12 research outputs found
Hydration-Shell Transformation of Thermosensitive Aqueous Polymers
Although
water plays a key role in the coilâglobule transition
of polymers and biomolecules, it is not clear whether a change in
water structure drives or follows polymer collapse. Here, we address
this question by using Raman multivariate curve resolution (Raman-MCR)
spectroscopy to investigate the hydration shell structure around polyÂ(<i>N</i>-isopropylacrylamide) (PNIPAM) and polyÂ(propylene oxide)
(PPO), both below and above the cloud point temperature at which the
polymers collapse and form mesoscopic polymer-rich aggregates. We
find that, upon clouding, the water surrounding long PNIPAM chains
transforms to a less ordered and more weakly hydrogen bonded structure,
while the water surrounding short PNIPAM and PPO chains remains similar
above and below the cloud point. Furthermore, microfluidic temperature
jump studies demonstrate that the onset of clouding precedes the hydration-shell
structural transformation, and thus the observed water structural
transformation is associated with ripening of aggregates composed
of long-chain polymers, on a time scale that is long compared to the
onset of clouding
Expulsion of Ions from Hydrophobic Hydration Shells
Raman
spectroscopy is combined with multivariate curve resolution
to quantify interactions between ions and molecular hydrophobic groups
in water. The molecular solutes in this study all have similar structures,
with a trimethyl hydrophobic domain and a polar or charged headgroup.
Our results imply that aqueous sodium and fluoride ions are strongly
expelled from the first hydration shells of the hydrophobic (methyl)
groups, while iodide ions are found to enter the hydrophobic hydration
shell, to an extent that depends on the methyl group partial charge.
However, our quantitative estimates of the corresponding ion binding
equilibrium constants indicate that the iodide concentration in the
first hydrophobic hydration shell is generally lower than that in
the surrounding bulk water, and so an iodide ion cannot be viewed
as having a true affinity for the molecular hydrophobic interface,
but rather is less strongly expelled from such an interface than fluoride
Influence of Cononsolvency on the Aggregation of Tertiary Butyl Alcohol in MethanolâWater Mixtures
The
term cononsolvency has been used to describe a situation in
which a polymer is less soluble (and so is more likely to collapse
and aggregate) in a mixture of two cosolvents than it is in either
one of the pure solvents. Thus, cononsolvency is closely related to
the suppression of protein denaturation by stabilizing osmolytes.
Here, we show that cononsolvency behavior can also influence the aggregation
of tertiary butyl alcohol in mixtures of water and methanol, as demonstrated
using both Raman multivariate curve resolution spectroscopy and molecular
dynamics simulations. Our results imply that cononsolvency results
from the cosolvent-mediated enhancement of the attractive (solvophobic)
mean force between nonpolar groups, driven by preferential solvation
of the aggregates, in keeping with WymanâTanford theory
Molecular Aggregation Equilibria. Comparison of Finite Lattice and Weighted Random Mixing Predictions
Molecular aggregation equilibria
are described using finite lattice
and mean field theoretical modeling strategies, both built upon a
random mixture reference system. The resulting predictions are compared
with each other for systems in which each aggregate consists of a
central solute molecule whose first coordination shell can accommodate
multiple bound ligands. Soluteâligand (direct) and ligandâligand
(cooperative) interactions are found to influence aggregate size distributions
in qualitatively different ways, as direct interactions produce a
shape-invariant transformation of the aggregate size distribution,
whereas cooperative interactions can lead to a vaporâliquid-like
transformation. When half the ligand binding sites are filled, the
corresponding aggregate size distributions are invariably unimodal
in the absence of cooperative interactions, but when the latter interactions
are attractive, the distributions are predicted to be bimodal below
and unimodal above a critical temperature. Mean field and finite lattice
predictions are found to be in globally good agreement with each other,
except under near-critical conditions, and even there, the predicted
average aggregate sizes and equilibrium constants are remarkably similar.
Potential applications of these theoretical predictions to the analysis
of experimental and molecular dynamics aggregation results are discussed
Quantifying the Nearly Random Microheterogeneity of Aqueous <i>tert</i>-Butyl Alcohol Solutions Using Vibrational Spectroscopy
The microheterogeneous structure of aqueous tert-butyl alcohol (TBA) solutions is quantified by combining experimental,
simulations, and theoretical results. Experimental Raman multivariate
curve resolution (Raman-MCR) CâH frequency shift measurements
are compared with predictions obtained using combined quantum mechanical
and effective fragment potential (QM/EFP) calculations, as well as
with molecular dynamics (MD), random mixture (RM), and finite lattice
(FL) predictions. The results indicate that the microheterogeneous
aggregation in aqueous TBA solutions is slightly less than that predicted
by MD simulations performed using either CHARMM generalized force
field (CGenFF) or optimized parameters for liquid simulations all
atom (OPLS-AA) force fields but slightly more than that in a self-avoiding
RM of TBA-like molecules. The results imply that the onset of microheterogeneity
in aqueous solutions occurs when solute contact free energies are
about an order of magnitude smaller than thermal fluctuations, thus
suggesting a fundamental bound of relevance to biological self-assembly
Micelle Structure and Hydrophobic Hydration
Despite
the ubiquity and utility of micelles self-assembled from
aqueous surfactants, longstanding questions remain regarding their
surface structure and interior hydration. Here we combine Raman spectroscopy
with multivariate curve resolution (Raman-MCR) to probe the hydrophobic
hydration of surfactants with various aliphatic chain lengths, and
either anionic (carboxylate) or cationic (trimethylammonium) head
groups, both below and above the critical micelle concentration. Our
results reveal significant penetration of water into micelle interiors,
well beyond the first few carbons adjacent to the headgroup. Moreover,
the vibrational C-D frequency shifts of solubilized deuterated <i>n</i>-hexane confirm that it resides in a dry, oil-like environment
(while the localization of solubilized benzene is sensitive to headgroup
charge). Our findings imply that the hydrophobic core of a micelle
is surrounded by a highly corrugated surface containing hydrated non-polar
cavities whose depth increases with increasing surfactant chain length,
thus bearing a greater resemblance to soluble proteins than previously
recognized
CO<sub>2</sub> Hydration Shell Structure and Transformation
The
hydration-shell of CO<sub>2</sub> is characterized using Raman
multivariate curve resolution (Raman-MCR) spectroscopy combined with <i>ab initio</i> molecular dynamics (AIMD) vibrational density
of states simulations, to validate our assignment of the experimentally
observed high-frequency OH band to a weak hydrogen bond between water
and CO<sub>2</sub>. Our results reveal that while the hydration-shell
of CO<sub>2</sub> is highly tetrahedral, it is also occasionally disrupted
by the presence of entropically stabilized defects associated with
the CO<sub>2</sub>-water hydrogen bond. Moreover, we find that the
hydration-shell of CO<sub>2</sub> undergoes a temperature-dependent
structural transformation to a highly disordered (less tetrahedral)
structure, reminiscent of the transformation that takes place at higher
temperatures around much larger oily molecules. The biological significance
of the CO<sub>2</sub> hydration shell structural transformation is
suggested by the fact that it takes place near physiological temperatures
Temperature-Dependent Hydrophobic Crossover Length Scale and Water Tetrahedral Order
Experimental Raman
multivariate curve resolution and molecular
dynamics simulations are performed to demonstrate that the vibrational
frequency and tetrahedrality of water molecules in the hydration-shells
of short-chain alcohols differ from those of pure water and undergo
a crossover above 100 °C (at 30 MPa) to a structure that is less
tetrahedral than pure water. Our results demonstrate that the associated
crossover length scale decreases with increasing temperature, suggesting
that there is a fundamental connection between the spectroscopically
observed crossover and that predicted to take place around idealized
purely repulsive solutes dissolved in water, although the water structure
changes in the hydration-shells of alcohols are far smaller than those
associated with an idealized âdewettingâ transition
Specific Ion Effects in Amphiphile Hydration and Interface Stabilization
Specific ion effects
can influence many processes in aqueous solutions:
protein folding, enzyme activity, self-assembly, and interface stabilization.
Ionic amphiphiles are known to stabilize the oil/water interface,
presumably by dipping their hydrophobic tails into the oil phase while
sticking their hydrophilic head groups in water. However, we find
that anionic and cationic amphiphiles adopt strikingly different structures
at liquid hydrophobic/water interfaces, linked to the different specific
interactions between water and the amphiphile head groups, both at
the interface and in the bulk. Vibrational sum frequency scattering
measurements show that dodecylsulfate (DS<sup>â</sup>) ions
do not detectably perturb the oil phase while dodecyltrimethylammonium
(DTA<sup>+</sup>) ions do. Raman solvation shell spectroscopy and
second harmonic scattering (SHS) show that the respective hydration-shells
and the interfacial water structure are also very different. Our work
suggests that specific interactions with water play a key role in
driving the anionic head group toward the water phase and the cationic
head group toward the oil phase, thus also implying a quite different
surface stabilization mechanism
Detection and Relative Quantification of Proteins by Surface Enhanced Raman Using Isotopic Labels
Detection and Relative Quantification of Proteins
by Surface Enhanced Raman Using Isotopic Label