Unravelling the Effects of Cholesterol on the Second-Order
Nonlinear Optical Responses of Di-8-ANEPPS Dye Embedded in Phosphatidylcholine
Lipid Bilayers
posted on 2021-09-07, 13:45authored byCharlotte Bouquiaux, Frédéric Castet, Benoît Champagne
Cholesterol is known for its role in maintaining the correct fluidity
and rigidity of the animals cell membranes and thus their functions.
Assessing the content and the role of cholesterol in lipid bilayers
is therefore of crucial importance for a deeper understanding and
control of membrane functioning. In this computational work, we investigate
bilayers built from three types of glycerophospholipid phosphatidylcholine
(PC) lipids, namely dipalmitoylphosphatidylcholine (DPPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and dioleoylphosphatidylcholine
(DOPC), and containing different amounts of cholesterol by analyzing
the second-harmonic generation (SHG) nonlinear optical (NLO) response
of a probe molecule, di-8-ANEPPS, inserted into the membranes. This
molecular property presents the advantage to be specific to interfacial
regions such as lipid bilayers. To unravel these effects, Molecular
Dynamics (MD) simulations have been performed on both DPPC and DOPC
lipids by varying the cholesterol mole fraction (from 0 to 0.66),
while POPC was only considered as a pure bilayer. In the case of the
structural properties of the bilayers, all the analyses converge toward
the same conclusion: as the mole fraction of cholesterol increases,
the systems become more rigid, confirming the condensing effect of
cholesterol. In addition, the chromophore is progressively more aligned
with respect to the normal to the bilayer. On the contrary, addition
of unsaturation disorders the lipid bilayers, with barely no impact
on the alignment of the chromophore. Then, using the frames obtained
from the MD simulations, the first hyperpolarizability β of
the dye in its environment has been computed at the TDDFT level. On
the one hand, the addition of cholesterol induces a progressive increase
of the diagonal component the β tensor parallel to the bilayer
normal. On the other hand, larger β values have been calculated
for the unsaturated than for the saturated lipid systems. In summary,
this study illustrates the relationship between the composition and
structure of the bilayers and the NLO responses of the embedded dye.