The adsorption behavior of hard and soft proteins under the effect of an external electric field was investigated by a combination of spectroscopic ellipsometry and molecular dynamics (MD) simulations. Optically transparent carbon electrodes (OTCE) were used as conductive, sorbent substrates. Lysozyme (LSZ) and ribonuclease A (RNase A) were selected as representative hard proteins whereas myoglobin (Mb), α-lactalbumin (α-LAC), bovine serum albumin (BSA), glucose oxidase (GOx), and immunoglobulin G (IgG) were selected to represent soft proteins. In line with recent publications from our group, the experimental results revealed that while the adsorption of all investigated proteins can be enhanced by the potential applied to the electrode, the effect is more pronounced for hard proteins. In contrast with the incomplete monolayers formed at open-circuit potential, the application of +800mV to the sorbent surface induced the formation of multiple layers of protein. These results also suggest that this effect can be related to the intrinsic polarizability of the protein (induction of dipoles), the resulting surface accessible solvent area (SASA), and structural rearrangements induced upon the incorporation on the protein layer. The described experiments are critical to understand the relationship between the structure of proteins and their tendency to form (under electric stimulation) layers with thicknesses that greatly surpass those obtained at open-circuit conditions.
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