Extreme situations require extreme molecules

Bolaamphiphiles can associate in single-layer membranes. However, phospholipids - the major constituent of biological membranes - associate in bilayer structures. “We asked the following fundamental questions”, explains Niki Baccile, co-principal investigator and corresponding author of the paper. “How do thinner single-layer bolaamphiphile membranes impact the plasticity of model phospholipid membranes? And how is the membrane plasticity and morphology affected by important external parameters, such as pH?”

The scientists combined microscopy techniques with neutrons scattering measurements at the ILL - small-angle neutron scattering (SANS) and neutron spin echo (NSE)  - to perform their investigations. “SANS is the ideal, non-destructive technology to investigate molecules in the nano-to-micrometer size range”, explains Lionel Porcar, main responsible of the ILL’s small-angle scattering instrument D22. His colleague Ingo Hoffmann from the neutron spectrometer IN15 adds, “NSE allows to decipher tiny molecular motions. The combination of SANS and NSE as complementary neutron techniques is essential to studying biological systems such as these vesicles.”

The team observed that the bolaamphiphile molecules mixed with the artificial membranes at all pH values. Using SANS, the team showed that the mixing process thinned out the membranes and transformed them from initially multilamellar structures (that is, multiple membranes surrounding each other like onion layers) to unilamellar ones. Complementarily, NSE - focusing on dynamics within the membranes - revealed that they became less rigid after incorporating the bolaamphiphile at neutral pH. Interestingly, under acidic conditions, bolaamphiphiles made the membranes more rigid. The team traces these important differences back to pH-dependent shape changes of the bolaamphiphiles.

Using sophisticated microscopy techniques, the scientists also observed how the bolaamphiphile-membrane mixtures gave rise to completely new structures. “We were intrigued to see that these resulting structures also varied depending on the pH value”, says Atul N. Parikh, co-principal investigator of the study. “For example, under neutral and acidic conditions, the vesicles started coming closer together, forming tubules and then subdivided to produce smaller vesicles inside larger ones.” At basic pH, the formation of vesicles was observed once again - but this time, they budded outside their larger “siblings”.