Ion-mediated changes of supported lipid bilayers and their coupling to the substrate. A case of bilayer slip?

Ion-mediated (Ca2+) changes in viscoelastic, structural and optical properties of negatively charged solid supported lipid bilayers (SLBs) on SiO2 surfaces were studied by means of quartz crystal microbalance with dissipation (QCM-D) monitoring and optical reflectometry. Despite the sensitivity of QCM-D to viscoeleastic/structural variations, it has not often been used to probe such changes for SLBs. SLBs were prepared from binary phospholipid mixtures of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC, neutral) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (POPG, negatively charged) on SiO2 sensor surfaces in a Ca2+-containing buffer. Interestingly, for bilayers containing POPG fractions above 35%, large QCM-D dissipation shifts occurred, when Ca2+ was removed from buffer in contact with the SLB (while maintaining 100 mM NaCl). The accompanying frequency changes were small. These Ca2+ mediated QCM-D responses are reversible, and a signal for considerable changes in the viscoelastic and structural properties of the SLB. Variation of Ca2+-concentration revealed a threshold concentration of around 0.4 mM for the changes in the SLB to occur. Below this value, at >35% POPG concentration in the SLB, the SLB appears to become more weakly attached to the SiO2 substrate, which is partly attributed to a weakening of the POPG-substrate interaction in the absence of Ca2+. A consequence of this is an oscillation-amplitude dependent dissipation, which we attribute to slip of the bilayer at higher oscillation amplitudes. Complementary experiments using a combined QCM-D/reflectometry instrument showed that the Ca2+-induced changes in the viscoelastic/structural properties of the SLB are accompanied by changes in the optical properties. We discuss different scenarios to explain the observed reversible effect of Ca2+-ions on the dissipative and optical properties of the mixed SLBs. Based on our results we propose the observed phenomenon to be a combination of geometric changes, internal structural changes, changes in the interfacial water layer, and a slip mechanism, i.e. friction between the SLB and the substrate.