Effect of Phloretin on the Binding of 1-Anilino-8-naphtalene sulfonate (ANS) to 1,2-Dimyristoyl-sn-glycero-3-phosphocoline (DMPC) Vesicles in the Gel and Liquid-Crystalline State

Abstract

Phloretin is a known modifier of the internal dipole potential of lipid membranes. We studied the interaction of phloretin with model lipid membranes and how it influences the membrane dipole organization using ANS as fluorescent probe. The fluorescence increase observed when ANS binds to DMPC liposomes in gel phase (13 C) was 2.5 times larger in the presence of phloretin. This effect was due to an increase in ANS affinity, which can be related to the known capability of phloretin in decreasing the dipole potential. Conversely, when the experiments were carried out at 33 C (liquid crystalline phase), phloretin completely inhibited the increase in ANS fluorescence. In addition, phloretin only affected the electrical properties of the membrane in the gel phase, whereas it modifies structural ones in the liquidcrystalline state. We postulate that phloretin was bound only to the DMPC interface in the gel phase decreasing the surface negative charge density without modifying the structural properties of the ANS binding sites. In the liquidcrystalline phase instead, it increased the accessibility of water to the ANS binding sites decreasing the intrinsic affinity and the fluorescence quantum yield of ANS.