LIPOSOME FUSION AND LIPID EXCHANGE ON ULTRAVIOLET-IRRADIATION OF LIPOSOMES CONTAINING A PHOTOCHROMIC PHOSPHOLIPID

A photochromic phospholipid, -butylphenylazo)phenylbutyroyl]phosphatid ylcholine (Bis-Azo PC) has been incorporated into liposomes of gel- an d liquid-crystalline-phase phospholipids. Liposomes of gel-phase phosp holipid are stable in the presence of the trans photostationary state Bis-Azo PC and can encapsulate fluorescent marker dye. On photoisomeri zation to the cis photostationary state, trapped marker is rapidly rel eased. Liposomes containing Bis-Azo PC can rapidly fuse together after UV isomerization, this process continuing in the dark. Exposure to wh ite light causes reversion of Bis-Azo PC to the trans form and halts d ye leakage and vesicle fusion. Both unilamellar and multilamellar lipo somes are able to fuse together on UV exposure. On UV photolysis, lipo somes containing Bis-Azo PC do not fuse with a large excess of unlabel ed liposomes, but transfer of Bis-Azo PC can be demonstrated spectroph otometrically. Vesicles of pure gel-phase lipid containing trapped mar ker dye but initially no Bis-Azo PC become leaky as a result of this l ipid transfer. Liposomes composed of liquid-crystalline-phase phosphat idylcholine-containing Bis-Azo PC neither leak trapped marker nor fuse together on photolysis, nor do liquid-crystalline-phase liposomes fus e with gel-phase liposomes under these conditions. These results are d iscussed together with some possible applications of liposome photodes tabilization.