Temperature-Dependent In-Plane Structure Formation
of an X‑Shaped Bolapolyphile within Lipid Bilayers
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Abstract
Polyphilic compound B12 is an X-shaped
molecule with a stiff aromatic
core, flexible aliphatic side chains, and hydrophilic end groups.
Forming a thermotropic triangular honeycomb phase in the bulk between
177 and 182 °C but no lyotropic phases, it is designed to fit
into DPPC or DMPC lipid bilayers, in which it phase separates at room
temperature, as observed in giant unilamellar vesicles (GUVs) by fluorescence
microscopy. TEM investigations of bilayer aggregates support the incorporation
of B12 into intact membranes. The temperature-dependent behavior of
the mixed samples was followed by differential scanning calorimetry
(DSC), FT-IR spectroscopy, fluorescence spectroscopy, and X-ray scattering.
DSC results support in-membrane phase separation, where a reduced
main transition and new B12-related transitions indicate the incorporation
of lipids into the B12-rich phase. The phase separation was confirmed
by X-ray scattering, where two different lamellar repeat distances
are visible over a wide temperature range. Polarized ATR-FTIR and
fluorescence anisotropy experiments support the transmembrane orientation
of B12, and FT-IR spectra further prove a stepwise “melting”
of the lipid chains. The data suggest that in the B12-rich domains
the DPPC chains are still rigid and the B12 molecules interact with
each other via π–π interactions. All results obtained
at temperatures above 75 °C confirm the formation of a single,
homogeneously mixed phase with freely mobile B12 molecules