Fluorescence study of the macrolide pentaene antibiotic filipin in aqueous solution and in a model system of membranes

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Abstract
The polyene antibiotic filipin (a pentaene) has been studied using photophysical techniques. The polyene self‐aggregates in water with a critical micellar concentration of 2 μM. Two approaches were used to evaluate the aggregate dimensions: (a) a lower limit of 10 nm for the aggregate radius was obtained from energy transfer experiments; (b) a formula for rationalizing the turbidity spectrum was derived, and from its application a spherical shape of radius about 50 nm was deduced.The low value for the fluorescence anisotropy of the aggregate (r= 0.02) is compatible with a very loose structure, i.e. the chromophore has very efficient depolarization dynamics that is not controlled by the aggregate size.The Stern‐Volmer plot of aggregated filipin fluorescence quenching by iodide is non‐linear, presenting a downward curvature. A model was used for the interpretation of these data, along with a study of the quenching in transient state; it was concluded that all the components of the decay are affected by the quencher, i.e. the aggregate has a very open structure with respect to the iodide ion.The partition constants of the polyene, Kp, between a model system of membranes (small unilamellar vesicles of dipalmitoylglycerophosphocholine) and the aqueous phase were determined from anisotropy measurements; the values obtained were Kp (gel phase) = (3.4 ± 0.8) × 103 and Kp (liquid crystal phase) = (7.7 ± 2.2) × 102. The observation that the polyene incorporation is efficient is at variance with the belief that the presence of sterols are essential for the interaction of polyene antibiotics with membranes [for review see Bolard, J. (1986) Biochim. Biophys. Acta 864, 257–304].