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How Heterogeneous Are Trehalose/Glycerol Cryoprotectant Mixtures? A Combined Time-Resolved Fluorescence and Computer Simulation Investigation
journal contribution
posted on 2016-10-10, 00:00 authored by Sandipa Indra, Ranjit BiswasHeterogeneity and molecular motions
in representative cryoprotectant mixtures made of trehalose and glycerol
are investigated in the temperature range 298 ≤ T (K) ≤ 353, via time-resolved fluorescence Stokes shift and
anisotropy measurements, and molecular dynamics simulations of four-point
density–time correlations and H-bond relaxations. Mixtures
containing 5 and 20 wt % of trehalose along with neat glycerol are
studied. Viscosity coefficients for these systems lie in the range
0.30 < η (P) < 23. Measured solute (Coumarin 153) rotation
and solvation times reveal a substantial departure from the hydrodynamic
viscosity dependence, suggesting the strong microheterogeneous nature
of these systems. Fluorescence anisotropy decays are highly nonexponential,
reflecting a non-Markovian character of the medium friction. A complete
missing of the Stokes shift dynamics in these systems at 298 K but
partial detection of it at other higher temperatures (shift magnitude
being ∼400–600 cm–1) indicates rigid
solute environments. An amorphous solid-like feature emerges in the
simulated radial distribution functions at these temperatures. Analyses
of mean squared displacements reveal rattling-in-a-cage motion, non-Gaussian
displacement distributions, and strong dynamic heterogeneity features.
Simulated dynamic structure factors and four-point correlations hint,
respectively, at very long α-relaxation and correlated time
scales at 298 K. This explains the long solute rotation times (∼80–200
ns) measured at 298 K. Stretched exponential decay of the simulated
H-bond relaxations with long time scales further highlights the strong
temporal heterogeneity and slow dynamics inherent to these systems.
In summary, this work provides the first insight into the molecular
motions and interspecies interaction in a representative cryoprotectant
mixture, and stimulates further study to investigate the interconnection
between cryoprotection and dynamic heterogeneity.
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Keywords
MixtureheterogeneityFluorescencerotationtime-resolved fluorescence Stokes shiftanisotropyH-bond relaxationsglycerolsoluteComputer Simulation Investigation Heterogeneityrepresentative cryoprotectant mixturestrehalosetime scaleshydrodynamic viscosity dependencerepresentative cryoprotectant mixturenon-Gaussian displacement distributionsStokes shift dynamics298 Kcorrelation
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