The non-monotonic effect of sucrose on interactions between lipid-bearing surfaces

The extremely low sliding friction of articular cartilage in synovial joints has been attributed to phospholipid boundary layers, lubricating via the hydration lubrication mechanism at their exposed, highly hydrated polar-head-groups, in a medium - the synovial fluid - where osmolytes, which may modify the hydration layer, are ubiquitous. Here, using a surface force balance (SFB), we carried out a systematic study to elucidate the effect of sucrose, a known osmotic regulator solute, with concentrations c_sucrose, ranging from 5 to 20 wt%, on the normal and shear forces between interacting phosphatidylcholine (PC) bilayers, both in the gel (1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC) and liquid (1,2-dimyristoyl-sn-glycero-3-phosphocholine, DMPC) phases, supported on atomically-smooth mica substrates. Several additional approaches including cryo-transmission electron microscope, atomic force microscopy, small- and wide-angle X-ray scattering, differential scanning calorimetry, dynamic light scattering and zeta potential measurements are exploited to get additional insight into the nature of the sucrose-dependent interactions. As c_sucrose is varied, a remarkable variation in the friction is observed: a marked reduction in friction is seen at low c_sucrose, but at higher sucrose levels the friction increases, for both gel and liquid phase lipids. This challenges the expectation that hydration lubrication is degraded by osmotic solutes, due to their competing for water of hydration, and reveals for the first time a non-monotonic effect of a sugar on the interactions, particularly frictional forces, between lipid bilayers. This non-monotonic effect correlates with the bilayer potential, and is attributed to a concentration-dependent affinity of the sugar to the PC headgroups.