Adsorption of β-lactoglobulin A on gold surface determined in situ by QCM-D measurements

Abstract

In this work, we described in detail the physicochemical properties of β-lactoglobulin isoform A (LGB-A) in a bulk solution and on a gold surface. DLS studies confirmed that the LGB trend for aggregation and the formation of larger oligomeric forms are dependent on environmental conditions. The isoelectric point of the system was determined using dynamic surface tension measurements and electrophoretic mobility, which are located at pH 5.45 and 4.8, respectively. The use of these methods has provided valuable information about the conformation stability of the LGB molecule in the electrolyte solution. The properties of self-assembling the LGB-A layer adsorbed on the gold surface were estimated in situ using a quartz crystal microbalance with dissipation monitoring (QCM-D) and Fourier transform infrared spectroscopy (FTIR) as a function of protein concentration, pH, and ionic strength. Based on the measurement of changes in the surface tension at the gas/liquid interface as well as the efficiency of adsorption at the liquid/solid interface using the QCM-D method, we estimated the preferred side-on orientation of the LGB-A molecules. The effectiveness of LGB-A adsorption on the gold surface was mostly controlled by electrostatic interactions that were confirmed by a very strong correlation with the zeta potential of the protein molecule and the gold surface. FTIR spectra shows significant changes in the secondary structure of the adsorbed protein. Particular changes concern the content in the structure β-sheet and random coil forms, with the α-helix content unchanged. The behaviour of the LGB molecule at the interface is in accordance with the direction of the dipole moment of the molecule as determined by molecular dynamics (MD).