Experiment and Simulation Reveal the Bending Properties of Nanoscopic Lipid Domains

Abstract

There has great interest in studying the mechanisms that support nanoscopic lipid domain size using model mixtures. To date, experimental studies have focused in large part on structural parameters such as thickness mismatch between phases, lacking access to mechanical properties in direct, experimental ways. The Neutron Spin Echo technique can directly measure the bending modulus of such bilayers systems by observing shape fluctuations. Furthermore, this method permits us to utilize hydrogen-deuterium contrast matching to make direct observations of the mechanical properties of nanoscopic domains themselves.I will present results detailing for the first time the bending modulus of an individual lipid domain. We have also measured both the structural and mechanical properties of each pure phase compositions of the three component lipid mixture which spontaneously forms these domains. Our results demonstrate that the liquid-ordered phase is more rigid than the liquid disordered phase. Interestingly, the value of the bending modulus in vesicles where these phases coexist as nanoscopic domains approaches that of the liquid ordered-phase, whereas our sample contrast matched so that we only probe the disordered phase in the coexisting system have a bending modulus similar to what is seen in the liquid-disordered only system. We analyze our results with the help of all atom MD simulations which accurately reproduce the structural properties of our system and agree with the trend in mechanical properties that we observe experimentally. These results have implications in determination of important properties such as the line tension which may drive domain size.