Experimental and theoretical studies of emodin interacting with a lipid bilayer of DMPC.

Emodin is one of the most abundant anthraquinone derivatives found in nature. It is the active principle of some traditional herbal medicines with known biological activities. In this work, we combined experimental and theoretical studies to reveal information about location, orientation, interaction and perturbing effects of Emodin on lipid bilayers, where we have taken into account the neutral form of the Emodin (EMH) and its anionic/deprotonated form (EM(-)). Using both UV/Visible spectrophotometric techniques and molecular dynamics (MD) simulations, we showed that both EMH and EM(-) are located in a lipid membrane. Additionally, using MD simulations, we revealed that both forms of Emodin are very close to glycerol groups of the lipid molecules, with the EMH inserted more deeply into the bilayer and more disoriented relative to the normal of the membrane when compared with the EM(-), which is more exposed to interfacial water. Analysis of several structural properties of acyl chains of the lipids in a hydrated pure DMPC bilayer and in the presence of Emodin revealed that both EMH and EM(-) affect the lipid bilayer, resulting in a remarkable disorder of the bilayer in the vicinity of the Emodin. However, the disorder caused by EMH is weaker than that caused by EM(-). Our results suggest that these disorders caused by Emodin might lead to distinct effects on lipid bilayers including its disruption which are reported in the literature.