Coupling of Leaflet Structure in Asymmetric Lipid Vesicles

Abstract

Lipid asymmetry is a hallmark of biological membranes. In particular, prototypical mammalian plasma membranes are known to be composed of an outer leaflet enriched in cholinephospholipids, while the majority of the aminophospholipids are confined to the inner leaflet. Asymmetric large unilamellar lipid vesicles (aLUVs), produced via cyclodextrin-mediated lipid exchange, are a new platform for more realistic mimics of biological membranes. These systems were shown to be stable over several days and have already been investigated by elastic scattering techniques (small-angle neutron and X-ray scattering; SANS/SAXS), providing insight into structural properties of the individual leaflets. One of the enduring questions concerning plasma membrane architecture and lipid asymmetry is the possibility of bilayer leaflets being coupled to each other, which may influence a number of physiological processes that require communication between interior and exterior of the cell. However, in the physiologically relevant fluid phase no evidence of structural coupling has yet been reported from scattering studies. In this work, we explore the role of hydrocarbon chain interdigitation as a potential trigger for transleaflet coupling. We use combinations of dipalmitoylphosphatidylcholine (DPPC) in the inner leaflet and mixed lipids with varying chain length mismatch in the outer leaflet, in particular C16:0/C18:1 PC (POPC), C18:0/C18:1 PC (SOPC), C18:0/C14:0 (SMPC), C14:0/C18:0 (MSPC) and C16:0/C14:0 PC (PMPC). This entails different interdigitation states of the mixed-chain lipids into the inner leaflet. We present consequences on transbilayer coupling as observed from leaflet specific structural data and thermotropic behavior of these systems.