Effects of Amphiphile
Topology on the Aggregation
of Oligocholates in Lipid Membranes: Macrocyclic versus Linear Amphiphiles
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Abstract
A macrocyclic and a linear trimer of a facially amphiphilic
cholate
building block were labeled with a fluorescent dansyl group. The environmentally
sensitive fluorophore enabled the aggregation of the two oligocholates
in lipid membranes to be studied by fluorescence spectroscopy. Concentration-dependent
emission wavelength and intensity revealed a higher concentration
of water for the cyclic compound. Both compounds were shown by the
red-edge excitation shift (REES) to be located near the membrane/water
interface at low concentrations, but the cyclic trimer was better
able to migrate into the hydrophobic core of the membrane than the
linear trimer. Fluorescent quenching by a water-soluble (NaI) and
a lipid-soluble (TEMPO) quencher indicated that the cyclic trimer
penetrated into the hydrophobic region of the membrane more readily
than the linear trimer, which preferred to stay close to the membrane
surface. The fluorescent data corroborated with the previous leakage
assays that suggested the stacking of the macrocyclic cholate trimer
into transmembrane nanopores, driven by the strong associative interactions
of water molecules inside the macrocycles in a nonpolar environment