4 research outputs found
Dynamics of Hydration Water in Sugars and Peptides Solutions
We
analyzed solute and solvent dynamics of sugars and peptides
aqueous solutions using extended depolarized light scattering (EDLS)
and broadband dielectric spectroscopies (BDS). Spectra measured with
both techniques reveal the same mechanism of rotational diffusion
of peptides molecules. In the case of sugars, this solute reorientational
relaxation can be isolated by EDLS measurements, whereas its contribution
to the dielectric spectra is almost negligible. In the presented analysis,
we characterize the hydration water in terms of hydration number and
retardation ratio ξ between relaxation times of hydration and
bulk water. Both techniques provide similar estimates of ξ.
The retardation imposed on the hydration water by sugars is ∼3.3
± 1.3 and involves only water molecules hydrogen-bonded (HB)
to solutes (∼3 water molecules per sugar OH-group). In contrast,
polar peptides cause longer range perturbations beyond the first hydration
shell, and ξ between 2.8 and 8, increasing with the number of
chemical groups engaged in HB formation. We demonstrate that chemical
heterogeneity and specific HB interactions play a crucial role in
hydration dynamics around polar solutes. The obtained results help
to disentangle the role of excluded volume and enthalpic contributions
in dynamics of hydration water at the interface with biological molecules
Description of Hydration Water in Protein (Green Fluorescent Protein) Solution
The structurally and dynamically
perturbed hydration shells that
surround proteins and biomolecules have a substantial influence upon
their function and stability. This makes the extent and degree of
water perturbation of practical interest for general biological study
and industrial formulation. We present an experimental description
of the dynamical perturbation of hydration water around green fluorescent
protein in solution. Less than two shells (∼5.5 Å) were
perturbed, with dynamics a factor of 2–10 times slower than
bulk water, depending on their distance from the protein surface and
the probe length of the measurement. This dependence on probe length
demonstrates that hydration water undergoes subdiffusive motions (τ
∝ <i>q</i><sup>–2.5</sup> for the first hydration
shell, τ ∝ <i>q</i><sup>–2.3</sup> for
perturbed water in the second shell), an important difference with
neat water, which demonstrates diffusive behavior (τ ∝ <i>q</i><sup>–2</sup>). These results help clarify the seemingly
conflicting range of values reported for hydration water retardation
as a logical consequence of the different length scales probed by
the analytical techniques used
Structure and Hydration of Highly-Branched, Monodisperse Phytoglycogen Nanoparticles
Phytoglycogen
is a naturally occurring polysaccharide nanoparticle
made up of extensively branched glucose monomers. It has a number
of unusual and advantageous properties, such as high water retention,
low viscosity, and high stability in water, which make this biomaterial
a promising candidate for a wide variety of applications. In this
study, we have characterized the structure and hydration of aqueous
dispersions of phytoglycogen nanoparticles using neutron scattering.
Small angle neutron scattering results suggest that the phytoglycogen
nanoparticles behave similar to hard sphere colloids and are hydrated
by a large number of water molecules (each nanoparticle contains between
250% and 285% of its mass in water). This suggests that phytoglycogen
is an ideal sample in which to study the dynamics of hydration water.
To this end, we used quasielastic neutron scattering (QENS) to provide
an independent and consistent measure of the hydration number, and
to estimate the retardation factor (or degree of water slow-down)
for hydration water translational motions. These data demonstrate
a length-scale dependence in the measured retardation factors that
clarifies the origin of discrepancies between retardation factor values
reported for hydration water using different experimental techniques.
The present approach can be generalized to other systems containing
nanoconfined water
Mechanical Properties of Nanoscopic Lipid Domains
The lipid raft hypothesis presents
insights into how the cell membrane
organizes proteins and lipids to accomplish its many vital functions.
Yet basic questions remain about the physical mechanisms that lead
to the formation, stability, and size of lipid rafts. As a result,
much interest has been generated in the study of systems that contain
similar lateral heterogeneities, or domains. In the current work we
present an experimental approach that is capable of isolating the
bending moduli of lipid domains. This is accomplished using neutron
scattering and its unique sensitivity to the isotopes of hydrogen.
Combining contrast matching approaches with inelastic neutron scattering,
we isolate the bending modulus of ∼13 nm diameter domains residing
in 60 nm unilamellar vesicles, whose lipid composition mimics the
mammalian plasma membrane outer leaflet. Importantly, the bending
modulus of the nanoscopic domains differs from the modulus of the
continuous phase surrounding them. From additional structural measurements
and all-atom simulations, we also determine that nanoscopic domains
are in-register across the bilayer leaflets. Taken together, these
results inform a number of theoretical models of domain/raft formation
and highlight the fact that mismatches in bending modulus must be
accounted for when explaining the emergence of lateral heterogeneities
in lipid systems and biological membranes