188 research outputs found
Heteronuclear collisions between laser-cooled metastable neon atoms
We investigate heteronuclear collisions in isotope mixtures of laser-cooled
metastable 3P2 neon. Experiments are performed with spin-polarized atoms in a
magnetic trap for all two-isotope combinations of the stable neon isotopes
20Ne, 21Ne, and 22Ne. We determine the rate coefficients for heteronuclear
ionizing collisions to beta_{21,20}=(3.9+/-2.7) x 10^{-11} cm^3/s,
beta_{22,20}=(2.6+/-0.7) x 10^{-11} cm^3/s, and beta_{21,22}=(3.9+/-1.9) x
10^{-11} cm^3/s. We also study heteronuclear elastic collision processes and
give upper bounds for heteronuclear thermal relaxation cross sections. This
work significantly extends the limited available experimental data on
heteronuclear ionizing collisions for laser-cooled atoms involving one or more
rare gas atoms in a metastable state
Variation in seed dormancy of the wetland sedge, Carex elongata, between populations and individuals in two consecutive years
Variation in dormancy of the wetland sedge, Carex elongata L., was tested using seeds from three wild populations and the garden-grown progeny of one population. Germination experiments, comprising four combinations of temperature and light, were conducted either with fresh-matured or cold-stratified seeds, to assess the relative contribution of populations and mother plants to the total variation. Between-year variation was tested at the population level and, partly, at the individual level, using seeds collected in two consecutive years. Among-population variation accounted for 72%, and among-individual variation for 23%, of the total variance in the first experiment. Significant differences were apparent between one wild population and its garden-grown descendants. Differences in germinability among populations were maintained in the two consecutive years, but a proportion of the variance was due to the contribution of the maternal environment. Weak evidence for genetic control at the individual level was shown by a correlation across years in one population. However, the lack of a main effect at the individual level in the first experiment makes it difficult to assess the relative contribution of the mother plants to the total variation. Our results imply that germination patterns of C. elongata have a genetic basis, but are markedly modified by environmental condition
How T Cells Do the “Search for the Needle in the Haystack”
In the body, a T cell is confronted with millions of antigen-presenting cells (APCs) in the search for potentially harmful antigen. To elicit an appropriate immune response, this search has to be performed as fast and as precise as possible. These two requirements, however, are at odds with each other: fast searches lack accuracy, whereas high fidelity decisions are typically time-consuming. Here, we use the archetypical search for the needle in the haystack as an analogy for the T cell's search problem. We provide a statistical framework to quantitatively estimate the constraints of search strategies for rare instances. Particularly, we propose a solution for balancing the demand for high speed with low error rates. It takes advantage of a two-phase search process, which combines a first rapid scan with a second high-fidelity check. Finally, we provide arguments that support a two-phase search model for identification of antigen-positive APCs by T cells
Localization of fixed dipoles at high precision by accounting for sample drift during illumination
Single molecule localization microscopy relies on the precise quantification
of the position of single dye emitters in a sample. This precision is improved
by the number of photons that can be detected from each molecule. It is
therefore recommendable to increase illumination times for the recording
process. Particularly recording at cryogenic temperatures dramatically reduces
photobleaching and thereby allows a massive increase in illumination times to
several seconds. As a downside, microscope instabilities may well introduce
jitter during such long illuminations, deteriorating the localization
precision. In this paper, we theoretically demonstrate that a parallel
recording of fiducial marker beads together with a novel fitting approach
accounting for the full drift trajectory allows for largely eliminating drift
effects for drift magnitudes of several hundred nanometers per frame.Comment: 12 pages, 7 figure
Cholesterol Slows down the Lateral Mobility of an Oxidized Phospholipid in a Supported Lipid Bilayer
We investigated the mobility and phase-partitioning of the fluorescent oxidized phospholipid analogue 1-palmitoyl-2-glutaroyl-sn-glycero-3-phospho-N-Alexa647-ethanolamine (PGPE-Alexa647) in supported lipid bilayers. Compared to the conventional phospholipid dihexadecanoylphosphoethanolamine (DHPE)-Bodipy we found consistently higher diffusion constants. The effect become dramatic when immobile obstacles were inserted into the bilayer. which essentially blocked the diffusion of DHPE-Bodipy but hardly influenced the movements of PGPE-Alexa647. In a supported lipid bilayer made of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), the differences in probe mobility leveled off with increasing cholesterol content. Using coarse-grained molecular dynamics simulations, we could ascribe this effect to increased interactions between the oxidized phospholipid and the membrane matrix, concomitant with a translation in the headgroup position of the oxidized phospholipid: at zero cholesterol content, its headgroup is shifted to the outside of the DOPC headgroup region, whereas increasing cholesterol concentrations pulls the headgroup into the bilayer plane
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