Curcumin is a yellow–orange dye widely used as a spice, food coloring and food preservative. It also exhibits a broad range of therapeutic effects against different disorders such as cancer, diabetes, or neurodegenerative diseases. As a compound insoluble in water curcumin accumulates in cell membranes and due to this location it may indirectly lead to the observed effects by structurally altering the membrane environment. To exert strong structural effects on membrane curcumin needs to adopt a transbilayer orientation. However, there is no agreement in literature as to curcumin’s orientation and its structural effects on membranes. Here, we investigated the effects of curcumin on lipid order, lipid phase transition, and local polarity in a model liposome membranes made of DMPC or DSPC using electron paramagnetic resonance (EPR) spin labeling technique. Curcumin affected lipid order at different depths within the membrane: it slightly increased the phospholipid polar headgroup mobility as monitored by spectral parameters of T-PC, while along the acyl chain the ordering effect was observed in terms of order parameter S. Also, rotational correlation times τ_2B and τ_2C of 16-PC in the membrane center were increased by curcumin. Polarity measurements performed in frozen suspensions of liposomes revealed enhancement of water penetration by curcumin in the membrane center (16-PC) and in the polar headgroup region (T-PC) while the intermediate positions along the acyl chain (5-PC and 10-PC) were not significantly affected. Curcumin at a lower concentration (5 mol%) shifted the temperature of the DMPC main phase transition to lower values and increased the transition width, and at a higher concentration (10 mol%) abolished the transition completely. The observed effects suggest that curcumin adopts a transbilayer orientation within the membrane and most probably form oligomers of two molecules, each of them spanning the opposite bilayer leaflets. The effects are also discussed in terms of curcumin’s protective activity and compared with those imposed on membranes by other natural dyes known for their protective role, namely polar carotenoids, lutein and zeaxanthin.

中文翻译：

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姜黄素对脂质膜的影响：EPR 自旋标记研究。

姜黄素是一种黄橙色染料，广泛用作香料、食用色素和食品防腐剂。它还对不同疾病（如癌症、糖尿病或神经退行性疾病）表现出广泛的治疗效果。由于不溶于水的化合物姜黄素在细胞膜中积累，并且由于这个位置，它可能通过结构改变膜环境间接导致观察到的影响。为了对膜姜黄素发挥强大的结构作用，需要采用跨双层取向。然而，关于姜黄素的取向及其对膜结构的影响，文献中并没有达成一致。在这里，我们研究了姜黄素对脂质顺序、脂质相变、使用电子顺磁共振 (EPR) 自旋标记技术在由 DMPC 或 DSPC 制成的模型脂质体膜中的局部极性和局部极性。姜黄素影响膜内不同深度的脂质顺序：通过 T-PC 的光谱参数监测，它略微增加了磷脂极性头部基团的迁移率，而沿着酰基链，根据顺序参数 S 观察到排序效应。此外，旋转相关性次 τ_2B和 τ _2C姜黄素增加了膜中心 16-PC 的含量。在脂质体的冷冻悬浮液中进行的极性测量显示，姜黄素在膜中心 (16-PC) 和极性头基区 (T-PC) 中增强了水渗透，而沿着酰基链的中间位置 (5-PC 和 10- PC) 没有受到显着影响。较低浓度 (5 mol%) 的姜黄素将 DMPC 主相变的温度转移到较低值并增加了转变宽度，而在较高浓度 (10 mol%) 时完全消除了转变。观察到的效果表明姜黄素在膜内采用跨双层取向，最有可能形成两个分子的低聚物，每个分子跨越相反的双层小叶。