Influence of Molecular Weight and pH on Adsorption of Chitosan at the Surface of Large and Giant Vesicles

摘要

This paper describes the mechanisms of adsorption of chitosan,a positively charged polyelectroly te,on the DOPC lipid membrane of large and giant unilamellar vesicles(respectively,LUVs and GUVs).We observe that the variation of the zeta potential of LUVs as a function of chitosan concentration is independent on the chitosan molecular weight(Mw).This result is interpreted in terms of electrostatic interactions,which induce a flat adsorption of the chitosan on the surface of the membrane.The role of electrostatic interactions is further studied by observing the variation of the zeta potential as a function of the chitosan concentration for two different charge densities tuned by the pH.Results show a stronger chitosan-membrane affinity at pH 6(lipids are negatively charged,and 40% chitosan amino groups are protonated)than at pH 3.4(100% of protonated amino groups but zwitterionic lipids are positively charged)which confirms that adsorption is of electrostatic origin.Then,we investigate the stability of decorated LUVs and GUVs in a large range of pH(6.0 < pH < 12,0)in order to complete a previous study made in acidic conditions [Quemeneur et al.Biomacromolecules 2007,8,2512-2519].A comparative study of the variation of the zeta potential as a function of the pH(2.0 < pH < 12.0)reveals a difference in behavior between naked and chitosan-decorated LUVs.This result is further confirmed by a comparative observation by optical microscopy of naked and chitosan-decorated GUVs in basic conditions(6.0 < pH < 12.0):at pH > 10.0,in the absence of chitosan,the vesicles present complex shapes,contrary to the chitosan-decorated vesicles which remain spherical,confirming thus that chitosan remains adsorbed on vesicles in basic conditions up to pH = 12.0.These results,in addition with our previous data,show that the chitosan-decorated vesicles are stable over a very broad range of pH(2.0 < pH < 12.0),which holds promise for their in vivo applications.Finally,the quantification of the chitosan adsorption on a LUV membrane is performed by zeta potential and fluorescence measurements.The fraction of membrane surface covered by chitosan is estimated to be lower than 40 %,which corresponds to the formation of a flat layer of chitosan on the membrane surface on an electrostatic basis.