Effect of Curcumin Addition on the Adsorption and Transport of a Cationic Dye across DPPG-POPG Liposomes Probed by Second Harmonic Spectroscopy.

The effect of addition of curcumin on the adsorption and transport characteristics of a cationic dye, LDS+, across negatively charged bilayers composed of POPG and DPPG lipids were investigated by the interface selective second harmonic (SH) spectroscopic technique. Curcumin induced changes in the SH electric field signal of the LDS+ ions (E2ω (LDS+)) were observed to depend critically on the bilayer acyl chain saturation/unsaturation ratio (S/U). Following earlier works, the increase in the E2ω (LDS+) signal is attributed to the release of the Na+ counterions present in the head group region of the bilayer by curcumin and the decay of the E2ω (LDS+) signal is attributed to the bilayer intercalated state of curcumin. While the changes observed in the E2ω (LDS+) signal in the presence of POPG liposomes were consistent with our earlier study ( Varshney, G. K. et al. Langmuir , 2016 , 32 , 10415 - 10421 ), they were significantly different for DPPG liposomes, following curcumin addition. While the increase in the E2ω (LDS+) signal in the presence of POPG liposomes, is marginal (∼10-20%) and instantaneous (<1 s) followed by a rapid decay (completed within ∼100 s), in the presence of DPPG liposomes it was observed to increase slowly and at saturation shows a substantial increase (100-200%), following curcumin addition. When liposomes consisting of a mixture of POPG and DPPG lipids are used, curcumin induced kinetic characteristics of the E2ω (LDS+) signal showed a mixture of the individual kinetic characteristics observed for the unsaturated (POPG) and saturated (DPPG) liposomes. The observed kinetic trends of the E2ω (LDS+) signal following curcumin addition are explained on the basis of the relative strength of the Na+-POPG and Na+-DPPG interaction. Higher ordering of the lipid acyl chain region in DPPG liposome makes the Na+-DPPG interaction much stronger than the Na+-POPG interaction. Further, it is proposed that, in POPG-DPPG liposomes, individual domains of POPG and DPPG lipids exist at low temperature as suggested by the observed temperature dependent kinetic characteristics of the E2ω (LDS+) signal following curcumin addition. These domains are dependent on the S/U ratio and phase state of the bilayer. The gel phase was observed to be more conducive for individual domain formation. Results presented in this work not only support the notion that biological activity of curcumin is associated with its bilayer altering properties, but more interestingly it provides a qualitative insight about how bilayer phase separation can be achieved by modulating the hydrophobic interactions between the lipid acyl chains.