How Far Is “Bulk Water” from Interfaces? Depends on the Nature of the Surface and What We Measure

Abstract

Using systematic molecular dynamics (MD) simulations, we revisit the question: At what distance from an interface do the properties of "bulk water" get recovered? We have considered three different kinds of interfaces: nonpolar (hydrophobic; isooctane−water interface), charged (negative; AOT bilayer), and polar (zwitterionic; POPC bilayer). In order to interrogate the extent of perturbation of the interfacial water molecules as a function of the distance from the interface, we utilize a diverse range of structural and dynamical parameters. To capture the structural perturbations, we look into local density (translational order), local tetrahedral order parameter, and dipolar orientation of the water molecules. We also explore the anisotropic diffusion of the water molecules in the direction perpendicular to the interface as well as the planar diffusion parallel to the interface in a distance dependent manner. In addition, the orientational time correlation functions have been computed to understand the extent of slowdown in the rotational dynamics. As expected, the electrostatic field emanating from the charged AOT interface seems to have the highest long-range effect on the orientational order and dynamics of the water molecules, whereas specific interactions like hydrogen bonding and electrostatic interaction lead to significant trapping and kinetic slowdown for both AOT and POPC (zwitterionic) very close to the interface. Our analysis highlights that not only the length-scale of perturbation depends on the nature of the interfaces and specific interactions but also the type of water property that we measure/calculate. Different water properties seem to have widely different length-scale of perturbation. Orientational order parameters seem to be perturbed to a much longer length-scale as compared to translational order parameters. The global orientational order of water can be perturbed even up to ∼4− 5 nm near the negatively charged AOT surface in the absence of any extra electrolyte. This observation has significant implication toward the interpretation of experimental measurements as well since different spectroscopic techniques would probe different parameters or water properties with possible mutual disagreement and inconsistency between different types of measurements. Thus, our study provides a broader and unifying perspective toward the aspect of "context dependent" structural and dynamical perturbation of "interfacial water".

Lipid/water interface of galactolipid bilayers in different lyotropic liquid-crystalline phases

“…( Gawrisch et al, 1978 ; Volkov et al, 2007a ; Volkov et al, 2007b ; Disalvo et al, 2008 ; Zhao et al, 2008 ; Beranova et al, 2012 ; Cheng et al, 2014 ; Pokorna et al, 2014 ), although more detailed information about its properties has been provided by computer modelling, e.g. ( Rog et al, 2009 ; Berkowitz and Vacha, 2012 ; Disalvo et al, 2014 ; Nickels et al, 2015 ; Pasenkiewicz-Gierula et al, 2016 ; Laage et al, 2017 ; Elola and Rodriguez, 2018 ; Martelli et al, 2018 ; Srivastava and Debnath, 2018 ; Tian and Chiu, 2018 ; Calero and Franzese, 2019 ; Kucerka et al, 2019 ; Srivastava et al, 2019 ; Deplazes et al, 2020 ; Luo et al, 2020 ; Szczelina et al, 2020 ; Hande and Chakrabarty, 2022 ).…”

Interplay of Hydropathy and Heterogeneous Diffusion in the Molecular Transport Within Lamellar Lipid Mesophases

“…In the previous formula, t is time and dt is the integration timestep, while ξ x , ξ y and ξ z are random variables distributed according to a Gaussian function with zero average and unit variance. Although molecular dynamics simulations indicate that diffusion in parallel and perpendicular directions to the lipid/water interface are distinct [8,39], for simplicity in Eq. ( 3) we consider, for a given position, the same diffusion coefficient for the random movement in any direction.…”

“…Theoretical considerations and computational simulations can provide a complementary insight into these systems, due to their faster and cheaper implementation, but also because they allow to address the effect of the various factors one at a time, which is not always possible in experiments. Although there is a vast literature focused on the modelling of diffusion in confined environments [34,35] and on the exotic water mobility at the interface with hydrophilic objects [36][37][38][39], little work has studied these topics within the context of lipid mesophases [8,28,29,40].…”

Spatially Resolved Hydration Thermodynamics in Biomolecular Systems

“…There is no abrupt physical boundary between the PHL and bulk water, and the answer to that question depends on the quantity (or observable) that is probed and on the technique used. 69 , 70 While some techniques, such as NMR, probe single-particle dynamics and are most sensitive to short-ranged interactions with the first hydration layer, other methods such as terahertz spectroscopy probe the collective response of water molecules in an extended H-bonded network and thus longer-ranged effects. 71 The modulations of the structure and dynamics of the PHL result from a combined effect of different local interactions between the protein surface and water molecules, and thus a more detailed picture would be desirable.…”

“…In contrast, the protein hydration layer (PHL) that surrounds a protein is more loosely interacting with the surface and is characterized by a heterogeneous distribution of structural and dynamic properties. ,, The question up to what length scale the presence of a protein (or other biomolecular) surface perturbs the water around it has been a matter of debate in the literature. There is no abrupt physical boundary between the PHL and bulk water, and the answer to that question depends on the quantity (or observable) that is probed and on the technique used. , While some techniques, such as NMR, probe single-particle dynamics and are most sensitive to short-ranged interactions with the first hydration layer, other methods such as terahertz spectroscopy probe the collective response of water molecules in an extended H-bonded network and thus longer-ranged effects . The modulations of the structure and dynamics of the PHL result from a combined effect of different local interactions between the protein surface and water molecules, and thus a more detailed picture would be desirable.…”