Adsorption properties of protein Papain at the solid|liquid (0.1 M KCl) interfaces of different hydrophobicity [highly oriented pyrolytic graphite (HOPG), bare gold, CH3, OH, and COOH-terminated self-assembled monolayers on gold] were studied by a combined quartz crystal microbalance and atomic force microscopy techniques. It was found that Papain forms an incomplete monolayer at hydrophobic interfaces (HOPG and CH3-terminated substrate), whereas on more hydrophilic ones, a complete monolayer formation was always observed with either the onset of the formation of a second layer (bare gold substrate) or adsorption in a multilayer fashion, possibly a bilayer formation (OH-terminated substrate). The surface concentration and compact monolayer film thickness was much lower on the COOH-terminated substrate compared to other surfaces studied. This result was explained by partial dissociation of the interfacial COOH groups leading to additional electrostatic interactions between the positively charged protein domains and negatively charged carboxylate anions, as well as to local pH changes promoting protein denaturation.

Institute of Chemistry of Organometallic Compounds CNR via Moruzzi 1 561 24 Pisa Italy

J Heyrovský Institute of Physical Chemistry of ASCR v v i Dolejškova 3 182 23 Prague Czech Republic

J Heyrovský Institute of Physical Chemistry of ASCR v v i Dolejškova 3 182 23 Prague Czech Republic and Faculty of Science J E Purkyně University České mládeže 8 400 96 Ústí nad Labem Czech Republic