Fluorescence properties of polyene antibiotics in phospholipid bilayer membranes

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Abstract
The fluorescence characteristics of the polyene antibiotic Nystatin have been studied by measurements of quantum yields, lifetimes, and anisotropies in a model bilayer membrane. These measurements have been performed as a function of temperature and fluorophore-to-phospholipid molar ratio. Comparisons with data available for the parinaric acids demonstrate that the photophysics of the two polyene chromophores is similar. The quantum yield decreases at increasing Nystatin densities while the lifetimes are constant. These observations combined with quantitative comparisons with calculations of the density dependence of dynamic quenching in two-dimensional systems show that Nystatin fluorescence is quenched in bilayer membranes by a static process. This is likely due to the formation of complexes within the bilayer. The quenching is equally efficient in both the gel and liquid crystalline phases of the bilayer which suggests that the formation of the complexes occurs at all temperatures. At high Nystatin densities, there is also a quenching of the polarization, but the mechanism whereby this occurs is not clear. The limiting anisotropy at low concentrations is between 0.33 and 0.35 at all temperatures, suggesting that the Nystatin monomer is highly restricted in its reorientational motion in both gel and liquid crystalline phases.