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Photophysical Behavior and Fluorescence Quenching of l‑Tryptophan in Choline Chloride-Based Deep Eutectic Solvents
journal contribution
posted on 2019-08-22, 20:07 authored by Anu Kadyan, Shreya Juneja, Siddharth PandeyIntrinsic fluorescence
from l-tryptophan (l-Trp)
is routinely used to obtain insight into the structural features and
dynamics of proteins and enzymes. In contrast to aqueous enzymology,
different parameters that control and influence the behavior of proteins
and enzymes in nonaqueous media depend heavily on the solvent. Detailed
analysis of the intrinsic fluorescence from l-Trp dissolved
in two deep eutectic solvents (DESs), reline and glyceline, prepared
by mixing salt choline chloride with H-bond donors urea and glycerol,
respectively, in a 1:2 molar ratio within 298.15–358.15 K temperature
range, is presented. Fluorescence emission maxima of l-Trp
dissolved in DESs show bathochromic shift with increasing temperature.
In comparison to water and several organic solvents, the fluorescence
quantum yields of l-Trp in both DESs are significantly higher.
While the rates of nonradiative decay in the two DESs are comparable
and increase with increasing temperature, radiative decay rates are
independent of temperature and are higher in glyceline than in reline,
resulting in a higher fluorescence quantum yield of l-Trp
in glyceline. Excited-state emission intensity decays of l-Trp fit best to a double exponential model irrespective of the identity
of the DES and temperature. Average lifetime decreases with increasing
temperature due to increased thermal deactivation; however, this decrease
is much slower in DESs as compared to that in water. Both steady-state
fluorescence anisotropy and rotational reorientation times for l-Trp are governed by the inherent complexity of the DESs as
solubilizing milieu resulting in noncompliance to simple hydrodynamic
treatment. Fluorescence quenching of l-Trp by acrylamide
in reline is purely dynamic in nature. This is in contrast to the
aqueous media where the decrease in fluorescence is a combined result
of both dynamic and static quenching. The quenching within reline
is fairly efficient considering the high viscosity of the medium.
Significantly lower activation energy of the bimolecular quenching
process as compared to the activation energy of the viscous flow indicates
facilitation of the electron/charge transfer quenching of l-Trp by acrylamide within the ionic environment offered by reline.
The effect of high viscosity is partly overcome by the strongly ionic
environment of reline during the electron/charge transfer between l-Trp and acrylamide. The results highlight the structural complexity
of these DESs especially within the cybotactic region of the probe,
which is absent in common molecular solvents of similar high viscosity.
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Choline Chloride-Based Deep Eutectic Solvents Intrinsic fluorescencesalt choline chlorideH-bond donors urearelineFluorescence emission maximal-TrpquenchingExcited-state emission intensityAverage lifetime decreasesfluorescence quantum yieldsradiative decay ratesDESs show bathochromic shiftactivation energy
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