Disruption of the deep eutectic solvent
(DES) nanostructure around
the dissolved solute upon addition of water is investigated by polarization-selective
two-dimensional infrared spectroscopy and molecular dynamics simulations.
The heterogeneous DES nanostructure around the solute is partially
retained up to 41 wt % of added water, although water molecules are
gradually incorporated in the solute’s solvation shell even
at lower hydration levels. Beyond 41 wt %, the solute is observed
to be preferentially solvated by water. This composition denotes the
upper hydration limit of the deep eutectic solvent above which the
solute senses an aqueous solvation environment. Interestingly, our
results indicate that the transition from a deep eutectic solvation
environment to an aqueous one around the dissolved solute can happen
at a hydration level lower than that reported for the “water
in DES” to “DES in water” transition.
Deep
eutectic solvents
(DESs) have gained popularity in recent years as an environmentally
benign, inexpensive alternative to organic solvents for diverse applications
in chemistry and biology. Among them, alcohol-based DESs serve as
useful media in various applications due to their significantly low
viscosity as compared to other DESs. Despite their importance as media,
little is known how their solvation dynamics change as a function
of the hydrocarbon chain length of the alcohol constituent. In order
to obtain insights into the chain-length dependence of the solvation
dynamics, we have performed two-dimensional infrared spectroscopy
on three alcohol-based DESs by systematically varying the hydrocarbon
chain length. The results reveal that the solvent dynamics slows down
monotonically with an increase in the chain length. This increase
in the dynamic timescales also shows a strong correlation with the
concomitant increase in the viscosity of DESs. In addition, we have
performed molecular dynamics simulations to compare with the experimental
results, thereby testing the capacity of simulations to determine
the amplitudes and timescales of the structural fluctuations on fast
timescales under thermal equilibrium conditions.
Deep
eutectic solvents, consisting of heterogeneous nanodomains
of hydrogen-bonded networks, have evolved into a range of viscous
fluids that find applications in several fields. As viscosity is known
to influence the dynamics of other neoteric solvents like ionic liquids,
understanding the effect of viscosity on deep eutectic solvents is
crucial to realize their full potential. Herein, we combine polarization-selective
pump–probe spectroscopy, two-dimensional infrared spectroscopy,
and molecular dynamics simulations to elucidate the impact of viscosity
on the dynamics of an alcohol-based deep eutectic solvent, ethaline.
We compare the solvent fluctuation and solute reorientation time scales
in ethaline with those in ethylene glycol, an ethaline constituent.
Interestingly, we find that the solute’s reorientation apparently
scales the bulk viscosity of the solvent, but the same is not valid
for the overall solvation dynamics. Using the variations in the estimated
intercomponent hydrogen bond lifetimes, we show that a dissolved solute
does not sense the bulk viscosity of the deep eutectic solvent; instead,
it senses domain-specific local viscosity determined by the making
and breaking of the hydrogen bond network. Our results indicate that
understanding the domain-specific local environment experienced by
the dissolved solute is of utmost importance in deep eutectic solvents.
The nanocrystal surface, which acts as an interface between the semiconductor lattice and the capping ligands, plays a significant role in the attractive photophysical properties of semiconductor nanocrystals for use in a wide range of applications. Replacing the long-chain organic ligands with short inorganic variants improves the conductivity and carrier mobility of nanocrystalbased devices. However, our current understanding of the interactions between the inorganic ligands and the nanocrystals is obscure due to the lack of experiments to directly probe the inorganic ligands. Herein, using twodimensional infrared spectroscopy, we show that the variations in the inorganic ligand dynamics within the heterogeneous nanocrystal ensemble can identify the diversities in the inorganic ligand−nanocrystal interactions. The ligand dynamics time scale in SCN − capped CdS nanocrystals identifies three distinct ligand populations and provides molecular insight into the nanocrystal surface. Our results demonstrate that the SCN − ligands engage in a dynamic equilibrium and stabilize the nanocrystals by neutralizing the surface charges through both direct binding and electrostatic interaction.
Dimethyl
sulfoxide (DMSO), a polar solvent molecule, is used in
a wide range of therapeutic and pharmacological applications. Different
intermolecular interactions, such as dimerization and hydrogen bonding
with water, are crucial to understanding the role of DMSO in applications.
Herein, we study DMSO in various solvation environments to decipher
the environment-dependent dimerization and hydrogen-bonding propensity.
We use a combination of infrared spectroscopy, quantum mechanical
calculations, and molecular dynamics simulations to reach our conclusions.
Although DMSO can exist in a dynamic equilibrium between monomers
and dimers, our results show that the relative intensity of the SO
stretch and the CH3 rocking modes is a spectroscopic indicator
of the extent of DMSO dimerization in solution. The dimerization (self-association)
is seen to be maximum in neat DMSO. When dissolved in different solvents,
the dimerization propensity decreases with increasing solvent polarity.
In the presence of a protic solvent, such as water, DMSO forms a hydrogen
bond with the solvent molecules, thereby reducing the extent of dimerization.
Further, we estimate the hydrogen-bond occupancy of DMSO. Our results
show that DMSO predominantly exists as doubly hydrogen-bonded in water.
The acid−base behavior of amino acids plays critical roles in several biochemical processes. Depending on the interactions with the protein environment, the pK a values of these amino acids shift from their respective solution values. As the side chains interact with the polypeptide backbone, a pH-induced change in the protonation state of aspartic and glutamic acids might significantly influence the structure and stability of a protein. In this work, we have combined two-dimensional infrared spectroscopy and molecular dynamics simulations to elucidate the pH-induced structural changes in an antimicrobial enzyme, lysozyme, over a wide range of pH. Simultaneous measurements of the carbonyl signals arising from the backbone and the acidic side chains provide detailed information about the pH dependence of the local and global structural features. An excellent agreement between the experimental and the computational results allowed us to obtain a residue-specific molecular understanding. Although lysozyme retains the helical structure for the entire pH range, one distinct loop region (residues 65−75) undergoes local structural deformation at low pH. Interestingly, combining our experiments and simulations, we have identified the aspartic acid residues in lysozyme, which are influenced the most/least by pH modulation.
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