Hydration Controlled Free Energy Landscapes of PEG-Mediated Stalk Formation

Polyethylene glycol (PEG) has received significant attention in recent decades because of its unique advantages in clinical therapeutics, including reduced immune recognition, prolonged systemic circulation, and enhanced stability of drug carriers. However, the role of PEG in cellular processes during the drug delivering at molecular levels remains poorly understood. Membrane fusion, one of the most fundamental processes in living systems, also underlies vesicular delivery, which is an emerging approach for drug loading into exosomes. In the present work, we investigate the role of PEG in regulating the thermodynamics and dynamics of stalk formation, a critical step in membrane fusion, in POPS and POPG systems using coarse-grained molecular dynamics simulations. It is found that the fluctuation in lipid membranes promotes the stalk formation. PEG-mediated stalk formation strongly correlates with the degree of hydration between membranes. At high hydration levels, the introduction of PEG promotes membrane–membrane contact and thus strongly favors the formation of highly hydrated stalk structures. In contrast, at low hydration levels, the presence of PEG slows the dynamics of water molecules in the proximal compartments, thereby disfavoring stalk formation. While PEG affects negligibly on lipid mobility and local water dynamics, it remarkably enhances the contact probability between membranes, leading to a reduction in the free energy barrier for stalk nucleation. Overall, these findings provide molecular-level insights into PEG-mediated membrane fusion and offer valuable theoretical guidance for the rational design of PEG-related drug delivery systems.