Membrane-Polymer Interactions in Lipid Vesicles

Doctoral thesis, 2013

Membrane related biological processes are commonly investigated in artificial biomimetic experimental systems. One of the most versatile models is based upon giant unilamellar phospholipid vesicles (liposomes), which are artificially generated spherical lipid structures in an aqueous environment. GUVs form an important platform for functional and mechanistic studies of biomembranes, which supports both fundamental and applied research. In my thesis I use a multidisciplinary approach in which method development, as well as micro- and nanoscale manipulations and biophysical investigations where conducted on polymer-membrane systems, predominantly in the context of artificial cells. A cell sized model for membrane-polymer interactions was developed, consisting of a thermoresponsive polymer and giant phospholipid vesicles. The study involved the formation of a hydrogel compartment within a giant vesicle, and the investigation of interactions of the so formed compartment with the liposome membrane. Poly(N-isopropyl acrylamide) copolymerized with vinyl ferrocene was utilized to modulate the polymer contracting properties and enable the dynamic formation of a dense homogeneous compartment within a GUV. The polymer-membrane interactions were also investigated theoretically to describe the force necessary to a pull a nanotube from the membrane. The force was expressed as a function of concentration and chain lengths. The study was supported by an experimental determination of the minimal concentrations required for polymers of different chain length to interact with the membrane. Furthermore, a novel technical approach is described, which involves an IR fiber laser assembly as a spot heating source, introducing various improvements to microscopy experiments on single liposomes at elevated temperatures. A similar direct laser heating procedure was utilized to generate single giant vesicles from multilamellar membrane reservoirs, allowing for the first time for a broader variety of membrane compositions to be processed in comparison with conventional methods. Additionally, an optofluidic open-volume technique to measure temperature in situ on the size scale of the vesicle systems is described. This method uses a recently developed microfluidic pipette and employs ratiometric measurement of two rhodamine dyes with different fluorescence temperature dependency profiles.