arXiv subjects

Hélène Berthoumieux

Publications and source records attributed to Hélène Berthoumieux.

13 recordsLinked to original sources

arXiv2025

Nonlocal dielectric properties of water: the role of electronic delocalisation

The nonlocal dielectric properties of liquid water are studied in the context of {\it ab initio} molecular dynamics simulations based on density functional theory. We calculate the dielectric response from the charge structure factor of the liquid using the fluctuation-dissipation theorem. We show that the dielectric response function of {\it ab initio} simulations differs significantly from that of classical force-fields, both qualitatively and quantitatively. In particular, it exhibits a larger amplitude and a wider range of responding wave numbers. We suggest that the difference is due to the localisation of the electronic charge density inherent in classical force files and Wannier post-treatment of DFT densities. The localised charge models do not reproduce the shape of the response function even for $q$ corresponding to intermolecular distances, and could lead to a significant underestimation of the dielectric response of the liquid by a factor of 10.

physics.chem-ph↗

arXiv2025

Dielectric properties of aqueous electrolytes at the nanoscale

Despite the ubiquity of aqueous electrolytes, the effect of salt on water organization remains controversial. We introduce a nonlocal and nonlinear field theory for the nanoscale polarization of ions and water and derive the electrolyte dielectric response as a function of salt concentration to first order in a loop expansion. By comparison with molecular dynamics simulations, we show that rising salt concentration induces a dielectric permittivity decrement and Debye screening in the longitudinal susceptibility but leaves the water structure remarkably unchanged.

cond-mat.soft↗

arXiv2025

pH gradient-driven deformation of a crista-like vesicle

The inner membrane of mitochondria presents folds, the cristae, which are the production place of ATP. This synthesis is driven by a flow of protons confined to the surface of the membrane, which also shapes the crista to ensure a high synthesis rate. We model a crista as a spherical vesicle submitted to a diffusive proton gradient flowing from the poles to the equator. Using Helfrich model, we introduce a pH-dependent spontaneous curvature for the membrane and determine the shape of the vesicle, when submitted to the pH gradient, in the regime of small deformations. Based on biophysical arguments, we define a functionality score for the vesicle and construct a phase diagram identifying the zones of "well-functioning" cristae, which we compare to experimental measurements.

physics.bio-ph↗

arXiv2024

Nonlinear conductivity of aqueous electrolytes: beyond the first Wien effect

The conductivity of strong electrolytes increases under high electric fields, a nonlinear response known as the first Wien effect. Here, using molecular dynamics simulations we show that this increase is almost suppressed in moderately concentrated aqueous electrolytes due to the alignment of the water molecules by the electric field. As a consequence of this alignement, the permittivity of water decreases and becomes anisotropic, an effect which can be measured in simulations and reproduced by a model of water molecules as dipoles. We incorporate the resulting anisotropic interactions between the ions into a Stochastic Density Field Theory and calculate ionic correlations as well as corrections to the Nernst-Einstein conductivity, which are in qualitative agreement with the numerical simulations.

cond-mat.soft↗

arXiv2023

Universal Casimir attraction between filaments at the cell scale

The electromagnetic Casimir interaction between dielectric objects immersed in salted water includes a universal contribution that is not screened by the solvent and therefore long-ranged. Here, we study the geometry of two parallel dielectric cylinders. We derive the Casimir free energy by using the scattering method. We show that its magnitude largely exceeds the thermal energy scale for a large parameter range. This includes length scales relevant for actin filaments and microtubules in cells. We show that the Casimir free energy is a universal function of the geometry, independent of the dielectric response functions of the cylinders, at all distances of biological interest. While multiple interactions exist between filaments in cells, this universal attractive interaction should have an important role in the cohesion of bundles of parallel filaments.

physics.bio-ph↗

arXiv2023

A universal attractive interaction between filaments at the cell scale

Actin filaments and microtubules both often form bundles of parallel filaments within cells. Here, we shed light on a universal attractive interaction between two such parallel filaments. Indeed, the electrodynamic Casimir interaction between dielectric objects immersed in salted water at room or body temperature includes a universal contribution that is unscreened by the solvent and therefore long-ranged. We study this interaction between two parallel cylinders immersed in salted water with strong Debye screening. We show that its magnitude can largely exceed the energy scale of thermal fluctuations in the case of actin filaments and microtubules in cells. While multiple interactions exist between filaments in cells, this universal attractive interaction should thus have an important role, e.g. in bundle formation and cohesion.

physics.bio-ph↗

arXiv2023

A model of membrane deformations driven by a surface pH gradient

Many cellular organelles are membrane-bound structures with complex membrane composition and shape. Their shapes have been observed to depend on the metabolic state of the organelle, and the mechanisms that couple biochemical pathways and membrane shape are still actively investigated. Here, we study a model coupling inhomogeneities in the lipid composition and membrane geometry via a generalized Helfrich free energy. We derive the resulting stress tensor, the Green's function for a tubular membrane and compute the phase diagram of the induced deformations. We then apply this model to study the deformation of mitochondria cristae described as membrane tubes supporting a pH gradient at its surface. This gradient in turn controls the lipid composition of the membrane via the protonation/deprotonation of cardiolipins, which are acid-based lipids known to be crucial for mitochondria shape and functioning. Our model predicts the appearance of tube deformations resembling the observed shape changes of cristea when submitted to a proton gradient.

cond-mat.soft↗

arXiv2023

Coupled interactions at the ionic graphene/water interface

We compute ionic free energy adsorption profiles at aqueous graphene interface by developing a self-consistent approach. To do so, we design a microscopic model for water and put the liquid on an equal footing with the graphene described by its electronic band structure. By evaluating progressively the electronic/dipolar coupled electrostatic interactions, we show that the coupling level including mutual graphene/water screening permits to recover remarkably the precision of extensive quantum simulations. We further derive the potential of mean force evolution of several alkali cations.

cond-mat.soft↗

arXiv2021

The nonlocal dielectric response of water in nanoconfinement

Recent experiments reporting a very low dielectric permittivity for nanoconfined water have renewed the interest to the structure and dielectric properties of water in narrow gaps. Here, we describe such systems with a minimal Landau-Ginzburg field-theory composed of a nonlocal bulk-determined term and a local water-surface interaction term. We show how the interplay between the boundary conditions and intrinsic bulk correlations encodes dielectric properties of confined water. Our theoretical analysis is supported by molecular dynamics simulations and comparison with the experimental data.

physics.chem-ph↗

arXiv2020

Mitochondrial cristae modeled as an out-of-equilibrium membrane driven by a proton field

As the places where most of the fuel of the cell, namely ATP, is synthesized, mitochondria are crucial organelles in eukaryotic cells. The shape of the invaginations of the mitochondria inner membrane, known as cristae, has been identified as a signature of the energetic state of the organelle. However, the interplay between the rate of ATP synthesis and the crista shape remains unclear. In this work, we investigate the crista membrane deformations using a pH-dependent Helfrich model, maintained out-of-equilibrium by a diffusive flux of protons. This model gives rise to shape changes of a cylindrical invagination, in particular to the formation of necks between wider zones under variable, and especially oscillating, proton flux.

physics.bio-ph↗

arXiv2019

Dielectric response of the water hydration shell of an ion: a field theory approach

The goal of this work is to propose a simple continuous model that captures the dielectric properties of water at the nanometric scale. We write an electrostatic energy as a functional of the polarisation field containing a term in $P^4$ and non-local Gaussian terms. Such an hamiltonian can reproduce two key properties of water: the saturation of the polarisation response of water in the presence of a strong electrostatic field and the nanometric dipolar correlations of the solvent molecules modifying the long range van der waals interaction. This model explores thus two fundamental aspects that have to be included in implicit models of electrolytes for a relevant description of electrostatic interactions at nanometric scales.

physics.chem-ph↗

arXiv2017

Gaussian field model for polar fluids as a function of density and polarization: toward a model for water

This work is concerned with a simple model for a polar fluid, a Gaussian field model based on the excess density and on the polarization. It is a convenient framework to implement the dielectric properties of correlated liquids that stem from nanometric correlations between molecules. It allows to study the effects of coupling terms between density and polarization on the structure of the fluid. Despite the simplicity of such a model, it can capture some interesting features of the response functions of water such as the quasi-resonant longitudinal dielectric susceptibility or the presence of two maxima in the structure factor. Explicit models of water generate extremely high computational cost and implicit models sometimes fail to treat properly the electrostatic interactions. A Gaussian field theory could therefore be an interesting alternative to describe water.

physics.chem-ph↗

arXiv2016

Fluctuations in reactive networks subject to extrinsic noise studied in the framework of the Chemical Langevin Equation

Theoretical and experimental studies have shown that the fluctuations of in vivo systems break the fluctuation-dissipation theorem. One can thus ask what information is contained in the correlation functions of protein concentrations and how they relate to the response of the reactive network to a perturbation. Answers to these questions are of prime importance to extract meaningful parameters from the in vivo fluorescence correlation spectroscopy data. In this paper we study the fluctuations of the concentration of a reactive species involved in a cyclic network that is in a non-equilibrium steady state perturbed by a noisy force, taking into account both the breaking of detailed balance and extrinsic noises. Using a generic model for the network and the extrinsic noise, we derive a Chemical Langevin Equation that describes the dynamics of the system, we determine the expressions of the correlation functions of the concentrations, estimate the deviation of the fluctuation-dissipation theorem and the range of parameters in which an effective temperature can be defined.

physics.chem-ph↗