The influence of the temperature on the surface topology, layer thickness and density of recrystallized bacterial S-layers from Bacillus sphaericus CCM2177 on polyelectrolyte multilayers in contact with liquid water was investigated. A quartz crystal microbalance with dissipation (QCM-D) was used to monitor the build-up of the polyelectrolyte multilayer and the adsorption of S-layer protein (1600 ng cm−2). The critical temperature (55 °C) at which the S-layer loses its 2-D structure was obtained from atomic force microscopy (AFM) and confirmed by neutron reflectometry (NR) experiments. The process of S-layer denaturation was found to be irreversible. Aggregates of denatured S-proteins resist lower loads than the crystalline nanostructure formed from folded S-proteins.
The combination of the QCM-D results with the scattering length density and film thickness (14 nm) obtained from neutron reflectometry studies permitted the estimation of the density of adsorbed S-protein together with the bound water (M = 1.16 g cm−3), the dry protein scattering length density (2.02 × 10−6 Å−2) and the S-protein mass density (1.48 g cm−3). The results confirmed that S-proteins form very loosely packed layers on polyelectrolyte multilayers incorporating a water volume fraction of around 68%.
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