New insights into the interaction of emodin with lipid membranes.

Emodin is a natural anthraquinone derivative found in nature, widely known as an herbal medicine. Here, the partition, location, and interaction of emodin with lipid membranes of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) are experimentally investigated with different techniques. Our studies have considered the neutral form of emodin (EMH) and its anionic/deprotonated form (EM), and their interaction with a more and less packed lipid membrane, DMPC at the gel and fluid phases, respectively. Though DSC results indicate that the two species, EMH and EM, similarly disrupt the packing of DMPC bilayers, spin labels clearly show that EMH causes a stronger bilayer disruption, both in gel and fluid DMPC. Fluorescence spectroscopy shows that both EMH and EM have a high affinity for DMPC: the binding of EM to both gel and fluid DMPC bilayers was found to be quite similar, and similar to that of EMH to gel DMPC, K = (1.4 ± 0.3)x10. However, EMH was found to bind twice more strongly to fluid DMPC bilayers, K = (3.2 ± 0.3)x10. Spin labels and optical absorption spectroscopy indicate that emodin is located close to the lipid bilayer surface, and suggest that EM is closer to the lipid/water interface than EMH, as expected. The present studies present a relevant contribution to the current understanding of the effect the two species of emodin, EMH and EM, present on different microregions of an organism, as local pH values can vary significantly, can cause in a neutral lipid membrane, either more or less packed, liked gel and fluid DMPC, respectively, and could be extended to lipid domains of biological membranes.