As a part of the German-Croatian bilateral project "The impact of myelin basic protein on the phase state, morphology and membrane structure" (DAAD-MZO), PhD student Tsu-Wang Sun visited RBI and presented his research in a lecture titled "Localized Dynamics in Membranes with Photoswitchable Lipid".

Lecture abstract:

Giant unilamellar vesicles (GUVs) serve as cell-sized model systems extensively studied to mimic the behavior of natural cell membranes and synthetic cells. The localization and segregation of specific elements, such as functional lipids and proteins, play a pivotal role in membrane regulation and function. Utilizing photo-responsive molecules presents a compelling strategy for achieving precise spatiotemporal regulation. In this context, azobenzene-phosphatidylcholine (azoPC) is a well-established photoswitchable lipid with an azobenzene moiety replacing one of its lipid tails. AzoPC's ability to respond reversibly to ultraviolet A and blue light holds the potential for inducing shape or phase changes in GUVs [1], making it a compelling avenue for investigating controlled reactions and mechanical energy. The photoisomerization of azoPC in the membrane significantly increases surface area and alters its elastic properties [2]. This study delves into the phase dynamics of membranes incorporating azoPC and the gel-phase phospholipid dipalmitoyl phosphocholine (DPPC), which has a melting point at 41°C. The domain patterns in GUVs can be melted and reorganized into coarsened patterns with temperature control. Despite a phase transition at 31.8°C identified by differential scanning calorimetry (DSC) in the equimolar mixture (DPPC:azoPC 1:1), microscopy observations intriguingly indicate that domain coalescence and homogenization occur only above 45°C. Further studies using the phase indicator Laurdan and fluorometer reveal an unexpected transition with phasor analysis [3], which may correspond to the higher melting temperature observed under microscopy. At room temperature, the GUVs display localized membrane protrusions and expansion upon UV irradiation, initially exhibiting twisting or crumpling (Fig. 1A). When incorporating azoPC in asymmetric fluid membranes (palmitoyloleoyl phosphatidylcholine, POPC), the vesicles exhibit intriguing tubulation and expanding responses (Fig. 1B). This investigation sheds light on the dynamic behaviors of photoswitchable lipids within lipid membranes, enhancing our understanding of cell membrane mechanics and controlled reactions