6,642 research outputs found
Temperature dependence of P680+ reduction in O2-evolving PS II membrane fragments at different redox states Si of the water oxidizing system
AbstractThe electron transfer kinetics from Z to P680+ was analyzed as a function of temperature in the range of 248<T< 295 K by measuring absorption changes induced at 830 nm by a laser flash train in dark adapted O2 evolving PS II membrane fragments from spinach. It was found: (i) that the kinetics of P680+ reduction and their dependence on the redox state Si of the catalytic site of water oxidation are only slightly affected by temperature within the physiological range of 270<T<295 K. (ii) In the dark relaxed state S, the electron transfer from Z to P680+ exhibits an activation energy of the order of 10 kJ/mol in 248<T<295 K. (iii) In the 2nd and subsequent flashes of the train the ability for a stable charge separation between P680+ and Q−A, markedly decreases below −10°C. This phenomenon is assumed to be due to a strong effect of temperature on the electron transfer from Q−A to QB. The results are briefly discussed in relation to possible effects of structural changes in the D-1/D-2 polypeptide complex on the reaction coordinate of electron transfer steps in PS II
In-situ measurements of dendrite tip shape selection in a metallic alloy
The size and shape of the primary dendrite tips determine the principal
length scale of the microstructure evolving during solidification of alloys.
In-situ X-ray measurements of the tip shape in metals have been unsuccessful so
far due to insufficient spatial resolution or high image noise. To overcome
these limitations, high-resolution synchrotron radiography and advanced image
processing techniques are applied to a thin sample of a solidifying Ga-35wt.%In
alloy. Quantitative in-situ measurements are performed of the growth of
dendrite tips during the fast initial transient and the subsequent steady
growth period, with tip velocities ranging over almost two orders of magnitude.
The value of the dendrite tip shape selection parameter is found to be
, which suggests an interface energy anisotropy of
for the present Ga-In alloy. The non-axisymmetric
dendrite tip shape amplitude coefficient is measured to be ,
which is in excellent agreement with the universal value previously established
for dendrites.Comment: 9 pages, 6 figures, submitted to "Physical Reviews Materials
Analysis of the electron transfer from Pheo− to QA in PS II membrane fragments from spinach by time resolved 325 nm absorption changes in the picosecond domain
AbstractAbsorption changes at 325 nm (ΔA325) induced by 15 ps laser flashes (λ = 650 nm) in PS II membrane fragments were measured with picosecond time-resolution. In samples with the reaction centers (RCs) kept in the open state (P I QA) the signals are characterized by a very fast rise (not resolvable by our equipment) followed by only small changes within our time window of 1.6 ns. In the closed state (P I Q−A) of the reaction center the signal decays with an average half-life time of about 250 ps. It is shown that under our excitation conditions (E = 2 × 1014 photons/cm2 per pulse) subtraction of the absorption changes in closed RCs (ΔAclosed325) from those in open RCs (ΔAopen325) leads to a difference signal which is dominated by the reduction kinetics of QA. From the rise kinetics of this signal and by comparison with data in the literature it is inferred that QA becomes reduced by direct electron transfer from Pheo− with a time constant of about 350 ± 100 ps
A deterministic cavity-QED source of polarization entangled photon pairs
We present two cavity quantum electrodynamics proposals that, sharing the
same basic elements, allow for the deterministic generation of entangled
photons pairs by means of a three-level atom successively coupled to two single
longitudinal mode high-Q optical resonators presenting polarization degeneracy.
In the faster proposal, the three-level atom yields a polarization entangled
photon pair via two truncated Rabi oscillations, whereas in the adiabatic
proposal a counterintuitive Stimulated Raman Adiabatic Passage process is
considered. Although slower than the former process, this second method is very
efficient and robust under fluctuations of the experimental parameters and,
particularly interesting, almost completely insensitive to atomic decay.Comment: 5 pages, 5 figure
The Fueling Diagram: Linking Galaxy Molecular-to-Atomic Gas Ratios to Interactions and Accretion
To assess how external factors such as local interactions and fresh gas
accretion influence the global ISM of galaxies, we analyze the relationship
between recent enhancements of central star formation and total
molecular-to-atomic (H2/HI) gas ratios, using a broad sample of field galaxies
spanning early-to-late type morphologies, stellar masses of 10^(7.2-11.2) Msun,
and diverse stages of evolution. We find that galaxies occupy several loci in a
"fueling diagram" that plots H2/HI vs. mass-corrected blue-centeredness, a
metric tracing the degree to which galaxies have bluer centers than the average
galaxy at their stellar mass. Spiral galaxies show a positive correlation
between H2/HI and mass-corrected blue-centeredness. When combined with previous
results linking mass-corrected blue-centeredness to external perturbations,
this correlation suggests a link between local galaxy interactions and
molecular gas inflow/replenishment. Intriguingly, E/S0 galaxies show a more
complex picture: some follow the same correlation, some are quenched, and a
distinct population of blue-sequence E/S0 galaxies (with masses below key
transitions in gas richness) defines a separate loop in the fueling diagram.
This population appears to be composed of low-mass merger remnants currently in
late- or post-starburst states, in which the burst first consumes the H2 while
the galaxy center keeps getting bluer, then exhausts the H2, at which point the
burst population reddens as it ages. Multiple lines of evidence suggest
connected evolutionary sequences in the fueling diagram. In particular,
tracking total gas-to-stellar mass ratios within the diagram provides evidence
of fresh gas accretion onto low-mass E/S0s emerging from central starbursts.
Drawing on a comprehensive literature search, we suggest that virtually all
galaxies follow the same evolutionary patterns found in our broad sample.Comment: 24 pages, 11 figures (table 4 available at
http://user.physics.unc.edu/~dstark/table4_csv.txt), accepted for publication
in Ap
A simplified model for Oâ‚‚ transport and reduction kinetics in porous cathodes of Li-O cells
Financial Competence and Expectations Formation: Evidence from Australia
We study the financial competence of Australian retirement savers using self-assessed and quantified measures. Responses to financial literacy questions show large variation and compare poorly with some international surveys. Basic and sophisticated financial literacy vary significantly with most demographics, self-assessed financial competence, income, superannuation accumulation and net worth. General numeracy scores are largely constant across gender, age, higher education and income. Financial competence also significantly affects expectations of stock market performance. Using a discrete choice model, we show that individuals with a higher understanding of risk, diversification and financial assets are more likely to assign a probability to future financial crises rather than expressing uncertainty. © 2011 The Economic Society of Australia
Effect of different endodontic regeneration protocols on wettability, roughness and chemical composition of surface dentin
Introduction
We investigated the changes in physiochemical properties of dentin surfaces after performing different endodontic regeneration protocols.
Methods
Human dentin slices were randomized into 4 treatment groups and 1 untreated control group (n = 10). One treatment group was irrigated with sodium hypochlorite (NaOCl) for 5 minutes followed by EDTA for 10 minutes. The other 3 treatment groups were irrigated with NaOCl; treated for 4 weeks with triple antibiotic paste (TAP), diluted triple antibiotic paste (DTAP), or calcium hydroxide (Ca[OH]2); and then irrigated with EDTA. After treatment, contact angles between a blood analog and dentin surfaces were evaluated. Surface roughness and chemical composition were characterized using optical profilometry and energy-dispersive X-ray spectroscopy, respectively. One-way analysis of variance followed by Fisher least significant difference tests were used for statistical analyses.
Results
All treatment groups showed a significant reduction in wettability and a significant increase in surface roughness when compared with untreated dentin. Dentin treated with Ca(OH)2 had significantly lower wettability compared with all other groups. No significant difference in wettability was found between dentin treated with DTAP and TAP protocols. Dentin treated with TAP had significantly higher surface roughness compared with all other groups. Untreated dentin and NaOCl + EDTA–treated dentin had significantly higher calcium and phosphorus as well as significantly lower carbon compared with dentin treated with Ca(OH)2, DTAP, and TAP.
Conclusions
Endodontic regeneration protocols had a significant effect on wettability, surface roughness, and chemical composition of surface dentin. The Ca(OH)2 protocol caused a significant reduction in dentin wettability compared with TAP or DTAP protocols
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