In situ SERS studies on the adsorption of tyrosinase on bare and alkanethiol-modified silver substrates

Copper-containing phenols oxidase enzyme tyrosinase (TY) from mushroom was adsorbed on bare silver substrates and on silver substrates modified with monolayers formed from various ω-functionalised alkanethiols: HS–(CH2)2–NH2 (CYS), HS–(CH2)2–SO3Na (MES), HS–(CH2)2–COOH (MPA), HS–(CH2)10–COOH (MUA) and HS–(CH2)15–COOH (MHA). Structures of formed adlayers have been determined by means of surface-enhanced Raman scattering (SERS) measurements. The results of our experiments suggest that bonding of TY to the silver surface takes place, to a large extent, via the aromatic rings oriented parallel to the metal surface. The integrity of monolayers formed from all studied thiols is not destroyed after immersion in a TY aqueous solution. Previous measurements carried out for laccase (other widely used copper-containing polyphenol oxidase enzyme) revealed that linkage monolayers formed from short-chain alkanethiols (CYS, MES, MPA) do not prevent direct interaction of this enzyme with the metal surface. It means that the structure of the linkage monolayer is significantly different when covered with various enzymes. We found that for the Ag/MPA/TY system a significant part of MPA molecules interact with the silver surface via both sulphur moiety and the carboxylic group. The other TY-induced changes of the thiolates linkage monolayer structure are very similar to the previously reported changes of the structure of these monolayers induced by the adsorption of bovine serum albumin [see A. Kudelski, Vib. Spectrosc. 41 (2006) 83].