Kinetic profile of the adsorption and conformational change of lysozyme on self-assembled monolayers as revealed by quartz crystal resonator

This report studied the adsorption behavior of lysozyme to hydrophobic and hydrophilic self-assembled monolayer (SAM(I) and SAM(II)) of thiol and gold surfaces, by using a quartz crystal microbalance (QCM) technique. This technique allows time-resolved simultaneous measurements of changes in frequency (fs) and energy dissipation (Rm) of the QCM during the adsorption process, which makes it possible to investigate the viscoelastic properties of the protein layer during the adsorption process. In the QCM measurements, the influence of the energy dissipation of the protein adsorption layer on the adsorption densities were discussed. An adsorption kinetics model including reversible and irreversible adsorption states was proposed to analyze the rate constants. The rate constants for reversible adsorption (ka1), irreversible adsorption (ka2), desorption (kd), and conformational transformation from reversible state to irreversible one (kf), were estimated from the real time responses of the QCM. The rate constants for lysozyme on methyl-terminated hydrophobic SAM(I) interface are ka1=(23.1±4.6) M−1 s−1, ka2=(3.72±0.49) M−1 s−1, kd=(4.4±0.6)×10−3 s−1 and kf=(7.3±1.9)×10−6 s−1. The rate constants for lysozyme on hydrophilic hydroxyl-terminated SAM(II) interface are ka1=(1.96±4.7) M−1 s−1, ka2=(0.51±0.09) M−1 s−1, kd=(11.2±0.7)10−3 s−1 and kf=(4.8±1.4)×10−6 s−1. The rate constants for lysozyme on bare gold are ka1=(18.7±5.1) M−1 s−1, ka2=(1.08±0.13) M−1 s−1, kd=(8.6±0.5)×10−3 s−1 and kf=(9.5±2.6)×10−6 s−1. The adsorption densities are in the order of SAM(I)>(Au)>SAM(II).