2,267 research outputs found
Molecular Model for Ice Clusters in a Supersaturated Vapor
A molecular model previously applied to prenucleation water clusters is used to examine ice Ih, embryos. The canonical partition function is evaluated for clusters having from 6 to 64 water molecules. The intermolecular vibrational free energies are extrapolated to clusters containing up to 120 molecules and free energies of formation, nucleation rates, and critical supersaturation ratios are calculated and compared with experiment. For the clusters studied, the ice Ih structure appears to be much less stable at all temperatures than the more spherical clathratelike cluster
Molecular Model for Prenucleation Water Clusters
A molecular model applicable to prenucleation water clusters is described. As an illustration the model is applied to water clusters having clathrate-like structures composed of five-menibered rings. This work was motivated by the apparent inadequacies of the corrected liquid drop model which (in addition to applying bulk properties to small clusters) predicts nucleation rates which may be as much as 1017 larger than experiment. We present the energy of formation at a temperature of 277°K for our molecular model for clusters ranging in size from 5 to 57 molecules. These results agree qualitatively with experiment and, we believe, provide a motivation for further development of the molecular approach
Pre-clinical evaluation of antiproteases as potential candidates for HIV-1 pre-exposure prophylaxis
Previous studies on highly HIV-1-exposed, yet persistently seronegative women from the Punwami Sex Worker cohort in Kenya, have shed light on putative protective mechanisms, suggesting that mucosal immunological factors, such as antiproteases, could be mediating resistance to HIV-1 transmission in the female reproductive tract. Nine protease inhibitors were selected for this study: serpin B4, serpin A1, serpin A3, serpin C1, cystatin A, cystatin B, serpin B13, serpin B1 and α-2-macroglobulin-like-protein 1. We assessed in a pilot study, the activity of these antiproteases with cellular assays and an ex vivo HIV-1 challenge model of human ecto-cervical tissue explants. Preliminary findings with both models, cellular and tissue explants, established an order of inhibitory potency for the mucosal proteins as candidates for pre-exposure prophylaxis when mimicking pre-coital use. Combination of all antiproteases considered in this study was more active than any of the individual mucosal proteins. Furthermore, the migration of cells out of ecto-cervical explants was blocked indicating potential prevention of viral dissemination following amplification of the founder population. These findings constitute the base for further development of these mucosal protease inhibitors for prevention strategies
Semiclassical ionization dynamics of the hydrogen molecular ion in an electric field of arbitrary orientation
Quasi-static models of barrier suppression have played a major role in our
understanding of the ionization of atoms and molecules in strong laser fields.
Despite their success, in the case of diatomic molecules these studies have so
far been restricted to fields aligned with the molecular axis. In this paper we
investigate the locations and heights of the potential barriers in the hydrogen
molecular ion in an electric field of arbitrary orientation. We find that the
barriers undergo bifurcations as the external field strength and direction are
varied. This phenomenon represents an unexpected level of intricacy even on
this most elementary level of the dynamics. We describe the dynamics of
tunnelling ionization through the barriers semiclassically and use our results
to shed new light on the success of a recent theory of molecular tunnelling
ionization as well as earlier theories that restrict the electric field to be
aligned with the molecular axis
Roche Lobe Overflow from Dwarf Stellar Systems
We use both analytical analyses and numerical simulations to examine the
evolution of residual gas within tidally-limited dwarf galaxies and globular
clusters. If the gas sound speed exceeds about 10% of the central velocity
dispersion, as is the case for ionized gas within small stellar systems, the
gas shall have significant density at the tidal radius, and the gas may be lost
on timescales as short as a few times the sound crossing time of the system. In
colder systems, the density at the tidal radius is much lower, greatly reducing
the mass loss rate, and the system may retain its gas for a Hubble time. The
tidally removed gas shall follow an orbit close to that of the original host
system, forming an extended stream of ionized, gaseous debris. Tidal mass loss
severely limits the ability of dwarf systems to continuously form stars. The
ordinary gas content in many dwarf galaxies is fully ionized during high
red-shift epochs, possibly preventing star formation in some systems, leading
to the formation of starless, dark-matter concentrations. In either the field
or in the center of galaxy clusters, ionized gas may be retained by dwarf
galaxies, even though its sound speed may be comparable to or even exceed the
velocity dispersion. These processes may help to explain some observed
differences among dwarf galaxy types, as well as observations of the haloes of
massive galaxies.Comment: 28 pages, LaTeX, AASTex macro
Orientational Defects in Ice Ih: An Interpretation of Electrical Conductivity Measurements
We present a first-principles study of the structure and energetics of
Bjerrum defects in ice Ih and compare the results to experimental electrical
conductivity data. While the DFT result for the activation energy is in good
agreement with experiment, we find that its two components have quite different
values. Aside from providing new insight into the fundamental parameters of the
microscopic electrical theory of ice, our results suggest the activity of traps
in doped ice in the temperature regime typically assumed to be controlled by
the free migration of L defects.Comment: 4 pages, 4 Figures, 1 Tabl
Character Strengths and Deep Connections Following Hurricanes Katrina and Rita: Spiritual and Secular Pathways to Resistance Among Volunteers
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/100170/1/jssr12043.pd
Understanding uncertainty in temperature effects on vector-borne disease: A Bayesian approach
Extrinsic environmental factors influence the distribution and population
dynamics of many organisms, including insects that are of concern for human
health and agriculture. This is particularly true for vector-borne infectious
diseases, like malaria, which is a major source of morbidity and mortality in
humans. Understanding the mechanistic links between environment and population
processes for these diseases is key to predicting the consequences of climate
change on transmission and for developing effective interventions. An important
measure of the intensity of disease transmission is the reproductive number
. However, understanding the mechanisms linking and temperature, an
environmental factor driving disease risk, can be challenging because the data
available for parameterization are often poor. To address this we show how a
Bayesian approach can help identify critical uncertainties in components of
and how this uncertainty is propagated into the estimate of . Most
notably, we find that different parameters dominate the uncertainty at
different temperature regimes: bite rate from 15-25 C; fecundity across
all temperatures, but especially 25-32 C; mortality from
20-30 C; parasite development rate at 15-16C and again at
33-35C. Focusing empirical studies on these parameters and
corresponding temperature ranges would be the most efficient way to improve
estimates of . While we focus on malaria, our methods apply to improving
process-based models more generally, including epidemiological, physiological
niche, and species distribution models.Comment: 27 pages, including 1 table and 3 figure
HIV-infected sex workers with beneficial HLA-variants are potential hubs for selection of HIV-1 recombinants that may affect disease progression
Cytotoxic T lymphocyte (CTL) responses against the HIV Gag protein are associated with lowering viremia; however, immune control is undermined by viral escape mutations. The rapid viral mutation rate is a key factor, but recombination may also contribute. We hypothesized that CTL responses drive the outgrowth of unique intra-patient HIV-recombinants (URFs) and examined gag sequences from a Kenyan sex worker cohort. We determined whether patients with HLA variants associated with effective CTL responses (beneficial HLA variants) were more likely to carry URFs and, if so, examined whether they progressed more rapidly than patients with beneficial HLA-variants who did not carry URFs. Women with beneficial HLA-variants (12/52) were more likely to carry URFs than those without beneficial HLA variants (3/61) (p < 0.0055; odds ratio = 5.7). Beneficial HLA variants were primarily found in slow/standard progressors in the URF group, whereas they predominated in long-term non-progressors/survivors in the remaining cohort (p = 0.0377). The URFs may sometimes spread and become circulating recombinant forms (CRFs) of HIV and local CRF fragments were over-represented in the URF sequences (p < 0.0001). Collectively, our results suggest that CTL-responses associated with beneficial HLA variants likely drive the outgrowth of URFs that might reduce the positive effect of these CTL responses on disease progression
The need to promote behaviour change at the cultural level: one factor explaining the limited impact of the MEMA kwa Vijana adolescent sexual health intervention in rural Tanzania. A process evaluation
Background - Few of the many behavioral sexual health interventions in Africa have been rigorously evaluated. Where biological outcomes have been measured, improvements have rarely been found. One of the most rigorous trials was of the multi-component MEMA kwa Vijana adolescent sexual health programme, which showed improvements in knowledge and reported attitudes and behaviour, but none in biological outcomes. This paper attempts to explain these outcomes by reviewing the process evaluation findings, particularly in terms of contextual factors.
Methods - A large-scale, primarily qualitative process evaluation based mainly on participant observation identified the principal contextual barriers and facilitators of behavioural change.
Results - The contextual barriers involved four interrelated socio-structural factors: culture (i.e. shared practices and systems of belief), economic circumstances, social status, and gender. At an individual level they appeared to operate through the constructs of the theories underlying MEMA kwa Vijana - Social Cognitive Theory and the Theory of Reasoned Action – but the intervention was unable to substantially modify these individual-level constructs, apart from knowledge.
Conclusion - The process evaluation suggests that one important reason for this failure is that the intervention did not operate sufficiently at a structural level, particularly in regard to culture. Recently most structural interventions have focused on gender or/and economics. Complementing these with a cultural approach could address the belief systems that justify and perpetuate gender and economic inequalities, as well as other barriers to behaviour change
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