Protein resistant surfaces are especially important for biosensoric applications, e.g., protein analytics and biomedical coatings for catheters and implant materials in order to prevent biofilm formation and resultant infections. Polyethylene glycols are presently the most frequently applied polymers for biologically inert surfaces. Due to structural similarity monofunctionalized polyglycerol derivates have been appropriately developed to enable a direct surface functionalization (self-assembled monolayer on gold). Surface plasma resonance (SPR) was chosen as an analytical tool to affirm the thickness of the absorbed biofilm layer. Thiol-functionalized dendritic polyglycerine proved to have very good protein resistant properties. Current research investigates cell and bacteria adsorption of these novel protein resistant surfaces and to set up a structure-activity relationship (Scheme 1).
Self-assembly of thiol-functionalized dendritic polyglycerol and protein resistant properties of polyglyerol nanolayers
Our group has synthesized several disulfide-functionalized hyperbranched PGs by coupling thioctic acid. The spontaneously formed self-assembled monolayers (SAMs) on a gold surface prevented the adsorption of proteins to the same level as for PEG SAMs and significantly better than dextran. A structure–activity relationship revealed that the more globular the structures, the more inert the assembly is towards protein adsorption, while the molecular weight changes did not appear to have a significant effect. A small improvement in the protein resistance was observed by substitution of the hydrogen bond donor groups by methyl groups, which was in agreement with the hypothesis referring to the absence of hydrogen-bond donors.
In order to study the mechanism of protein resistance in more detail we recently prepared a large variety of PG derivatives to assess their protein-resistance behavior. Two different approaches to modify the gold surfaces, the "anhydride" method and direct chemisorption of alkanethiolates, were used. In the first case, a library of mono-amino oligoglycerols (linear, dendrons, and hyperbranched) with terminal hydroxyl or methoxy functionality were synthesized and subsequently coupled to gold surfaces coated with mercaptoundecanoic acid. Surface plasmon resonance (SPR) spectroscopy with parallel adsorption measurements of four model proteins (fibrinogen, albumin, lysozyme, and pepsin) revealed that the capability of PG dendrons to resist non-specific protein adsorption strongly depends on the coating density, surface functionality, size, and structural freedom of the PG structures. The coating density using the anhydride method was considerably lower for higher generation ([G.3] and [G.4]) which resulted in a lower protein resistance. By comparison of the perfect [G.3] PG dendron and its hyperbranched analog we concluded that the conformational freedom is critical for achieving a high protein resistance (reduction of the protein adsorption from 47 to 17%). In the case of the methylated hyperbranched PG the best protein resistant surface (0.5% fibrinogen adsorption) was obtained, although contact angle measurements indicated a quite hydrophobic surface. In the direct chemisorption of alkanethiolates approach, there was better coating efficiency. Self-assembled monolayers with different terminal functionalities (OH, OCH3) were prepared by chemisorption of the different PG derivatives onto gold chips from ethanolic solution. Surprisingly, gold surfaces modified with [G.1]-OH thiolate already showed a dramatic decrease of the fibrinogen adsorption on the SAMs, indicating that already relatively small oligomers can be protein resistant.