The interaction between dodecyltrimethylammonium bromide (DTAB) and curcumin has been studied in pH 5.0 sodium phosphate buffer using absorption and fluorescence measurements. With increasing DTAB concentration (C(DTAB)) from 0 to 20 mM, the absorption peak of curcumin at 430 nm, corresponding to the conjugated structure of curcumin, first weakens gradually into a shoulder but increases back into one peak with much higher absorption intensity. On the contrary, as C(DTAB) increases, the initial small absorption shoulder of curcumin at 355 nm, corresponding to the feruloyl unit of curcumin, first increases gradually into a clear peak but decreases back into one shoulder until almost disappeared finally. By remaining at nearly the same wavelength, the fluorescence of curcumin first decreases at C(DTAB) lower than 5 mM and then increases gradually up to C(DTAB) = 10 mM, which is followed by sharp increases of fluorescence intensity with marked blue-shifts at higher C(DTAB). The values of anisotropy and microviscosity of curcumin obtained from the fluorescence polarization technique also showed pronounced changes at different surfactant concentrations. The interaction mechanisms of DTAB with curcumin have been presented at low, intermediate, and high surfactant concentrations, which is relating to interaction forces, surfactant aggregations, as well as structural alterations of curcumin.

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The spectroscopic properties of liposomal curcumin in pH 7.0 sodium phosphate buffer were studied at various curcumin concentrations and temperatures. At 25 °C, liposomal curcumin exhibited much higher values than free curcumin in absorption maximum, fluorescence maximum, and fluorescence anisotropy. When curcumin concentration was increased from 2 to 20 μM, the values of fluorescence anisotropy of liposomal curcumin decreased gradually, consistent with the reduction of phase transition temperature of liposome. This observation revealed that liposomal curcumin can disrupt the packing of phospholipid bilayer and give a loose and disordered structure. On the other hand, as the temperature was increased from 25 to 80 °C, the relative intensity of maximum absorption of liposomal curcumin showed a more pronounced decrease above the phase transition temperature than lower temperatures, suggesting a weaker curcumin protection from the liquid crystalline phase of phospholipid bilayer than the rigid gel phase. However, it was observed that the fluorescence anisotropy of liposomal curcumin had higher values as the temperature increased. This phenomenon was explained as the result of location change of curcumin toward the core of phospholipid bilayer, although the structure of the phospholipid bilayer tended to be looser at higher temperatures.

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