On the Effect of Ions on Water Dynamics in Dilute and Concentrated Aqueous Salt Solutions

International audienceAqueous ionic solutions are ubiquitous in chemistry and in biology. Experiments show that ions affect water dynamics, but a full understanding of several questions remains needed: why some salts accelerate water dynamics while others slow it down, why the effect of a given salt can be concentration dependent, whether the effect of ions is rather local or more global. Numerical simulations are particularly suited to disentangle these different effects, but current force fields suffer from limitations and often lead to a poor description of dynamics in several aqueous salt solutions. Here, we develop an improved classical force field for the description of alkali halides which yields dynamics in excellent agreement with experimental measurements for water reorientational and translational dynamics. These simulations are analyzed with an extended jump model, which allows to compare the effects of ions on local hydrogen-bond exchange dynamics and on more global properties like viscosity. Our results unambiguously show that the ion-induced changes in water dynamics are usually mostly due to a local effect on the hydrogen-bond exchange dynamics; in contrast , the change in viscosity leads to a smaller effect, which governs the retardation only for a minority of salts and at high concentrations. We finally show how the respective importance of these two effects can be directly determined from experimental measurements alone, thus providing guidelines for the selection of an elec-trolyte with specific dynamical properties

Are there dynamical effects in enzyme catalysis? Some thoughts concerning the enzymatic chemical step

AbstractWe offer some thoughts on the much debated issue of dynamical effects in enzyme catalysis, and more specifically on their potential role in the acceleration of the chemical step. Since the term ‘dynamics’ has been used with different meanings, we find it useful to first return to the Transition State Theory rate constant, its assumptions and the choices it involves, and detail the various sources of deviations from it due to dynamics (or not). We suggest that much can be learned about the key current questions for enzyme catalysis from prior extensive studies of dynamical and other effects in the case of reactions in solution. We analyze dynamical effects both in the neighborhood of the transition state and far from it, together with the situation when quantum nuclear motion is central to the reaction, and we illustrate our discussion with various examples of enzymatic reactions

Reorientation dynamics of nanoconfined water: Power-law decay, hydrogen-bond jumps, and test of a two-state model

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/136/4/10.1063/1.3679404.The reorientation dynamics of water confined within nanoscale, hydrophilicsilica pores are investigated using molecular dynamics simulations. The effect of surface hydrogen-bonding and electrostatic interactions are examined by comparing with both a silica pore with no charges (representing hydrophobic confinement) and bulk water. The OH reorientation in water is found to slow significantly in hydrophilic confinement compared to bulk water, and is well-described by a power-law decay extending beyond one nanosecond. In contrast, the dynamics of water in the hydrophobic pore are more modestly affected. A two-state model, commonly used to interpret confined liquid properties, is tested by analysis of the position-dependence of the water dynamics. While the two-state model provides a good fit of the orientational decay, our molecular-level analysis evidences that it relies on an over-simplified picture of water dynamics. In contrast with the two-state model assumptions, the interface dynamics is markedly heterogeneous, especially in the hydrophilic pore and there is no single interfacial state with a common dynamics

Reorientational Dynamics of Water Confined in Zeolites

International audienceWe present a detailed molecular dynamics study of water reorientation and hydrogen-bond dynamics in a strong confinement situation, within the narrow pores of an all-silica Linde type A (LTA) zeolite. Two water loadings of the zeolite are compared with the bulk case. Water dynamics is found to be retarded in this extreme hydrophobic confinement and the slowdown is more pronounced at higher water loading. We show that water reorientation proceeds mainly by large-amplitude angular jumps, whose mechanism is similar to that determined in the bulk. The slowdown upon hydrophobic confinement is found to arise predominantly from an excluded-volume effect affecting the large fraction of water molecules lying at the interface with the zeolite matrix, with an additional minor contribution coming from a structuring effect induced by the confinement

Simulations of the infrared, Raman, and 2D-IR photon echo spectra of water in nanoscale silica pores

Vibrational spectroscopy is frequently used to characterize nanoconfined liquids and probe the effect of the confining framework on the liquid structure and dynamics relative to the corresponding bulk fluid. However, it is still unclear what molecular-level information can be obtained from such measurements. In this paper, we address this question by using molecular dynamics (MD) simulations to reproduce the linear infrared (IR), Raman, and two-dimensional IR (2D-IR) photon echo spectra for water confined within hydrophilic (hydroxyl-terminated) silica mesopores. To simplify the spectra the OH stretching region of isotopically dilute HOD in D2O is considered. An empirical mapping approach is used to obtain the OH vibrational frequencies, transition dipoles, and transition polarizabilities from the MD simulations. The simulated linear IR and Raman spectra are in good general agreement with measured spectra of water in mesoporous silica reported in the literature. The key effect of confinement on the water spectrum is a vibrational blueshift for OH groups that are closest to the pore interface. The blueshift can be attributed to the weaker hydrogen bonds (H-bonds) formed between the OH groups and silica oxygen acceptors. Non-Condon effects greatly diminish the contribution of these OH moieties to the linear IR spectrum, but these weaker H-bonds are readily apparent in the Raman spectrum. The 2D-IR spectra have not yet been measured and thus the present results represent a prediction. The simulated spectra indicates that it should be possible to probe the slower spectral diffusion of confined water compared to the bulk liquid by analysis of the 2D-IR spectra

Origins of the non-exponential reorientation dynamics of nanoconfined water

This is the published version. Copyright 2014 American Institute of PhysicsThe dynamics of water are dramatically modified upon confinement in nanoscale hydrophilic silica pores. In particular, the OH reorientation dynamics of the interfacial water are non-exponential and dramatically slowed relative to the bulk liquid. A detailed analysis of molecular dynamics simulations is carried out to elucidate the microscopic origins of this behavior. The results are analyzed in the context of the extended jump model for water that describes the reorientation as a combination of hydrogen-bond exchanges, or jumps, and rotation of intact hydrogen bonds, with the former representing the dominant contribution. Within this model, the roles of surface and dynamical heterogeneities are considered by spatially resolving the hydrogen-bond jump dynamics into individual sites on the silica pore surface. For each site the dynamics is nearly mono-exponential, indicating that dynamical heterogeneity is at most a minor influence, while the distribution of these individual site jump times is broad. The non-exponential dynamics can also not be attributed to enthalpic contributions to the barriers to hydrogen-bond exchanges. Two entropic effects related to the surface roughness are found to explain the retarded and diverse dynamics: those associated with the approach of a new hydrogen-bond acceptor and with the breaking of the initial hydrogen-bond

Balance Impairment in Radiation Induced Leukoencephalopathy Patients Is Coupled With Altered Visual Attention in Natural Tasks

Background: Recent studies have shown that alterations in executive function and attention lead to balance control disturbances. One way of exploring the allocation of attention is to record eye movements. Most experimental data come from a free viewing of static scenes but additional information can be leveraged by recording eye movements during natural tasks. Here, we aimed to provide evidence of a correlation between impaired visual alteration in natural tasks and postural control in patients suffering from Radiation-Induced Leukoencephalopathy (RIL).Methods: The study subjects were nine healthy controls and 10 patients who were diagnosed with RIL at an early stage, with isolated dysexecutive syndrome without clinically detectable gait or posture impairment. We performed a balance evaluation and eye movement recording during an ecological task (reading a recipe while cooking). We calculated a postural score and oculomotor parameters already proposed in the literature. We performed a variable selection using an out-of-bag random permutation and a random forest regression algorithm to find: (i) if visual parameters can predict postural deficit and, (ii) which are the most important of them in this prediction. Results were validated using the leave-one-out cross-validation procedure.Results: Postural scores indeed were found significantly lower in patients with RIL than in healthy controls. Visual parameters were found able to predict the postural score of RIL patients with normalized root mean square error (RMSE) of 0.16. The present analysis showed that horizontal and vertical eye movements, as well as the average duration of the saccades and fixations influenced significantly the prediction of the postural score in RIL patients. While two patients with very low MATTIS-Attention sub score showed the lowest postural scores, no statistically significant relationship was found between the two outcomes.Conclusion: These results highlight the significant relationship between the severity of balance deficits and the visual characteristics in RIL patients. It seems that increased balance impairment is coupled with a reduced focusing capacity in ecological tasks. Balance and eye movement recordings during a natural task could be a useful aspect of multidimensional scoring of the dysexecutive syndrome

6. L'eau liquide : structure et dynamique aux temps très courts

La prochaine fois que vous boirez un verre d’eau, arrêtez-vous un instant pour l’observer. Vous serez sûrement étonné d’apprendre que sous cette surface paisible se déchaîne une tempête microscopique, dans laquelle les molécules d’eau s’agitent sans discontinuer, faisant et défaisant les liaisons qui les relient à leurs voisines. En l’espace d’une seconde, chaque molécule change ainsi de partenaires près d’un millier de milliards de fois ! Fig. 1 – Structure moléculaire de l’..

Water and hydrogen-bond dynamics in aqueous systems (from bulk to biomolecular environments)

Des aspects clés de la dynamique de réorientation de l'eau ont été étudiés à l'aide de simulations numériques et de mod les analytiques, en forte connexion avec les résultats expérimentaux. Ce travail porte essentiellement sur le mécanisme moléculaire de la réorientation, qui implique des sauts angulaires de grande amplitude permettant l'échange de partenaires de liaisons hydrogènes. Ainsi, dans le cas du bulk, j'ai pu éclaircir quelques aspects fondamentaux de la dépendance en température de la réorientation, et ai proposé une interprétation de résultats expérimentaux récents de spectroscopie non linéaire. Par ailleurs, j'ai rationalisé l'effet de solutés très variés, comme des hydrophobes, des amphiphiles, et des surfaces étendues, sur la dynamique de l'eau. J'ai montré que la réorientation de l'eau n'est que faiblement ralentie par la présence de groupes hydrophobes, alors que les groupes hydrophiles peuvent avoir un effet bien plus important sur la dynamique. Ceci forme donc un cadre unique pour comprendre ultérieurement la dynamique de l'eau dans des systèmes plus complexes, en particulier les milieux biologiques.PARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF