18 research outputs found
Insight into the Molecular Mechanisms of Protein Stabilizing Osmolytes from Global Force-Field Variations
A prominent class of osmolytes that
are able to stabilize proteins
in their native fold consist of small highly water-soluble molecules
with a large dipole moment and hydrophobic groups attached to the
positively charged end of the molecule, for which we coin the term
dipolar/hydrophobic osmolytes. For TMAO, which is a prime member of
this class, we perform large-scale water-explicit MD simulations and
determine the bulk solution activity coefficient as well as the affinity
to a stretched polyglycine chain for varying TMAO dipolar strength
and hydrophobicity. Double optimization with respect to experimental
values for the activity coefficient and the polyglycine transfer free
energy is achieved. The resulting optimal TMAO force field shows excellent
transferability to different concentrations and also reproduces transfer
free energies of various amino acids, including the tryptophan anomaly,
for which TMAO acts as a denaturant. By globally analyzing the thermodynamic
and structural properties of suboptimal TMAO force fields, we identify
the frustration between dipolar and hydrophobic interactions as the
working mechanism and the design principle of dipolar/hydrophobic
osmolytes
The Complete Phase Diagram of Monolayers of Enantiomeric <i>N</i>‑Stearoyl-threonine Mixtures with Preferred Heterochiral Interactions
Langmuir monolayers
of chiral amphiphiles are well-controlled model
systems for the investigation of phenomena related to stereochemistry.
Here, we have investigated mixed monolayers of one pair of enantiomers
(l and d) of the amino-acid-based amphiphile N-stearoyl-threonine. The monolayer characteristics were
studied by pressure–area isotherm measurements and grazing
incidence X-ray diffraction (GIXD) over a wide range of mixing ratios
defined by the d-enantiomer mole fraction xD. While the isotherms provide insights into thermodynamical
aspects, such as transition pressure, compression/decompression hysteresis,
and preferential homo- and heterochiral interactions, GIXD reveals
the molecular structural arrangements on the Ångström
scale. Dominant heterochiral interactions in the racemic mixture lead
to compound formation and the appearance of a nonchiral rectangular
lattice, although the pure enantiomers form a chiral oblique lattice.
Miscibility was found to be limited to mixtures with 0.27 ≲ xD ≲ 0.73, as well as to both outer edges
(xD ≲ 0.08 and xD ≳ 0.92). Beyond this range, coexistence of oblique
and rectangular lattices occurs, as is clearly seen in the GIXD patterns.
Based on the results, a complete phase diagram with two eutectic points
at xD ≈ 0.25 and xD ≈ 0.75 is proposed. Moreover, N-stearoyl-threonine was found to have a strong tendency to form a
hydrogen-bonding network between the headgroups, which promotes superlattice
formation
Grazing-Incidence Neutron-Induced Fluorescence Probes Density Profiles of Labeled Molecules at Solid/Liquid Interfaces
We
report on the use of characteristic prompt γ-fluorescence
after neutron capture induced by an evanescent neutron wave to probe
densities and depth profiles of labeled molecules at solid/liquid
interfaces. In contrast to classical scattering techniques and X-ray
fluorescence, this method of “grazing-incidence neutron-induced
fluorescence” combines direct chemical specificity, provided
by the label, with sensitivity to the interface, inherent to the evanescent
wave. We demonstrate that the formation of a supported lipid membrane
can be quantitatively monitored from the characteristic fluorescence
of <sup>157</sup>Gd<sup>3+</sup> ions bound to the headgroup of chelator
lipids. Moreover, we were able to localize the <sup>157</sup>Gd<sup>3+</sup> ions along the surface normal with nanometer precision.
This first proof of principle with a well-defined model system suggests
that the method has a great potential for biology and soft matter
studies where spatial resolution and chemical sensitivity are required
Generic Role of Polymer Supports in the Fine Adjustment of Interfacial Interactions between Solid Substrates and Model Cell Membranes
To
understand the generic role of soft, hydrated biopolymers in adjusting
interfacial interactions at biological interfaces, we designed a defined
model of the cell–extracellular matrix contacts based on planar lipid
membranes deposited on polymer supports (polymer-supported membranes).
Highly uniform polymer supports made out of regenerated cellulose
allow for the control of film thickness without changing the surface
roughness and without osmotic dehydration. The complementary combination
of specular neutron reflectivity and high-energy specular X-ray reflectivity
yields the equilibrium membrane–substrate distances, which
can quantitatively be modeled by computing the interplay of van der
Waals interaction, hydration repulsion, and repulsion caused by the
thermal undulation of membranes. The obtained results help to understand
the role of a biopolymer in the interfacial interactions of cell membranes
from a physical point of view and also open a large potential to generally
bridge soft, biological matter and hard inorganic materials
Water Structuring Induces Nonuniversal Hydration Repulsion between Polar Surfaces: Quantitative Comparison between Molecular Simulations, Theory, and Experiments
Polar surfaces in water typically repel each other at
close separations,
even if they are charge-neutral. This so-called hydration repulsion
balances the van der Waals attraction and gives rise to a stable nanometric
water layer between the polar surfaces. The resulting hydration water
layer is crucial for the properties of concentrated suspensions of
lipid membranes and hydrophilic particles in biology and technology,
but its origin is unclear. It has been suggested that surface-induced
molecular water structuring is responsible for the hydration repulsion,
but a quantitative proof of this water-structuring hypothesis is missing.
To gain an understanding of the mechanism causing hydration repulsion,
we perform molecular simulations of different planar polar surfaces
in water. Our simulated hydration forces between phospholipid bilayers
agree perfectly with experiments, validating the simulation model
and methods. For the comparison with theory, it is important to split
the simulated total surface interaction force into a direct contribution
from surface–surface molecular interactions and an indirect
water-mediated contribution. We find the indirect hydration force
and the structural water-ordering profiles from the simulations to
be in perfect agreement with the predictions from theoretical models
that account for the surface-induced water ordering, which strongly
supports the water-structuring hypothesis for the hydration force.
However, the comparison between the simulations for polar surfaces
with different headgroup architectures reveals significantly different
decay lengths of the indirect water-mediated hydration-force, which
for laterally homogeneous water structuring would imply different
bulk-water properties. We conclude that laterally inhomogeneous water
ordering, induced by laterally inhomogeneous surface structures, shapes
the hydration repulsion between polar surfaces in a decisive manner.
Thus, the indirect water-mediated part of the hydration repulsion
is caused by surface-induced water structuring but is surface-specific
and thus nonuniversal
Combination of MD Simulations with Two-State Kinetic Rate Modeling Elucidates the Chain Melting Transition of Phospholipid Bilayers for Different Hydration Levels
The
phase behavior of membrane lipids plays an important role in
the formation of functional domains in biological membranes and crucially
affects molecular transport through lipid layers, for instance, in
the skin. We investigate the thermotropic chain melting transition
from the ordered <i>L</i><sub>β</sub> phase to the
disordered <i>L</i><sub>α</sub> phase in membranes
composed of dipalmitoylphosphatidylcholine (DPPC) by atomistic molecular
dynamics simulations in which the membranes are subject to variable
heating rates. We find that the transition is initiated by a localized
nucleus and followed by the propagation of the phase boundary. A two-state
kinetic rate model allows characterizing the transition state in terms
of thermodynamic quantities such as transition state enthalpy and
entropy. The extrapolated equilibrium melting temperature increases
with reduced membrane hydration and thus in tendency reproduces the
experimentally observed dependence on dehydrating osmotic stress
Neutron Reflectometry Elucidates Protein Adsorption from Human Blood Serum onto PEG Brushes
Poly(ethylene glycol)
(PEG) brushes are reputed for their ability
to prevent undesired protein adsorption to material surfaces exposed
to biological fluids. Here, protein adsorption out of human blood
serum onto PEG brushes anchored to solid-supported lipid monolayers
was characterized by neutron reflectometry, yielding volume fraction
profiles of lipid headgroups, PEG, and adsorbed proteins at subnanometer
resolution. For both PEGylated and non-PEGylated lipid surfaces, serum
proteins adsorb as a thin layer of approximately 10 Å, overlapping
with the headgroup region. This layer corresponds to primary adsorption
at the grafting surface and resists rinsing. A second diffuse protein
layer overlaps with the periphery of the PEG brush and is attributed
to ternary adsorption due to protein–PEG attraction. This second
layer disappears upon rinsing, thus providing a first observation
of the structural effect of rinsing on protein adsorption to PEG brushes
DSC thermograms of myelin (20 Mm) under different aqueous media.
<p>In the cases where homogenization takes place–bi-distilled water (black line) and near physiological medium (red line)–a maximum is reached after which a stabilization of the C<sub>p</sub> is observed near physiological temperature. In the case of high [Ca<sup>2+</sup>] (blue line), a continuous variation of C<sub>p</sub> according to a phase redistribution of components is observed.</p
Number (<i>n</i>) of correlated membranes for the different phases as a function of temperature for whole myelin (empty) and DIGs (filled).
<p>In black is marked the <i>n</i> for the native period, in red for the expanded period and in blue for the compact period. In the presence of physiological (diamonds) and high [Ca<sup>2+</sup>] (circles) media.</p
Neutron diffraction (planar system) periodicities compared to SAXS on equivalent conditions.
<p>Neutron diffraction (planar system) periodicities compared to SAXS on equivalent conditions.</p