Effects of Amphiphile Topology on the Aggregation of Oligocholates in Lipid Membranes: Macrocyclic versus Linear Amphiphiles

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. Disciplines Chemistry CommentsReprinted (adapted) with permission from Langmuir 28 (2012) 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.

Aggregation and Dynamics of Oligocholate Transporters in Phospholipid Bilayers Revealed by Solid-State NMR Spectroscopy

“…Filling the interior of CT with lipid chains, however, does not seem favorable from an energetic point of view. An alternative possibility, which is more consistent with the previous fluorescent data, 32 is that water molecules may be present in the interior of the macrocycle in the DLPC bilayer but may not form a continuous conduit to the membrane surface. In comparison to a TM channel with rapidly exchanging water molecules with the surface water, the CT molecules in DLPC bilayers under such a circumstance would experience much fewer water molecules in a given period of time, causing water cross peaks to develop more slowly.…”

“…Fluorescent data suggested that the linear trimer had a stronger preference for the membrane surface than the cyclic analogue. 32 Even though the linear trimer can aggregate intermolecularly at high concentrations and migrate into the hydrophobic core of the membrane, the aggregates seem to be quite unstable and in rapid equilibrium with the surface-bound species, which are expected to be quite mobile. The dynamics of LT observed in the 13 C− 1 H dipolar coupling data is consistent with its surface binding.…”

“…The study yielded significant insight into the relationship between the amphiphile topology and its aggregation in the membrane environment. 32 Solid-state NMR spectroscopy is a powerful technique to study structure, dynamics, and topology of membrane-bound proteins and other macromolecules. 33,34 13 C and 15 N chemical shifts give information about the conformation of the membranebound species while lipid−protein interactions can be investigated by using various 2D correlation NMR experiments.…”

“…The main support for the hydrophobically driven pore formation so far was from leakage assays. , Spectroscopic evidence was also obtained from the excimer emission of pyrene-labeled macrocycles such as 4 . , More recently, we took advantage of the environmental sensitivity of the dansyl group and studied compounds 5 and 6 by a number of fluorescence techniques. The study yielded significant insight into the relationship between the amphiphile topology and its aggregation in the membrane environment …”

Preparation and characterization polyvinylidene fluoride membranes from water and ethanol coagulants via in situ free radical polymerization

“…In colloid and materials research, self‐assembly of amphiphilic copolymer in solution is always a key focus . Even the simplest class of amphiphiles, the head or tail surfactants, at a specific and narrow concentration range of amphiphile in solution, named the critical micelle concentration, can form a wide variety of self‐assembled colloidal‐sized particles termed micelles depending on their relative volumes of the hydrophilic and hydrophobic components . For the formation of micelles, the hydrophilic portion of the amphiphile from solution minimizing unfavorable interactions between the surrounding organic solvent molecules and the hydrophilic groups of the amphiphile is effectively removed .…”