Protein Adsorption Studied by Atomic Force Microscopy
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A wide range of techniques provides information on protein adsorption onto solid surfaces. Vibrational sum frequency spectroscopy (VSFS)
The interaction force between two solid surfaces can be measured as a function of distance by the surface force apparatus (SFA). Claesson et al. employed SFA to study a wide range of proteins (globular, unordered, fibrous) and determined protein conformation, orientation, and the operative forces. Small compact globular and soft globular proteins could be distinguished by measuring their compressibility. Blomberg et al. used SFA to study the adsorption of lysozyme on mica as a function of protein concentration, determining the protein's adsorption orientation, ability to form multilayers, and adsorption irreversibility.
Conformation information on adsorbed protein is also available from several spectroscopic methods. Circular dichroism spectroscopy (CD), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy all probe protein secondary structure. These techniques have been useful in demonstrating how proteins alter their structure upon adsorption. Proteins have been shown to have greater structural perturbation on hydrophobic surfaces, compared to hydrophilic surfaces using CD, FTIR, and Raman spectroscopy.
A significant amount of literature on protein adsorption is concerned with the kinetics and total mass of protein adsorbed onto a solid surface. The quartz crystal microbalance (QCM) measures the changes in resonance frequency and dissipation factor of an oscillating quartz crystal and can provide information on the adsorbed mass and temporal variations in surface viscoelastic properties. Otzen et al. employed QCM to study the adsorption of protein S6 onto a methyl-terminated quartz surface and found that the adsorption kinetics of protein S6 depends on the equilibrium fraction of denatured protein in the bulk, rather than on the kinetics of bulk denaturation. Upon comparison with optical techniques, such as ellipsometry and optical waveguide lightmode spectroscopy (OWLS), Hook et al. showed that QCM reports higher adsorbed mass, this being attributed to water bound to the adsorbed protein.