PH- and calcium ion-dependent release from egg phosphatidylcholine liposomes incorporating hydrophobically modified...

Liposomes, which release their contents in response to change in environmental pH and Ca2+ concentration, were prepared by modifying the surface of egg phosphatidylcholine (EPC) liposome with hydrophobically modified alginate (Hm AL). The liposomes prepared using the ratio of 1:7 (Hm AL:EPC) was stable when the pH of release medium was higher than 6.5. They started to release at pH 6.0, and the degree of release in 300 sec was more than 80% at pH 4.5. The collapse of Hm AL chains, caused by the lack of intramolecular electrostatic repulsion, would impose a mechanical energy on the liposomal membrane, promoting a release from the liposomes. Liposomes bearing less amount of Hm AL exhibited less degree of release, but the release profiles were found similar whatever Hm AL to EPC ratio was. On the other hand, the liposomes bearing Hm AL released calcein significantly (10-30%) in 300 sec when Ca2+ was contained in the release medium (5 mM and 10 mM). The collapse or the gellation of Hm AL, caused by an electrostatic interaction between Ca2+ and the polysaccharide, would be responsible for the release.