Abstract
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 τ2B and τ2C 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.
Highlights
Curcumin is a yellow–orange dye derived from the rhizome of Curcuma longa
A significant decrease of the (h0/h−) parameter shows that the anisotropy of the motion of the nitroxide moiety of T-PC is decreased in the presence of curcumin which can be interpreted as the increase in the mobility of the polar headgroups
This effect is DMPC membrane in the presence and absence of curcumin at 25 °C. h0/h− is a ratio of the height of central and high-field peaks and ΔH+ is a peak-to-peak width of the low-field line either the phosphate group or the nearby carbonyls of DMPC
Summary
Curcumin is a yellow–orange dye derived from the rhizome of Curcuma longa (check Fig. 1 for its chemical structure). A xanthophyll molecule orients within the membrane mostly perpendicular to the surface being anchored in polar headgroup regions on both sides of the membrane by its hydroxyl groups, while location of apolar β-carotene is not clearly determined [15, 17, 21, 22]. This is the reason why xanthophylls are more effective than β-carotene in affecting membrane properties and protecting membrane lipids against peroxidation [16, 21, 23, 24]. Depending on curcumin location in the membrane, different models of its protective activity are proposed: as a physical barrier, a so-called carpet, preventing peptide or oxidants penetration if associated with the membrane at the surface or by membrane stiffening and increasing the energy barrier to ion or peptide embedding if positioned perpendicular to the membrane surface and penetrating between the alkyl chains of lipids [11]
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