Acetylcholinesterase Adsorption on Modified Gold: Effect of Surface Chemistry on Enzyme Binding and Activity

Surface chemistry plays a crucial role in the performanceof biosensorsand biocatalysts, where enzymes directly interact with a solid support.In this work, we investigated the effect of surface charge and hydrophobicityon the binding and activity of acetylcholinesterase (AChE) followingdirect adsorption to modified gold surfaces. Surface modificationsincluded self-assembled monolayers (SAMs) terminated with -COO-, -NH3 (+), -OH, and-CH3 functional groups at varying mole %. We alsoinvestigated the effects of positively and negatively charged helicalpeptides covalently coupled to the SAM. Using spectroscopic ellipsometry,we measured the surface concentration of AChE on each modified surfaceafter 1 h of adsorption. We found that surface concentration was directlyproportional to surface hydrophobicity (r = 0.76).The highest binding was observed on the more hydrophobic surfaces.We also measured the specific activity of AChE on each surface usinga colorimetric assay and found that activity was inversely proportionalto surface hydrophobicity (r = -0.71). Thehighest activity was observed on the more hydrophilic surfaces. Plottingspecific activity versus surface concentration showed a similar relationship,with the highest activity observed at low AChE densities (& SIM;20%of a monolayer) on surfaces terminated with 50% -COO- or -NH3 (+) and 50% -CH3 functional groups. Interestingly, this is similar to the approximatecomposition of hydrophobic versus hydrophilic amino acid residueson the surface of AChE. These surfaces also exhibited the highesttotal activity: a & SIM;100% improvement over bare gold due to acombination of moderate binding and high activity retention. Thiswork highlights the importance of developing new attachment strategiesbeyond direct adsorption that promote, tune, and optimize both highbinding and high activity retention.