Characteristics and photo-responsive release property of liposome containing 7-acetoxy coumarin

A photo-responsive liposome was developed by loading 7-acetoxy coumarin (ATC) in egg phosphatidylcholine (EPC) liposome. ATC was derivetized from 7-hydroxy coumarin using sodium acetate. The ATC-loaded liposomes were prepared by suspending the dry mixture film of ATC/EPC in distilled water and sonicating the suspension. When the ATC to EPC ratio was less than 1:8, homogeneous liposomal suspensions were obtained without any precipitate. On 1H NMR spectra, the unsaturated chains of liposomal EPCs were somewhat deteriorated by a subsequent irradiation, the irradiation of lambda = 365 nm and then the irradiation of lambda = 254 nm. On TEM photos, the size of liposomes incorporating ATC markedly increased by the subsequent irradiation. Photo-dimerization (under lambda = 365 nm) and de-dimerization (under lambda = 254 nm) of ATC took a place even in EPC liposomes. Upon the irradiation of lambda = 254 nm, liposome containing ATC dimers exhibited an enhanced release of 5(6)-carboxyfluorescein compared with liposome containing ATC monomer and liposome free of ATC. The de-dimerization of ATC dimers is believed to fluidize the liposomal membrane and promote the release.