Driving a Stimuli-Responsive Wedge in the Packing of Phospholipid Membranes Using Bolaamphiphile Intercalants

Bolaamphiphiles─amphiphilic molecules with polar groups at each of the two ends of a hydrophobic tail with pH-sensitive spontaneous molecular curvatures, endow membranes of extremophiles with an exquisite balance between stability (or robustness) and adaptability (or plasticity). But how the presence (or real-time insertion) of bolaamphiphiles influences lamellar lipid membranes is poorly understood. Using a combination of time-resolved confocal fluorescence microscopy, in situ small-angle X-ray and neutron scattering (SAXS and SANS), and neutron spin echo (NSE) measurements, we monitor here the pH-dependent interactions of nanoscopic vesicles of a representative bolaamphiphile─a glucolipid consisting of a single glucose headgroup and a C18:1 (oleyl) fatty acid tail (G-C18:1)─with the membranes of an essentially cylindrical fluid-phase phospholipid (dioleoylphosphatidylcholine, DOPC). We found that the two mesophases interact spontaneously at all pH values, producing large-scale morphological remodeling. Under neutral and acidic conditions, when the bolaamphiphile assumes a cylindrical shape, vesicles fuse with one another, producing invaginations, inner tubulation, and vesicle-in-vesicle aggregates. Under basic pH, by contrast, when the carboxylic acid is deprotonated and the molecule is inverted-conical in shape, the bolaamphiphile causes phospholipid membranes to undergo poration, budding, and vesiculation. This pH-dependent environmentally sensitive membrane remodeling without the disruption of the essential bilayer motif illustrates how local molecular-level packing perturbations can translate into global system-level morphological changes, enabling membranes to acquire environmental sensitivity and real-time adaptability. These results support the notion that molecular fluxes─which add (or remove) amphiphilic molecules to biological membranes─can endow de novo functionalities (e.g., pH sensitivity) and influence global morphologies of cell-sized vesicles.