71 research outputs found
X-ray Diffraction and Molecular Dynamics Study of Medium-range Order in Ambient and Hot Water
We have developed x-ray diffraction measurements with high energy-resolution
and accuracy to study water structure at three different temperatures (7, 25
and 66 C) under normal pressure. Using a spherically curved Ge crystal an
energy resolution better than 15 eV has been achieved which eliminates
influence from Compton scattering. The high quality of the data allows a
precise oxygen-oxygen pair correlation function (PCF) to be directly derived
from the Fourier transform of the experimental data resolving shell structure
out to ~12 {\AA}, i.e. 5 hydration shells. Large-scale molecular dynamics (MD)
simulations using the TIP4P/2005 force-field reproduce excellently the
experimental shell-structure in the range 4-12 {\AA} although less agreement is
seen for the first peak in the PCF. The Local Structure Index [J. Chem. Phys.
104, 7671 (1996)] identifies a tetrahedral minority giving the
intermediate-range oscillations in the PCF and a disordered majority providing
a more featureless background in this range. The current study supports the
proposal that the structure of liquid water, even at high temperatures, can be
described in terms of a two-state fluctuation model involving local structures
related to the high-density and low-density forms of liquid water postulated in
the liquid-liquid phase transition hypothesis.Comment: Submitted to Phys. Chem. Chem. Phy
Ab initio van der Waals interactions in simulations of water alter structure from mainly tetrahedral to high-density-like
The structure of liquid water at ambient conditions is studied in ab initio
molecular dynamics simulations using van der Waals (vdW) density-functional
theory, i.e. using the new exchange-correlation functionals optPBE-vdW and
vdW-DF2. Inclusion of the more isotropic vdW interactions counteracts highly
directional hydrogen-bonds, which are enhanced by standard functionals. This
brings about a softening of the microscopic structure of water, as seen from
the broadening of angular distribution functions and, in particular, from the
much lower and broader first peak in the oxygen-oxygen pair-correlation
function (PCF), indicating loss of structure in the outer solvation shells. In
combination with softer non-local correlation terms, as in the new
parameterization of vdW-DF, inclusion of vdW interactions is shown to shift the
balance of resulting structures from open tetrahedral to more close-packed. The
resulting O-O PCF shows some resemblance with experiment for high-density water
(A. K. Soper and M. A. Ricci, Phys. Rev. Lett., 84:2881, 2000), but not
directly with experiment for ambient water. However, an O-O PCF consisting of a
linear combination of 70% from vdW-DF2 and 30% from experiment on low-density
liquid water reproduces near-quantitatively the experimental O-O PCF for
ambient water, indicating consistency with a two-liquid model with fluctuations
between high- and low-density regions
Disaccharide topology induces slow down in local water dynamics
Molecular level insight into water structure and structural dynamics near proteins, lipids and
nucleic acids is critical to the quantitative understanding of many biophysical processes. Un-
fortunately, understanding hydration and hydration dynamics around such large molecules is challenging because of the necessity of deconvoluting the effects of topography and chemical heterogeneity. Here we study, via classical all atom simulation, water structure and structural dynamics around two biologically relevant solutes large enough to have significant chemical and topological heterogeneity but small enough to be computationally tractable: the disaccharides Kojibiose and Trehalose. We find both molecules to be strongly amphiphilic (as quantified from normalized local density fluctuations) and to induce nonuniform local slowdown in water translational and rotational motion. Detailed analysis of the rotational slowdown shows that while the rotational mechanism is similar to that previously identified in other aqueous systems by Laage, Hynes and coworkers, two novel characteristics are observed: broadening of the transition state during hydrogen bond exchange (water rotation) and a subpopulation of water for which rotation is slowed because of hindered access of the new accepting water molecule to the transition state. Both of these characteristics are expected to be generic features of water rotation around larger biomolecules and, taken together, emphasize the difficulty in transferring insight into water rotation around small molecules to much larger amphiphilic
solutes.This work is part of the research program of the âStichting voor Fundamenteel Onderzoek der
Materie (FOM)â which is financially supported by the âNederlandse organisatie voor Wetenschap-
pelijk Onderzoek (NWO)â. Further financial support was provided by a Marie Curie Incoming
International Fellowship (RKC). We gratefully acknowledge SARA, the Dutch center for high-
performance computing, for computational time and Huib Bakker and Daan Frenkel for useful
critical reviews on an earlier version of this work. We thank two anonymous reviewers for their
excellent work, especially for bringing to our attention calculations done on the transition state geometry of dimers and the overstructuring of the O-O radial distribution function of SPC/E water
A review of the predictability and prediction of ENSO
A hierarchy of El Niño-Southern Oscillation (ENSO) prediction schemes has been developed during the Tropical Ocean-Global Atmosphere (TOGA) program which includes statistical schemes and physical models. The statistical models are, in general, based on linear statistical techniques and can be classified into models which use atmospheric (sea level pressure or surface wind) or oceanic (sea surface temperature or a measure of upper ocean heat content) quantities or a combination of oceanic and atmospheric quantities as predictors. The physical models consist of coupled ocean-atmosphere models of varying degrees of complexity, ranging from simplified coupled models of the âshallow waterâ type to coupled general circulation models. All models, statistical and physical, perform considerably better than the persistence forecast in predicting typical indices of ENSO on lead times of 6 to 12 months. The TOGA program can be regarded as a success from this perspective. However, despite the demonstrated predictability, little is known about ENSO predictability limits and the predictability of phenomena outside the tropical Pacific. Furthermore, the predictability of anomalous features known to be associated with ENSO (e.g., Indian monsoon and Sahel rainfall, southern African drought, and off-equatorial sea surface temperature) needs to be addressed in more detail. As well, the relative importance of different physical mechanisms (in the ocean or atmosphere) has yet to be established. A seasonal dependence in predictability is seen in many models, but the processes responsible for it are not fully understood, and its meaning is still a matter of scientific discussion. Likewise, a marked decadal variation in skill is observed, and the reasons for this are still under investigation. Finally, the different prediction models yield similar skills, although they are initialized quite differently. The reasons for these differences are also unclear
The ethnic dimensions of suburbanisation in Estonia
Large-scale suburbanisation is a relatively recent phenomenon in East Central Europe and is responsible for major socio-spatial changes in metropolitan areas. Little is known about the ethnic dimensions of suburbanisation, despite the existence of often sizeable Russian minority populations in some member-states of the former Soviet Union. We use individual-level Estonian census data from the year 2000 in order to investigate the ethnic dimensions of suburbanisation. The results show that ethnic minorities have a considerably lower probability of suburbanising compared to the majority population, and minorities are less likely to move to rural municipalities-the main sites of suburban change-in the suburban ring of cities.PostprintPeer reviewe
Subsurface circulation in the Somali Current
Direct velocity measurements were made at intermediate depths along the East African coast from March to July 1979. Strong time-dependent flows with multiple reversals in direction were found in the upper 1000 meters between 3°N and 4°S. At 700 meters, there may be a connection between the southwestward coastal current and an equatorial jet observed at 49°E, the latter turning south near the coast. North of 3°N little spatial organization of the flow can be recognized. Copyright © 1980 AAAS
Types of spatial mobility and the ethnic context of destination neighbourhoods in Estonia (discussion paper)
Most studies of the ethnic composition of destination neighbourhoods after residential moves do not take into account the types of moves people have made. However, from an individual perspective, different types of moves may result in neighbourhood environments that differ in terms of their ethnic composition from those in which individuals previously lived. We investigate how the ethnic residential context changes for individuals as a result of different types of mobility (immobility, intra-urban mobility, suburbanisation, and long-distance migration) for residents of the segregated post-Soviet city of Tallinn. We compare the extent to which Estonian- and Russian-speakers integrate in residential terms. Using unique longitudinal Census data (2000-2011) we tracked changes in the individual ethnic residential context of both groups. We found that the moving destinations of Estonian- and Russianspeakers diverge. When Estonians move, their new neighbourhood generally possesses a lower percentage of Russian-speakers compared with when Russian-speakers move, as well as compared with their previous neighbourhoods. For Russian-speakers, the percentage of other Russian-speakers in their residential surroundings decreases only for those who move to the surburbs or who move over longer distances to rural villages. By applying a novel approach of tracking the changes in the ethnic residential context of individuals for all mobility types, we were able to demonstrate that the two largest ethnolinguistic groups in Estonia tend to behave as âparallel populationsâ and that residential integration in Estonia is therefore slow.OTBArchitecture and The Built Environmen
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