Surface Coating of Liposomes with Hydrophobized Polysaccharides

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Abstract
Coating the outermost surface of a liposomal membrane with several different hydrophobized polysaccharides was investigated by fluorescence depolarization, gel chromatography, and dynamic light scattering methods. The binding of cholesterol-bearing pullulan to the liposomal surface was biphasic. The first process was finished within minutes while the subsequent slow stages took over several hours. The binding isotherms followed Langmuir-type adsorption. The binding constant (K) increased with increases in the substitution degree of the cholesteryl moiety and the molecular weight of the pullulan derivatives used, while the maximum amount of the polysaccharide coating (qs) was almost the same. The apparent liposome size increased by 20-30 nm upon coating. Chemical structure of the parent polysaccharide had only a slight effect on the binding constant, while the structures of the hydrophobic moiety had a significant effect on the coating behavior of the liposomes. In the case of dodecyl diglyceryl group-bearing pullulan, both K and qs were smaller than those of other cholesterol-bearing polysaccharides. The addition of hexadecyl-bearing pullulan to the liposome induced aggregation of the liposomes. The cholesteryl moiety is an excellent hydrophobic anchor for polysaccharide coating liposomal surfaces compared with simple monoalkyl or dialkyl chains.