Immobilization of metallothionein on gold/mica surfaces:: Relationship between surface morphology and protein-substrate interaction

We have studied the surface coverage as well as the morphological and mechanical properties of metallothionein (MT) layers immobilized on gold and mica surfaces by means of three different strategies based on (1) unspecific adsorption/chemisorption (MT-Au system), (2) covalent binding (MT-modified Au system), and (3) physisorption (MT-mica). These systems have been studied by quartz crystal microbalance and atomic force microscopy techniques. The monolayers of metallothionein obtained by direct adsorption onto gold surfaces are disordered and relatively rough in contrast to those obtained by covalent binding of the protein to a previously functionalized substrate, which are smoother and show a close-packed ordered molecular arrangement. For the MT-mica system the MT layer is as smooth as the MT-modified Au layer although the imaged protein dimensions are larger, indicating a strong protein-substrate interaction. These different properties are explained on the basis of the different MT-substrate interactions governing the MT adsorption process and the MT adsorption sites available in each case, Besides, for the MT-Au system the proteins are imaged in the attractive and repulsive force regimes, the proteins being elastically deformed in the latter regime. Our results also show that covalent binding strategies not only allow the protein-substrate interaction to be controlled but can also lead to the protein layer ordering.