Adsorption of a small protein to a methyl-terminated hydrophobic surfaces:: effect of protein-folding thermodynamics and kinetics

We have studied the adsorption kinetics of a small monomeric protein S6 using the quartz crystal microbalance with dissipation monitoring (QCM-D) technique. Competitive adsorption from various proportions of native (Nat) and denatured (Den) protein in the bulk phase was carried out using a range of chemical denaturant concentrations. The ratio between Nat and Den in bulk has a profound affect on the adsorption behavior, most obvious from a significant (one order of magnitude) increase in the rate of a lag- and consolidation-adsorption phase when Nat is the major species present in bulk, signaling that these adsorption phases originates from the Den fraction of proteins in the bulk. To determine whether the kinetics of protein unfolding in the bulk phase are rate-limiting for adsorption of Nat, the adsorption kinetics of wildtype S6 with the mutant VA85 (whose unfolding kinetics are around 30 times more rapid under conditions of equal thermodynamic stability) were compared. It is concluded that adsorption of Nat is not rate-limited by the rate of unfolding in solution, leading to the speculation that Nat, upon binding on the surface, is trapped as a misfolded state whose structural dynamics do not correlate with those of Nat. (C) 2002 Elsevier Science B.V. All rights reserved.