Characterisation of membrane mimetics on a dual polarisation interferometer

Dual polarisation interferometry (DPI) has been used to characterise the formation of hybrid bilayer membranes (HBM) on a silicon-oxynitride surface. This technique allows the simultaneous determination of multiple physical properties of an HBM, as the HBM is being formed in a single experiment: mass, thickness in the z-direction (normal to the surface), tilt angle of the first layer and refractive index. Decanoic acid was covalently attached to an amine modified silicon-oxynitride sensor chip surface via 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride condensation reaction. The decanoic acid layer was 0.92 ± 0.12 nm thick, indicating a tilt angle of 57° from surface normal, and possessed a mass of 1.05 ± 0.10 ng mm−2 and a refractive index (RI) of 1.450 ± 0.020. Phospholipid vesicles made from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) were injected over the fatty acid surface to form an HBM. The DPPC HBM was 4.32 ± 0.68 nm thick, with a total mass of 3.18 ± 0.60 ng mm−2 and a RI of 1.404 ± 0.007. The DMPC HBM was 2.12 ± 0.34 nm thick, with a total mass of 2.25 ± 0.51 ng mm−2, and a RI of 1.435 ± 0.007. DPI thus provides an insight into HBM formation and differences between the structural organisation of HBMs of different composition.