45 research outputs found
Analytic treatment of nuclear spin-lattice relaxation for diffusion in a cone model
We consider nuclear spin-lattice relaxation rate resulted from a diffusion
equation for rotational wobbling in a cone. We show that the widespread point
of view that there are no analytical expressions for correlation functions for
wobbling in a cone model is invalid and prove that nuclear spin-lattice
relaxation in this model is exactly tractable and amenable to full analytical
description. The mechanism of relaxation is assumed to be due to dipole-dipole
interaction of nuclear spins and is treated within the framework of the
standard Bloemberger, Purcell, Pound - Solomon scheme. We consider the general
case of arbitrary orientation of the cone axis relative the magnetic field. The
BPP-Solomon scheme is shown to remain valid for systems with the distribution
of the cone axes depending only on the tilt relative the magnetic field but
otherwise being isotropic. We consider the case of random isotropic orientation
of cone axes relative the magnetic field taking place in powders. Also we
consider the cases of their predominant orientation along or opposite the
magnetic field and that of their predominant orientation transverse to the
magnetic field which may be relevant for, e.g., liquid crystals. Besides we
treat in details the model case of the cone axis directed along the magnetic
field. The latter provides direct comparison of the limiting case of our
formulas with the textbook formulas for free isotropic rotational diffusion.
The dependence of the spin-lattice relaxation rate on the cone half-width
yields results similar to those predicted by the model-free approach.Comment: 29 p., 7 fig. arXiv admin note: substantial text overlap with
arXiv:1101.249
Insights into malaria susceptibility using genome-wide data on 17,000 individuals from Africa, Asia and Oceania
The human genetic factors that affect resistance to infectious disease are poorly understood. Here we report a genome-wide association study in 17,000 severe malaria cases and population controls from 11 countries, informed by sequencing of family trios and by direct typing of candidate loci in an additional 15,000 samples. We identify five replicable associations with genome-wide levels of evidence including a newly implicated variant on chromosome 6. Jointly, these variants account for around one-tenth of the heritability of severe malaria, which we estimate as -23% using genome-wide genotypes. We interrogate available functional data and discover an erythroid-specific transcription start site underlying the known association in ATP2B4, but are unable to identify a likely causal mechanism at the chromosome 6 locus. Previously reported HLA associations do not replicate in these samples. This large dataset will provide a foundation for further research on thegenetic determinants of malaria resistance in diverse populations.Peer reviewe
Study of reflection and transmission of plane waves at thermoelastic-diffusive solid/liquid interface
Perspectiva bayesiana na seleção de genótipos de feijão-caupi em ensaios de valor de cultivo e uso
Model uncertainty in the ecosystem approach to fisheries
Fisheries scientists habitually consider uncertainty in parameter values, but often neglect uncertainty about model structure. The importance of this latter source of uncertainty is likely to increase with the greater emphasis on ecosystem models in the move to an ecosystem approach to fisheries (EAF). It is therefore necessary to increase awareness about pragmatic approaches with which fisheries modellers and managers can account for model uncertainty and so we review current ways of dealing with model uncertainty in fisheries and other disciplines. These all involve considering a set of alternative models representing different structural assumptions, but differ in how those models are used. The models can be used to identify bounds on possible outcomes, find management actions that will perform adequately irrespective of the true model, find management actions that best achieve one or more objectives given weights assigned to each model, or formalise hypotheses for evaluation through experimentation. Data availability is likely to limit the use of approaches that involve weighting alternative models in an ecosystem setting, and the cost of experimentation is likely to limit its use. Practical implementation of the EAF should therefore be based on management approaches that acknowledge the uncertainty inherent in model predictions and are robust to it. Model results must be presented in a way that represents the risks and trade-offs associated with alternative actions and the degree of uncertainty in predictions. This presentation should not disguise the fact that, in many cases, estimates of model uncertainty may be based on subjective criteria. The problem of model uncertainty is far from unique to fisheries, and coordination among fisheries modellers and modellers from other communities will therefore be useful