Viscoelastic change following adsorption and subsequent molecular reorganisation of a nitroreductase enzyme on a gold surface: A QCM study

A quartz crystal shear mode resonator operating at 5 MHz has been used to study viscoelastic changes which result from adsorption from solution of a layer of the enzyme nitroreductase on a gold electrode. The enzyme is anchored to the gold via thiol end-groups of cysteine residues the number of which, per molecule, has been varied by genetic modification. Using an appropriate model for the acoustic shear wave load on the resonator, the viscoelasticity of the adsorbed enzyme layer have been monitored as the layer develops. A negative viscosity is deduced for all the enzymes and is attributed to amplification of shear waves in the enzyme layer by energy transfer from a compression mode also produced by the crystal. In addition to gradual changes in the viscoelastic parameters as adsorption proceeds, spontaneous changes in viscoelasticity are observed, which indicate a rapid reorganization of the enzyme layer. These various changes, reflecting alterations in the mechanical response of the adsorbed enzyme to shear forces acting at 5 MHz, suggest that the quartz crystal shear wave resonator has considerable potential for studying structural motions in adsorbed proteins at frequencies not readily accessible by other techniques.