Molecular Shuttles with a Photoresponsive Macrocycle: Investigating Interactions and Operation within Lipid Membranes

Rotaxane-based molecular shuttles using their motion to move cargo within lipid bilayers could be used to position probes and actuators at specific membrane regions, enabling new ways to study and modulate biological and artificial membranes. As an initial step towards that goal, we designed a rotaxane whose ring is decorated with two azobenzene photoswitches. We investigated its operation in solution and within lipid bilayers of large unilamellar vesicles. In solution, two-dimensional exchange spectroscopy showed that the photoswitches can be stochastically shuttled along the axle at 0.114 Hz. Remarkably, the azobenzene units retain their photoreversibility and fatigue resistance when integrated into the rotaxane. Moreover, a 55% increase in the hydrodynamic radius (RH) of the vesicles was induced by consecutive cycles of azobenzene photoswitching at a 10 mol% concentration of rotaxane. Initial molecular dynamic simulations suggest water accumulates near the rotaxanes in the bilayer. Therefore, it is plausible that continuous rearrangement of the lipid packing induced by each azobenzene photoisomerization could facilitate further water penetration and vesicle swelling. However, other mechanisms could also contribute to the increase in RH. Future large-scale simulations and complementary experimental techniques will offer a better understanding of the observed phenomenon and determine if shuttling occurs within the bilayer. Our findings provide new insights into the interaction of rotaxanes with the surrounding lipids and their impact on membrane properties, aiding in developing systems for precise manipulation of lipid membranes for applications in biomedicine and bioengineering.