The mechanical properties of model biological membranes

The material properties of cellular membranes have been widely studied because of their importance in proper cellular function. Often lipid vesicles, synthetic pure lipid analogs, are studied as simplified cellular membrane models, as uncovering the principles that guide vesicle membrane mechanics should provide insight into the properties of more complicated biological membranes. Due to their nature as self-assembled materials, the mechanical properties of lipid membranes are wholly dictated by weak intermolecular interactions. As a result, altering membrane interactions (through, for example, altering lipid composition) produces membranes with different mechanical parameters. Conversely, measuring membrane mechanical properties offers unique insight into molecular-level interactions.; This thesis describes experimental work that attempts to infer relationships between membrane characteristics on the molecular-level and the material properties of the resulting macroscopic films. Among the most notable observations of this work are the reductions in lipid membrane mechanical stability the result from increasing membrane temperature, phase separating from a miscible liquid crystalline (L_α) membrane to a two-phase state, and increasing vesicle surface charge. The effect of temperature is explained based on the increased probability of nucleating a rupture event due to greater available thermal energy. For the case of phase separation, the cause of the reduced stability is unclear. However, the fact that the effect was detected in two extremely different lipid systems—which fluorescence microscopy observations suggest to possess markedly different domain configurations—raises the possibility that the reduction in stability is a generic effect of phase coexistence. A possible explanation is that phase boundaries may serve as defect nucleation sites. Finally, in perhaps the thesis' most important contribution, the reduction in stability for charged vesicles was shown to scale with both anionic lipid fraction and the efficiency of ion binding. This strongly implicates that the effect is electrostatic in origin. A model based on an effective electrostatically-induced tension is presented and shown to successfully model stability data from vesicles made with two different anionic lipids and three different electrolytes.