Poly-L-lysine induced shape change of negatively charged giant vesicles

Abstract

Decoration of biomembrane with polymer may improve its physical properties, biocompatibility, and stability. In this study, we employ the inverted fluorescence microscopy to characterize the polylysine (PLL) induced shape transformation of the negatively charged giant unilamellar vesicles (GUVs) in low ionic medium. It is found that PLL may be adsorbed to the 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1, 2-dioleoyl-sn-glycero-3-phosphatidic acid (DOPA) binary mixture vesicles, resulting in the attachment between the membranes, the formation of the ropes, and rupture of the GUVs. The response of GUVs generally is enhanced with the increase of the negatively charged DOPA in the membranes. The experimental observations are concluded as follows. Firstly, for the PLL induced attachment of GUVs, the attachment area grows gradually with time. Secondly, ropes can only be found in relatively large GUVs. However, the hollow structure is not discernable from the fluorescence imaging. Thirdly, after the rupture of GUVs, some phase-separated-like highly fluorescence lipid domains form in the adjacent intact vesicles. Through careful discussion and analysis, we show that on the one hand, the positively charged PLL adheres to the negatively charged membrane surface, bridging the neighboring GUVs and drawing the originally electrical repulsive vesicles together. The contact zone between GUVs expands with the increasing adsorption of PLL in this area. And the local high fluorescence areas in the GUVs originate from the PLL induced membrane attachment as well. Some membrane segments from ruptured vesicles are adsorbed to the particular areas of GUV, forming a few lipid patch structures above the latter membrane. On the other hand, PLL is adsorbed to the membrane area enriched in the negatively charged DOPA, reversing the surface charge of the upper leaflet and deteriorating the stability of the lipid bilayer. The original equilibrium of the system is broken by the change of the electrical interaction between the neighboring lipid domains as well as the interaction between the domain and water-dispersed PLL. The lipid packing density and inter-lipid force are affected by the PLL adsorption. Lipid membranes have to bud to release the stress built in the spontaneous curvature incompatibility in the two leaflets. The system may become stable again after buds grown into rods with a certain length. All in all, this study deepens the understanding of the interaction mechanism between lipid membrane and oppositely charged polymer. The conclusions obtained will provide valuable reference for the further studies on the polymer-GUV application areas including drug delivery, control release, cell deformation, micro-volume reaction, and gene therapy.