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Evolution of Aggregate Structure in Solutions of Anionic Monorhamnolipids: Experimental and Computational Results
journal contribution
posted on 2017-07-22, 13:48 authored by Ryan J. Eismin, Elango Munusamy, Laurel L. Kegel, David E. Hogan, Raina M. Maier, Steven D. Schwartz, Jeanne E. PembertonThe evolution of
solution aggregates of the anionic form of the
native monorhamnolipid (mRL) mixture produced by Pseudomonas
aeruginosa ATCC 9027 is explored at pH 8.0 using both experimental
and computational approaches. Experiments utilizing surface tension
measurements, dynamic light scattering, and both steady-state and
time-resolved fluorescence spectroscopy reveal solution aggregation
properties. All-atom molecular dynamics simulations on self-assemblies
of the most abundant monorhamnolipid molecule, l-rhamnosyl-β-hydroxydecanoyl-β-hydroxydecanoate
(Rha-C10-C10), in its anionic state explore the formation of aggregates
and the role of hydrogen bonding, substantiating the experimental
results. At pH 8.0, at concentrations above the critical aggregation
concentration of 201 μM but below ∼7.5 mM, small premicelles
exist in solution; above ∼7.5 mM, micelles with hydrodynamic
radii of ∼2.5 nm dominate, although two discrete populations
of larger lamellar aggregates (hydrodynamic radii of ∼10 and
90 nm) are also present in solution in much smaller number densities.
The critical aggregation number for the micelles is determined to
be ∼26 monomers/micelle using fluorescence quenching measurements,
with micelles gradually increasing in size with monorhamnolipid concentration.
Molecular dynamics simulations on systems with between 10 and 100
molecules of Rha-C10-C10 indicate the presence of stable premicelles
of seven monomers with the most prevalent micelle being ∼25
monomers and relatively spherical. A range of slightly larger micelles
of comparable stability can also exist that become increasing elliptical
with increasing monomer number. Intermolecular hydrogen bonding is
shown to play a significant role in stabilization of these aggregates.
In total, the computational results are in excellent agreement with
the experimental results.
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Keywords
monomerMolecular dynamics simulationsmonorhamnolipid201 μ Mevolutionhydrodynamic radiifluorescence quenching measurementsmicellemoleculetime-resolved fluorescence spectroscopypHpremicellesurface tension measurementsmMsolution aggregation propertiesconcentrationPseudomonas aeruginosa ATCC 9027nmroleaggregateRha-C 10-C
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