Kinetics of the homogeneous exchange of lysozyme adsorbed on a titanium oxide surface

The desorption and exchange processes of the protein lysozyme adsorbed on TiO2 particles in the presence of lysozyme in solution are investigated by means of a I-125 radio-labeling technique. Lysozyme is a compact and highly positively charged protein, making this study complementary to previous work devoted to the understanding of the exchange mechanisms of adsorbed proteins on solid surfaces. It is found that at least three populations of adsorbed molecules exist on the surface: (i) a rapidly exchangeable and/or desorbable population (The rate constants associated with the exchange and rapid desorption processes are equal to 6.28 x 10(4) L . mol(-1). h(-1) and 2.7 x 10(-4) s(-1), respectively), (ii) a population which does not exchange but can only desorb from the surface over a time scale of the order of 4 h and whose rate constant is equal to 6.8 x 10(-5) s(-1), and, (iii) finally, an irreversibly adsorbed population. It is also demonstrated that the homogeneous exchange process can be modeled by a reaction of order one with respect to molecules in solution. Moreover, it is suggested that both the desorption and exchange processes are of order one with respect to the adsorbed lysozyme molecules.