Liposomes incorporating hydrophobically modified silk fibroin: pH-dependent release

pH-sensitive liposomes were prepared by modifying the surface of egg phosphatidylcholine (EPC) liposomes with hydrophobically modified silk fibroin (HmSF). For the hydrophobic modification of SF, palmitic acid residues were covalently attached to SF through amide bond. According to the result of TNBS assay, about 90% of amino groups of SF participated in the conjugation reaction with palmitic acid N-hydroxysuccinimide ester, and the number of palmitic acid residue per SF molecule was calculated to be 15.3. The fluorescence quenching of calcein in liposomal suspension decreased from about 80-50%, when the ratio of HmSF to phospholipid increased from 1:20 to 1:5. The maximum degree of release from liposomes incorporating HmSF (of which HmSF to phospholipid ratio was 1:20) for 5 min was about 95% at pH 4.5, 25% at pH 5.0, and 5% at pH 5.5, and no appreciable amount of release was observed in the range of pH 6.5 to pH 8.0. Under acidic conditions, HmSF is likely to perturb the packing of phospholipid in liposomal membrane, possibly due to the unionization of the carboxylic groups of the silk protein.