Electrochemical quartz crystal nanobalance (EQCN) studies of the adsorption behaviour of an enzyme, mandelate racemase, and its substrate, mandelic acid, on Pt

The adsorption behaviour of the enzyme, mandelate racemase (MR), and its substrate, (S)-mandelic acid (MA), was studied at a polycrystalline Pt surface in pH 7.4 phosphate buffer at 294 K using the electrochemical quartz crystal nanobalance (EQCN) technique of simultaneous cyclic voltammetry (CV) and frequency measurements. It was shown that the EQCN frequency measurements did not directly monitor the molar mass of the adsorbed protein at anodic potentials, but instead measured changes in the surface oxide in the absence and presence of adsorbed protein. However, at a potential characteristic of the double layer for platinum, EQCN frequency measurements gave a measure of the extent of solvent displacement by the adsorbed protein. The adsorption process was modelled using the Langmuir adsorption isotherm. The values for the Gibbs energy of adsorption, ΔGADS, obtained with these EQCN frequency measurements gave excellent agreement within experimental uncertainty with those obtained from the simultaneous CV measurements for both the enzyme and substrate, and showed the enzyme to have a greater affinity for the surface than the substrate molecule. The maximum surface concentrations from CV and EQCN frequency measurements gave excellent agreement for the two techniques and showed MA to be adsorbed in a tilted orientation with monolayer coverage. On the other hand, the surface concentrations showed the presence of ∼2.5 monolayers of enzyme from charge transfer CV measurements, while the frequency results showed the presence of a densely packed monolayer from the “footprint” of solvent displacement by the adsorbed enzyme molecules on the electrode surface. The two techniques provide valuable complementary information on the interfacial behaviour of molecules.