Curcumin Alters Membrane Phase-Separation, Particle Size Distribution, Permeability, and Anisotropy Differently in Unsaturated or Saturated Small Unilamellar Vesicles

Abstract

Curcumin is an extract of turmeric, and is an effective reactive oxygen species scavenger, with potential anti-cancer benefits. In this study, we examined the effects of curcumin on membrane bilayer permeability, fusion, anisotropy, hydrodynamic radius and negative staining EM using two lipid vesicle delivery systems: 1) 99% Soy phosphatidylcholine (Soy PC) and 2) DMPC/DPPG/Cholesterol (1:1:0.7 mol/mol/mol) (MPC). Curcumin was added to these two vesicle populations at 10:1, 50:1, and 100:1 (lipid mol/curc mol). The structure of curcumin suggests it would most likely intercalate into the head group region of the membrane bilayer, and thus our hypothesis was that the unsaturated fatty acids of Soy PC vesicles would accommodate curcumin with less membrane perturbation than the saturated MPC vesicles. Curcumin increased membrane permeability to carboxyfluorescein leakage (CF) in both Soy PC and MPC small unilamellar vesicles in a concentration-dependent manner; however, Soy PC vesicles were 35% more permeable to CF than MPC vesicles. The fusion of the Soy PC and MPC was examined by vesicle mixing using the Tb/DPA-assay. Both Soy PC and MPC vesicles fused within seconds of addition, and the rate of fusion was dependent on buffer calcium concentration. Curcumin had membrane concentration-dependent effects on the fluorescence anisotropy of DPH-PC and TMA-DPH Soy PC and MPC vesicles. Lipid vesicle hydrodynamic radius and TEM negative stained vesicles indicate a unimodal distribution for Soy PC vesicles (r = 100-120 nm), while MPC vesicles had a bimodal distribution (r = 100-130 nm and 3000-4000 nm). Thus curcumin can more readily intercalate into more unsaturated lipid vesicle bilayers, and we conclude that for maximal delivery of curcumin to cells using a liposomal therapy, a more unsaturated vesicle system is suggested.