353 research outputs found
The Nucleation and Growth of Calcium Phosphate Crystals at Protein and Phosphatidylserine Liposome Surfaces
The kinetics of calcium phosphate crystal growth at the surfaces of proteins and phospholipids has been investigated using free drift and constant composition methods in supersaturated calcium phosphate solutions (relative supersaturations: with respect to hydroxyapatite, HAP, σHAP = 15.0, and with respect to octacalcium phosphate, OCP, σOCP = 1. 9). Fibrinogen and collagen molecules adsorbed at hydrophobic surfaces as well as uncross-linked collagen fibrils induce ion binding and subsequent nucleation of calcium phosphate. The formation of OCP on phosphatidylserine vesicles introduced to highly supersaturated calcium phosphate solutions probably involves the interaction of the calcium ions with the ionized carboxylic groups of the phospholipid
Periodicities in data observed during the minimum and the rising phase of solar cycle 23; years 1996 - 1999
Three types of observations: the daily values of the solar radio flux at 7
frequencies, the daily international sunspot number and the daily Stanford mean
solar magnetic field were processed in order to find all the periodicities
hidden in the data. Using a new approach to the radio data, two time series
were obtained for each frequency examined, one more sensitive to spot magnetic
fields, the other to large magnetic structures not connected with sunspots.
Power spectrum analysis of the data was carried out separately for the minimum
(540 days from 1 March 1996 to 22 August 1997) and for the rising phase (708
days from 23 August 1997 to 31 July 1999) of the solar cycle 23. The Scargle
periodograms obtained, normalized for the effect of autocorrelation, show the
majority of known periods and reveal a clear difference between the
periodicities found in the minimum and the rising phase. We determined the
rotation rate of the `active longitudes' in the rising phase as equal to 444.4
4 nHz (26\fd0 \pm 0\fd3). The results indicate that appropriate and
careful analysis of daily radio data at several frequencies allows the
investigation of solar periodicities generated in different layers of the solar
atmosphere by various phenomena related to the periodic emergence of diverse
magnetic structures.Comment: 15 pages, 8 figure
Ab initio study on the magneto-structural properties of MnAs
The magnetic and structural properties of MnAs are studied with ab initio
methods, and by mapping total energies onto a Heisenberg model. The stability
of the different phases is found to depend mainly on the volume and on the
amount of magnetic order, confirming previous experimental findings and
phenomenological models. It is generally found that for large lattice constants
the ferromagnetic state is favored, whereas for small lattice constants
different antiferromagnetic states can be stabilized. In the ferromagnetic
state the structure with minimal energy is always hexagonal, whereas it becomes
orthorhombically distorted if there is an antiferromagnetic component in the
hexagonal plane. For the paramagnetic state the stable cell is found to be
orthorhombic up to a critical lattice constant of about 3.7 Angstrom, above
which it remains hexagonal. This leads to the second order structural phase
transition between paramagnetic states at about 400 K, where the lattice
parameter increases above this critical value with rising temperature due to
the thermal expansion. For the paramagnetic state an analytic approximation for
the magnitude of the orthorhombic distortion as a function of the lattice
constant is given. Within the mean field approximation the dependence of the
Curie temperature on the volume and on the orthorhombic distortion is
calculated. For orthorhombically distorted cells the Curie temperature is much
smaller than for hexagonal cells. This is mainly due to the fact that some of
the exchange coupling constants in the hexagonal plane become negative for
distorted cells. With these results a description of the susceptibility as
function of temperature is given
Effect of temperature on the rate of ageing : an experimental study of the Blowfly Calliphora stygia
All organisms age, the rate of which can be measured by demographic analysis of mortality rates. The rate of ageing is thermally sensitive in ectothermic invertebrates and we examined the effects of temperature on both demographic rates of ageing and on cellular senescence in the blowfly, Calliphora stygia. The short lifespan of these flies is advantageous for demographic measurements while their large body size permits individual-based biochemical characterisation. Blowflies maintained at temperatures from 12°C to 34°C had a five to six-fold decrease in maximum and average longevity, respectively. Mortality rates were best described by a two-phase Gompertz relation, which revealed the first-phase of ageing to be much more temperature sensitive than the second stage. Flies held at low temperatures had both a slower first-phase rate of ageing and a delayed onset of second-phase ageing, which significantly extended their longevity compared with those at high temperatures. Blowflies that were transferred from 29°C to 15°C had higher first-phase mortality rates than those of flies held at constant 15°C, but their onset of second-phase ageing was deferred beyond that of flies held constantly at this temperature. The accumulation of fluorescent AGE pigment, a measure of cellular oxidative damage, increased steadily over time in all blowflies, irrespective of the temporal pattern of mortality. Pigment accumulated steadily during periods of ‘negligible senescence’, as measured by minimal rate of mortality, and the rate of accumulation was significantly affected by temperature. Thus accumulation of AGE pigment is more representative of chronological age than a reflection of biological age or a cause of mortality
Transiting exocomets detected in broadband light by TESS in the Pictoris system
We search for signs of falling evaporating bodies (FEBs, also known as
exocomets) in photometric time series obtained for Pictoris after
fitting and removing its Scuti type pulsation frequencies. Using
photometric data obtained by the TESS satellite we determine the pulsational
properties of the exoplanet host star Pictoris through frequency
analysis. We then prewhiten the 54 identified Scuti p-modes and
investigate the residual photometric time series for the presence of FEBs. We
identify three distinct dipping events in the light curve of Pictoris
over a 105-day period. These dips have depths from 0.5 to 2 millimagnitudes and
durations of up to 2 days for the largest dip. These dips are asymmetric in
nature and are consistent with a model of an evaporating comet with an extended
tail crossing the disk of the star. We present the first broadband detections
of exocomets crossing the disk of Pictoris, consistent with the
predictions made 20 years earlier by Lecavelier Des Etangs et al. (1999). No
periodic transits are seen in this time series. These observations confirm the
spectroscopic detection of exocomets in Calcium H and K lines that have been
seen in high resolution spectroscopy.Comment: Accepted version updated with correct references to Rappaport et al.
(2018) and various typo
Effectiveness of actions associated with the environmental protection and the sustainable development
The problem, which for many centuries measured globe is "coexistence" between man and
nature. The essential element of this process is the continuous satisfaction of human needs with the
use of what nature and the process of globalization have given of themselves. Therefore, the authors
of this article research has identified the following problems, namely. Do man can imagine himself
without essential environment? Is the action and the human conscience allows him to "surrender"
the gifts of fauna and flora? Is in the current progress in the development of society is the human
factor that causes people to become empathetic to the devastation of the environment? The present
article attempts to answer these questions, and the responses are the hypotheses
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