7 research outputs found
Sitting at the edge: How biomolecules use hydrophobicity to tune their interactions and function
Water near hydrophobic surfaces is like that at a liquid-vapor interface,
where fluctuations in water density are substantially enhanced compared to that
in bulk water. Here we use molecular simulations with specialized sampling
techniques to show that water density fluctuations are similarly enhanced, even
near hydrophobic surfaces of complex biomolecules, situating them at the edge
of a dewetting transition. Consequently, water near these surfaces is sensitive
to subtle changes in surface conformation, topology, and chemistry, any of
which can tip the balance towards or away from the wet state, and thus
significantly alter biomolecular interactions and function. Our work also
resolves the long-standing puzzle of why some biological surfaces dewet and
other seemingly similar surfaces do not.Comment: 12 pages, 4 figure
Structure, Stability, and Rupture of Free and Supported Liquid Films and Assemblies in Molecular Simulations
A Molecular Thermodynamic Model of Coacervation in Solutions of Polycations and Oppositely Charged Micelles
To guide the rational design of personal care formulations,
we
formulate a molecular thermodynamic model that predicts coacervation
from cationic polymers and mixed micelles containing neutral and anionic
surfactants and added salt. These coacervates, which form as a result
of dilution of conditioning shampoos during use, deposit conditioning
agents and other actives to the scalp or skin and also provide lubrication
benefits. Our model accounts for mixing entropy, hydrophobic interactions
of polycation with water, free energies of bindings of oppositely
charged groups to micelles and polycations, and electrostatic interactions
that capture connectivity of charged groups on the polycation chain
and the micelle. The model outputs are the compositions of surfactants,
polycation, salt, and water in the coacervate and in its coexisting
dilute phase, along with the binding fractions and coacervate volume
fraction. We study the effects of overall composition (of surfactant,
polycation, and added salt), charge fractions on micelles and polycations,
and binding free energies on the phase diagram of coacervates. Then,
we perform coacervation experiments for three systems: sodium dodecyl
sulfate (SDS)–JR30M, sodium methyl cocoyl taurate (Taurate)–JR30M,
and sodium lauryl alaninate (Alaninate)–JR30M, where JR30M
is a cationic derivative of hydroxyethylcellulose (cat-HEC), and rationalize
their coacervation data using our model. For comparison with experiment,
we also develop a parametrization scheme to obtain the requisite binding
energies and Flory–Huggins χ parameter. We find that
our model predictions agree reasonably well with the experimental
data, and that the sulfate-free surfactants of Taurate and Alaninate
display much larger 2-phase regions compared to SDS with JR30M
Sitting at the Edge: How Biomolecules use Hydrophobicity to Tune Their Interactions and Function
Water near extended hydrophobic surfaces is like that
at a liquid–vapor
interface, where fluctuations in water density are substantially enhanced
compared to those in bulk water. Here we use molecular simulations
with specialized sampling techniques to show that water density fluctuations
are similarly enhanced, even near hydrophobic surfaces of complex
biomolecules, situating them at the edge of a dewetting transition.
Consequently, water near these surfaces is sensitive to subtle changes
in surface conformation, topology, and chemistry, any of which can
tip the balance toward or away from the wet state and thus significantly
alter biomolecular interactions and function. Our work also resolves
the long-standing puzzle of why some biological surfaces dewet and
other seemingly similar surfaces do not
Probing Additive Loading in the Lamellar Phase of a Nonionic Surfactant: Gibbs Ensemble Monte Carlo Simulations Using the SDK Force Field
Understanding
solute uptake into soft microstructured materials,
such as bilayers and worm-like and spherical micelles, is of interest
in the pharmaceutical, agricultural, and personal care industries.
To obtain molecular-level insight on the effects of solutes loading
into a lamellar phase, we utilize the Shinoda–Devane–Klein
(SDK) coarse-grained force field in conjunction with configurational-bias
Monte Carlo simulations in the osmotic Gibbs ensemble. The lamellar
phase is comprised of a bilayer formed by triethylene glycol mono-<i>n</i>-decyl ether (C10E3) surfactants surrounded by water with
a 50:50 surfactant/water weight ratio. We study both the unary adsorption
isotherm and the effects on bilayer structure and stability caused
by <i>n</i>-nonane, 1-hexanol, and ethyl butyrate at several
different reduced reservoir pressures. The nonpolar <i>n</i>-nonane molecules load near the center of the bilayer. In contrast,
the polar 1-hexanol and ethyl butyrate molecules both load with their
polar bead close to the surfactant head groups. Near the center of
the bilayer, none of the solute molecules exhibits a significant orientational
preference. Solute molecules adsorbed near the polar groups of the
surfactant chains show a preference for orientations perpendicular
to the interface, and this alignment with the long axis of the surfactant
molecules is most pronounced for 1-hexanol. Loading of <i>n</i>-nonane leads to an increase of the bilayer thickness, but does not
affect the surface area per surfactant. Loading of polar additives
leads to both lateral and transverse swelling. The reduced Henry’s
law constants of adsorption (expressed as a molar ratio of additive
to surfactant per reduced pressure) are 0.23, 1.4, and 14 for <i>n</i>-nonane, 1-hexanol, and ethyl butyrate, respectively, and
it appears that the SDK force field significantly overestimates the
ethyl butyrate–surfactant interactions