Investigations on membrane perturbation by chrysin and its copper complex using self-assembled lipid bilayers

The mechanism of membrane interactions of most of the flavonoids in the presence of transition-metal ions is not well-understood. To understand this phenomenon, the present work aims to synthesize a chrysin-copper complex at room temperature and investigate its influence on the electrical characteristics of planar lipid bilayers. The chrysin-copper complex was characterized by various spectroscopic techniques and was found to have a metal/ligand ratio of 1:2 and of cationic nature. Its ability to inhibit 1,1'-diphenyl-2-picrylhydrazyl (DPPH) radicals was not significant at alkaline pH because of the involvement of the 5-hydroxy group in coordination with the copper ion compared to its parent flavonoid, chrysin (p < 0.05). The addition of different concentrations (20-100 μM) of chrysin and the chrysin-copper complex to lipid bilayers decreases the resistance, indicating a strong surface interaction and partial insertion into the bilayer near the lipid-water interface. The dose-dependent reduction in resistance as a result of the chrysin-copper complex is more pronounced in comparison to chrysin, implying that the bulkier and charged chrysin-copper complex displays greater ability to distort the lipid bilayer architecture. These conclusions were further confirmed by curcumin-loaded liposome permeabilization studies, where both chrysin and its Cu(II) complex increased the fluidity in a dose-dependent manner. However, the extent of fluidization by the chrysin-copper complex was nearly twice that of chrysin alone (p < 0.05). The implications of these surface interactions of chrysin and its copper complex on cell membranes were studied using a hypotonic hemolysis assay. Our results demonstrate that, at low concentrations (20 μM), the chrysin-copper complex exhibited twice the protection against hypotonic stress-induced membrane disruption when compared to chrysin. However, this stabilizing effect gradually decreased and became comparable to chrysin at higher concentrations. This biphasic behavior of the chrysin-copper complex could further be explored for therapeutic applications.