Viscoelastic properties of adsorbed and cross-linked polypeptide and protein layers at a solid–liquid interface

Abstract

The real-time changes in viscoelasticity of adsorbed poly(L-lysine) (PLL) and adsorbed histone (lysine rich fraction) due to cross-linking by glutaraldehyde and corresponding release of associated water were investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D) and attenuated total reflection Fourier transform infrared spectroscopy (ATR/FTIR). The kinetics of PLL and histone adsorption were measured through changes in mass adsorbed onto a gold-coated quartz surface from changes in frequency and dissipation and using the Voigt viscoelastic model. Prior to cross-linking, the shear viscosity and shear modulus of the adsorbed PLL layer were ∼ 3.0 × 10 −3 Pa s and ∼ 2.5 × 10 5 Pa , respectively, while after cross-linking, they increased to ∼ 17.5 × 10 −3 Pa s and ∼ 2.5 × 10 6 Pa , respectively. For the adsorbed histone layer, shear viscosity and shear modulus increased modestly from ∼ 1.3 × 10 −3 to ∼ 2.0 × 10 −3 Pa s and from ∼ 1.2 × 10 4 to ∼ 1.6 × 10 4 Pa , respectively. The adsorbed mass estimated from the Sauerbrey equation (perfectly elastic) and the Voigt viscoelastic model differ appreciably prior to cross-linking whereas after cross-linking they converged. This is because trapped water molecules were released during cross-linking. This was confirmed experimentally via ATR/FTIR measurements. The variation in viscoelastic properties increased substantially after cross-linking presumably due to fluctuation of the randomly cross-linked network structure. An increase in fluctuation of the viscoelastic properties and the loss of imbibed water could be used as a signature of the formation of a cross-linked network and the amount of cross-linking, respectively.