Conformation and self-assembly of a nystatin nitrobenzoxadiazole derivative in lipid membranes

Nystatin is a polyene (tetraene) macrolide antibiotic presenting antifungal activity that acts at the cellular membrane level. In the present study, we report the interaction of this antibiotic labelled at its amine group with 7-nitrobenz-2-oxa-1,3-diazole (NBD-Nys) with sterol-free and ergosterol- and cholesterol-containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) large unilamellar vesicles (LUV). The mean tetraene to NBD separating distance determined from fluorescence energy transfer measurements increased from 18 to 25.6 A upon antibiotic binding to the lipid vesicles, indicating that the monomeric labelled antibiotic adopts a more extended conformation in its lipid-bound state than in aqueous solution. The oligomeric state of membrane-bound NBD-Nys was also studied by resonance energy homotransfer between the NBD fluorophores. The decrease measured in its steady state fluorescence anisotropy upon increasing the surface concentration of the NBD-Nys is shown to be consistent with a random distribution of molecules on the surface of the liposomes. This data contradicts the sharp increase measured for nystatin mean fluorescence lifetime in the presence of 10 mol% ergosterol-containing POPC LUV within the same antibiotic concentration range and which is known to report nystatin oligomerization in the lipid vesicles. Therefore, we conclude that the amine group of nystatin is an essential requisite for the supramolecular organization/pore formation of this antibiotic.