Does the Sauerbrey equation hold true for binding of peptides and globular proteins to a QCM?: A systematic study of mass dependence of peptide and protein binding with a piezoelectric sensor.

Abstract

The term ‘microbalance’ arose after Sauerbrey showed a mass per unit area dependence on sensor frequency change for thin, solid films on a QCM. Others have extrapolated this relationship to interactions with biological ‘soft’ matter using acoustic wave devices. We rigorously examined the relationship between QCM frequency change and the molecular weight of protein and peptide analytes on a RAP♦id 4™ system using more than 120 individual assays. A series of amino acid, peptides and proteins with molecular weight from 372 to 150,000Da constituting a molecular weight ladder were biotinylated with a target biotin/protein ratio close to one to minimize avidity effects. Analyte concentration and contact time were chosen so as to attain near saturation of an anti-biotin antibody surface. The series resistance and resonant frequency changes (dF and dR) arising from a 5-parameter fit of the imaginary component of the impedance signal were analysed, giving a linear relationship (R2=0.98) between frequency response and analyte molecular weight, even down to level of a single amino-acid. As predicted by theory, there was also a linear relationship between the changes in density and viscosity of the liquid in contact with the sensor and both dF and dR. The resistance and resonance frequency changes recorded for mixtures of deuterium oxide and glycerol were the sum of changes induced by each individual liquid. Hence the Sauerbrey equation for mass per unit area dependence of QCM signal does hold true for peptides and proteins in a liquid.