50 research outputs found
Spin-Lattice Relaxation of Coupled Metal-Radical Spin-Dimers in Proteins: Application to Fe2+-Cofactor (QAâ, QBâ,Ïâ ) Dimers in Reaction Centers from Photosynthetic Bacteria
AbstractThe spin-lattice relaxation times (T1) for the reduced quinone acceptors QAâ and QBâ, and the intermediate pheophytin acceptor Ïâ, were measured in native photosynthetic reaction centers (RC) containing a high spin Fe2+ (S=2) and in RCs in which Fe2+ was replaced by diamagnetic Zn2+. From these data, the contribution of the Fe2+ to the spin-lattice relaxation of the cofactors was determined. To relate the spin-lattice relaxation rate to the spin-spin interaction between the Fe2+ and the cofactors, we developed a spin-dimer model that takes into account the zero field splitting and the rhombicity of the Fe2+ ion. The relaxation mechanism of the spin-dimer involves a two-phonon process that couples the fast relaxing Fe2+ spin to the cofactor spin. The process is analogous to the one proposed by R. Orbach (Proc. R. Soc. A. (Lond.). 264:458â484) for rare earth ions. The spin-spin interactions are, in general, composed of exchange and dipolar contributions. For the spin dimers studied in this work the exchange interaction, Jo, is predominant. The values of Jo for QAâFe2+, QBâFe2+, and ÏâFe2+ were determined to be (in kelvin) â0.58, â0.92, and â1.3Ă10â3, respectively. The |Jo| of the various cofactors (obtained in this work and those of others) could be fitted with the relation exp(âÎČJd), where d is the distance between cofactor spins and ÎČJ had a value of (0.66-0.86) Ă
â1. The relation between Jo and the matrix element |Vij|2 involved in electron transfer rates is discussed
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Development and pathology of echinostoma caproni in experimentally infected mice
In the present article, several parasitological features of mice, each experimentally infected with 75 metacercariae of Echinostoma caproni (Trematoda: Echinostomatidae), were studied during the first 12 wk postinfection. Moreover, the early pathological responses also were analyzed and compared with data previously published on other host species of E. caproni to gain further insight into the factors determining worm rejection or establishment of chronic infections. The results obtained show that the pattern of E. caproni infection in mice is consistent with a highly compatible hostâparasite system. This combination is characterized by a high worm establishment, high egg output, and long survival of the worms. However, some differences with respect to other highly compatible hosts have been observed, particularly in relation to the survival of the adult worms. Histological studies suggest that the kinetics of goblet cells, mucosal neutrophils, and mononuclear inflammatory cells in the mesentery seem to be essential in determining the course of E. caproni infection in mice.Sotillo Gallego, Javier, [email protected] ; Monteagudo Castro, Jose Carlos, [email protected] ; Marcilla Diaz, Antonio, [email protected]
Gravitational Waves from Hyper-Accretion onto Nascent Black Holes
We examine the possibility that hyper-accretion onto newly born, black holes
occurs in highly intermittent, non-asymmetric fashion favorable to
gravitational wave emission in a neutrino cooled disk. This picture of
near-hole accretion is motivated by magneto-rotationally induced,
ultra-relativistic disk dynamics in the region of the flow bounded from below
by the marginally bound geodesic radius. For high spin values, a largely
coherent magnetic field in this region has the dynamical implication of compact
mass segregation at the displacement nodes of the non-axisymmetric, MRI modes.
When neutrino stress competes favorably for the disk dynamical structure, the
matter clumps may be rather dense and sufficiently long-lived to excite the
Quasi-Normal Ringing (a.k.a. QNR) modes of the Kerr geometry upon their
in-fall. We find that such accretion flow may drive bar-like, quadrupole
(l,m=2,2) modes in nearly resonant fashion for spin parameters . The
ensuing build up in strain amplitude of the undamped oscillations warrants a
brisk rate of energy deposition into gravitational waves. A detectability
assessment for the LIGO interferometers through the match filtering technique
is given by integrating the energy flux over a one second epoch of resonant
hyper-accretion at 1 \msun \sec^{-1}. Thus, a 15 \msun Kerr black hole
spinning at ( Hz), and located at 27
Mpc (e.g., GRB980425), will deliver a characteristic strain amplitude, , large enough to be detectable by LIGO II. If resonant
hyper-accretion were sustainable for a longer period (or at higher rates)
possibly associated with a second broad hump in a GRB light-curve, these
objects could be detected by LIGO I at very low redshifts.Comment: 12 pages, 4 figures, M.N.R.A.S. submissio
Kepler-21b: A 1.6REarth Planet Transiting the Bright Oscillating F Subgiant Star HD 179070
We present Kepler observations of the bright (V=8.3), oscillating star HD
179070. The observations show transit-like events which reveal that the star is
orbited every 2.8 days by a small, 1.6 R_Earth object. Seismic studies of HD
179070 using short cadence Kepler observations show that HD 179070 has a
frequencypower spectrum consistent with solar-like oscillations that are
acoustic p-modes. Asteroseismic analysis provides robust values for the mass
and radius of HD 179070, 1.34{\pm}0.06 M{\circ} and 1.86{\pm}0.04 R{\circ}
respectively, as well as yielding an age of 2.84{\pm}0.34 Gyr for this F5
subgiant. Together with ground-based follow-up observations, analysis of the
Kepler light curves and image data, and blend scenario models, we
conservatively show at the >99.7% confidence level (3{\sigma}) that the transit
event is caused by a 1.64{\pm}0.04 R_Earth exoplanet in a 2.785755{\pm}0.000032
day orbit. The exoplanet is only 0.04 AU away from the star and our
spectroscopic observations provide an upper limit to its mass of ~10 M_Earth
(2-{\sigma}). HD 179070 is the brightest exoplanet host star yet discovered by
Kepler.Comment: Accepted to Ap
Kinetics of echinostoma caproni (trematoda: echinostomatidae) antigens in feces and serum of experimentally infected hamsters and rats
This study reports on the kinetics of antibody production to Echinostoma caproni and the dynamics of antigens in feces and sera in 2 experimental hosts (hamsters and rats) that display different degrees of susceptibility with this echinostome. Echinostoma caproni produced chronic infections in hamsters, whereas rats lost the infection at 49â56 days postinfection (DPI). Hamsters developed higher antibody responses than rats, probably in relation to different intestinal absorptions of worm antigens in each host species. The levels of coproantigens were indicative of the course of infection in each host. Positive coproantigen levels were detected at 1â2 DPI in both hosts, and the values remained positive until the end of the experiment in hamsters; in rats, the coproantigen levels reverted to negative values, coinciding with the loss of infection. High levels of circulating antigens were detected in hamsters from 21 DPI to the end of the study. In contrast, low levels of E. caproni seroantigens were detected in rats only. These observations may reflect the differences in local inflammatory responses induced by E. caproni in each host species.Toledo Navarro, Rafael, [email protected] ; Espert Fernandez, Ana M., [email protected] ; Marcilla Diaz, Antonio, [email protected] ; Esteban Sanchis, Jose Guillermo, [email protected]
The influence of metallicity on stellar differential rotation and magnetic activity
Observations of Sun-like stars over the last half-century have improved our
understanding of how magnetic dynamos, like that responsible for the 11-year
solar cycle, change with rotation, mass and age. Here we show for the first
time how metallicity can affect a stellar dynamo. Using the most complete set
of observations of a stellar cycle ever obtained for a Sun-like star, we show
how the solar analog HD 173701 exhibits solar-like differential rotation and a
7.4-year activity cycle. While the duration of the cycle is comparable to that
generated by the solar dynamo, the amplitude of the brightness variability is
substantially stronger. The only significant difference between HD 173701 and
the Sun is its metallicity, which is twice the solar value. Therefore, this
provides a unique opportunity to study the effect of the higher metallicity on
the dynamo acting in this star and to obtain a comprehensive understanding of
the physical mechanisms responsible for the observed photometric variability.
The observations can be explained by the higher metallicity of the star, which
is predicted to foster a deeper outer convection zone and a higher facular
contrast, resulting in stronger variability.Comment: Submitted to Ap
Ï Earth: A 3.14 day Earth-sized Planet from K2's Kitchen Served Warm by the SPECULOOS Team
We report on the discovery of a transiting Earth-sized (0.95 R_â) planet around an M3.5 dwarf star at 57 pc, EPIC 249631677. The planet has a period of ~3.14 days, i.e., ~Ï, with an installation of 7.45 S_â. The detection was made using publicly available data from K2's Campaign 15. We observed three additional transits with SPECULOOS Southern and Northern Observatories, and a stellar spectrum from Keck/HIRES, which allowed us to validate the planetary nature of the signal. The confirmed planet is well suited for comparative terrestrial exoplanetology. While exoplanets transiting ultracool dwarfs present the best opportunity for atmospheric studies of terrestrial exoplanets with the James Webb Space Telescope, those orbiting mid-M dwarfs within 100 pc such as EPIC 249631677b will become increasingly accessible with the next generation of observatories