The formation of excimer and exciplex in the structural investigations of lipid vesicles: deuterium isotope effect

Abstract

Ethyl pyrene (EP), which forms EP excimer and EP-dimethylaniline (DMA) exciplex in lipid vesicles prepared with 1,2-dimyristoyl phosphatidylcholine (DMPC) and its deuterated analogue (DMPC(D)), has been used to monitor the structural properties of membrane lipid. The excimer or exciplex formation is evaluated by examining the logarithm of excimer or exciplex to monomer fluorescence quantum yield ratio as a function of temperature. The EP excimer is influenced by the gel to liquid-crystalline phase transition in both DMPC and DMPC(D). Deuterium isotope substitution in lipid enhances the excimer yield. The EPDMA exciplex is also dependent on membrane lipids, and is sensitive to the lipid phase transition in DMPC, but not in DMPC(D). These findings reveal that in the liquid-cyrstalline phase the two hydrocarbon tails in the two lipid dispersions are structurally comparable, while in the gel phase the tails in DMPC(D) have a significantly less ordered configuration than those in DMPC, making DMPC(D) less rigid in its structure. The present results not only support our previous suggestion, but also indicate that the looser structure and reduced stability in deuterated molecules may be associated with a less structural ordering in them.