Direct real-time imaging of protein adsorption onto hydrophilic and hydrophobic surfaces

Atomic force microscopy has been used to follow in real time the adsorption from solution of two of the gliadin group of wheat seed storage proteins onto hydrophilic (mica) and hydrophobic (graphite) surfaces. The liquid cell of the microscope was used initially to acquire images of the substrate under a small quantity of pure solvent (1% acetic acid). Continuous imaging as an injection of gliadin solution entered the liquid cell enabled the adsorption process to be followed in situ from zero time. For omega-gliadin, a monolayer was formed on the mica substrate during a period of approximately 2000 s, with the protein molecules oriented in parallel to the mica surface. In contrast, the omega-gliadin had a relatively low affinity for the graphite substrate, as demonstrated by slow and weak adsorption to the surface. With gamma-gliadin, random deposition onto the mica surface was observed forming monodispersed structures, whereas on the graphite surface, monolayer islands of protein were formed with the protein molecules in a perpendicular orientation. Sequential adsorption experiments indicated strong interactions between the two proteins that, under certain circumstances, caused alterations to the surface morphologies of preadsorbed species. The results are relevant to our understanding of the interactions of proteins within the hydrated protein bodies of wheat grain and how these determine the processing properties of wheat gluten and dough.