forms of GlcAc bind to lysozyme in a competitive fashion was indicated by observing the proton magnetic resonance spectra in the presence of 2-acetamido-d3-2-deoxy-agr-d-glucopyranose. The methyl glycosides, methyl-agr-GlcAc and methyl-ß-GlcAc, were also shown to bind competitively with both anomers of GlcAc. Quantitative analysis of the chemical shift data observed for the association of GlcAc with lysozyme was complicated by the mutarotation of GlcAc between its agr- and ß-anomeric forms. However, in the case of the methyl glucosides, where the conformation of each anomer is frozen, it was possible to analyze the chemical shift data in a straightforward manner, and the dissociation constant as well as the chemical shift of the acetamido methyl protons of the enzyme-inhibitor complex was determined for both anomers. The results indicate that the two anomers of methyl-GlcAc bind to lysozyme with slightly different affinities but that the acetamido methyl groups of both anomers experience identical magnetic environments in the enzyme-inhibitor complex
Piezoelectric copolymer hydrophones for ultrasonic field characterization
Hydrophones to be used in the characterization of medical ultrasonic transducers have
been fabricated using a new polyvinylidene fluoride/trifluoroethylene (VF2/VF3)
copolymer. The copolymer has an advantage over VF2 in that it does not require
prestretching before poling. Thin copolymer films can be cast from solution and then
poled using the corona discharge method. As there is a need for small‐diameter
hydrophones to provide good spatial resolution in measuring highly focused ultrasonic
beams, hydrophones with diameter as small as 0.1 mm have been made. Both
needle‐type and line hydrophones have been tested and their performance reported. In
the case of line hydrophones, the output signal is proportional to the line integral of the
acoustic pressure and a computer tomographic technique has been used to reconstruct the
beam profiles
Could CuB be the site of redox linkage in cytochrome c oxidase?
This paper explores the proton pumping function of cytochrome c oxidase [ferrocytochrome-c:oxygen oxidoreductase (EC 1.9.3.1)] based upon redox linkage at the "high-potential" CU(B) center. A model is proposed that is derived from a redox-linked ligand exchange mechanism previously described for the Cu(A) site. Qualitative analysis of this mechanism indicates that such a mechanism is feasible. However, the relatively short distance between Cu(B) and cytochrome a3 implies that the uncoupling electron transfers are quite facile. In addition, the position of the Cu(B) center with respect to the inner mitochondrial membrane argues against redox linkage at the Cu(B) site
A study of inner zone electron data and their comparison with trapped radiation models
A summary and intercomparison of recent inner radiation zone electron data are presented. The morphology of the inner radiation zone is described and the data compared with the current generation of inner zone trapped electron models. An analytic representation of the inner zone equatorial pitch angle distribution is presented. This model was based upon data from eight satellites and was used to reduce all data to the form of equatorial flux. Although no Starfish-free high energy electron measurements were available from the inner portion of the inner radiation zone, it was found that the AE-6 model provided a good description of the present solar maximum environment
The nature of CuA in cytochrome c oxidase
The isolation and purification of yeast cytochrome c oxidase is described. Characterization of the purified protein indicates that it is spectroscopically identical with cytochrome c oxidase isolated from beef heart. Preparations of isotopically substituted yeast cytochrome c oxidase are obtained incorporating [1,3-15N2]histidine or [beta,beta- 2H2]cysteine. Electron paramagnetic resonance and electron nuclear double resonance spectra of the isotopically substituted proteins identify unambiguously at least 1 cysteine and 1 histidine as ligands to CuA and suggest that substantial spin density is delocalized onto a cysteine sulfur in the oxidized protein to render the site Cu(I)-S
Large-Scale Structure in Brane-Induced Gravity II. Numerical Simulations
We use N-body simulations to study the nonlinear structure formation in
brane-induced gravity, developing a new method that requires alternate use of
Fast Fourier Transforms and relaxation. This enables us to compute the
nonlinear matter power spectrum and bispectrum, the halo mass function, and the
halo bias. From the simulation results, we confirm the expectations based on
analytic arguments that the Vainshtein mechanism does operate as anticipated,
with the density power spectrum approaching that of standard gravity within a
modified background evolution in the nonlinear regime. The transition is very
broad and there is no well defined Vainshtein scale, but roughly this
corresponds to k_*~ 2 at redshift z=1 and k_*~ 1 at z=0. We checked that while
extrinsic curvature fluctuations go nonlinear, and the dynamics of the
brane-bending mode C receives important nonlinear corrections, this mode does
get suppressed compared to density perturbations, effectively decoupling from
the standard gravity sector. At the same time, there is no violation of the
weak field limit for metric perturbations associated with C. We find good
agreement between our measurements and the predictions for the nonlinear power
spectrum presented in paper I, that rely on a renormalization of the linear
spectrum due to nonlinearities in the modified gravity sector. A similar
prediction for the mass function shows the right trends. Our simulations also
confirm the induced change in the bispectrum configuration dependence predicted
in paper I.Comment: 19 pages, 13 figures. v2: corrected typos, added more simulations,
better test of predictions in large mass regime. v3: minor changes, published
versio
Quantum Circulant Preconditioner for Linear System of Equations
We consider the quantum linear solver for with the circulant
preconditioner . The main technique is the singular value estimation (SVE)
introduced in [I. Kerenidis and A. Prakash, Quantum recommendation system, in
ITCS 2017]. However, some modifications of SVE should be made to solve the
preconditioned linear system . Moreover, different from
the preconditioned linear system considered in [B. D. Clader, B. C. Jacobs, C.
R. Sprouse, Preconditioned quantum linear system algorithm, Phys. Rev. Lett.,
2013], the circulant preconditioner is easy to construct and can be directly
applied to general dense non-Hermitian cases. The time complexity depends on
the condition numbers of and , as well as the Frobenius norm
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