Photophysical studies on the 4'-N,N-dimethylamino-3-hydroxyflavone fluorophore were performed in hydrogen-bonding solvents. Both in hydrogen-bonding acids and bases, clear evidence of excited state intramolecular proton transfer (ESIPT) emerged from steady-state and time-resolved spectroscopies. The same was also observed for the fluorophores residing in the hydrophilic shell region of aqueous micelles, where they come into close contact with water molecules at the micelle-water interface. Slow ∼100 ps ESIPT time-constants were determined in these systems that correlated well with solvation dynamics. The slow ESIPT time-constants are attributed to activated barrier crossing from the solvent-relaxed enol form to tautomer form in the excited state energy surface of the flavone. In contrast to the barrier-less ESIPT occurring in early (<1 ps) time-scales, this activated proton-transfer event necessarily requires extensive reorganization of flavone···solvent intermolecular hydrogen bonds, a process heavily modulated by the relatively slower dynamics of solvent relaxation around the excited fluorophore.
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.
Two azoreductases (I and II) were purified to homogeneity from extracts of Shigella dysenteriae (type 1). Azoreductase I was a dimer of identical subunits of M(r) 28,000, whereas azoreductase II was a monomer of 11,000 M(r). Both were flavoproteins, each containing 1 mol of FMN per mol enzyme. Both NADH and NADPH functioned as electron donors for the azoreductases. Azoreductase I used Ponceau SX, Tartrazine, Amaranth and Orange II as substrates. Azoreductase II utilized all the dyes except Amaranth.
Excited state intramolecular proton transfer (ESIPT) time-constants of 4'-N,N-dimethylamino-3-hydroxyflavone (DMA3HF) in high n-alcohols--1-butanol, 1-hexanol and 1-decanol--were measured to be 90 ps, 130 ps and 190 ps, respectively, which are unusually slow. At the same time, the solvation time-constants of the DMA3HF enol in the same set of solvents were measured as 100 ps, 150 ps and >300 ps, respectively. Thus, both the ESIPT and enol solvation time-constants in high n-alcohols increase monotonically with the alkyl chain-length of the solvent, although the increase is not strictly proportional. It appears that the H-bonding capacity of the solvent is the single major factor influencing both processes, causing them to become closely correlated. Solvation causes a drastic change in the solvent molecular configuration around the excited enol, E*, inducing the breakage of DMA3HF···solvent inter-molecular H-bonding, which in turn promotes ESIPT. Following previously reported theoretical work on ESIPT, a qualitative description of the S1 potential energy surface can be formulated, where the involvement of solvent relaxation with the ESIPT process is explained.
Deep
eutectic solvents have emerged as inexpensive green alternatives
to conventional solvents for diverse applications in chemistry and
biology. Despite their importance as useful media in various applications,
little is known about the microscopic solvation structures of deep
eutectic solvents around solutes. Herein, we show that the electrostatic
field, which can be estimated both from infrared experiments and theory,
can act as a unified concept to report on the microscopic heterogeneous
solvation of deep eutectic solvents. Using a fluorophore containing
the carbonyl moiety as the solute and the electrostatic field as a
descriptor of the solvation structure of the deep eutectic solvents,
we report the residue-specific distribution, orientation, and hydrogen
bonding in deep eutectic solvents constituting of choline chloride
and alcohols of varying chain-lengths. We observe that an increase
in alcohol chain-length not only affects the alcohol’s propensity
to form hydrogen bond to the solute but also alters the spatial arrangement
of choline cations around the solute, thereby leading to a microheterogeneity
in the solvation structure. Moreover, to extend our electrostatic
field based strategy to other deep eutectic solvents, we report an
emission spectroscopy based method. We show that this method can be
applied, in general, to all deep eutectic solvents, irrespective of
their constituents. Overall, this work integrates experiments with
molecular dynamics simulations to provide insights into the heterogeneous
DES solvation.
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