Protein-ligand interactions at poly (amidoamine) dendrimer monolayers on gold

Avidin-biotin interactions as a typical protein-ligand model were investigated on the monolayers of a fourth-generation poly(amidoamine) dendrimer that were constructed on the self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA) on gold. Surface plasmon resonance (SPR) spectroscopic analysis revealed a resonance angle shift of 0.34degrees +/- 0.03degrees for the formation of dendrimer monolayers on reactive SAMs, which indicates that about 89% of the gold surface is covered with dendrimer molecules. The dendrimer monolayers were functionalized with biotin, and the efficacy of dendrimer monolayers as a biomolecular interface was evaluated in terms of the surface density of biotin ligands and the avidin binding level. For comparisons, the mixed SAMs and polymeric layers of poly-I.-lysine (PLL) on MUA SAMs were prepared and examined by a similar procedure. The specific binding of avidin to the biotinylated dendrimer monolayers, approached a surface density of 5.0 +/- 0.2 ng(.)mm(-2), which corresponds to about 88% surface coverage by avidin, showing a much higher level than those from mixed SAMs (2.3 +/- 0.1 ng(.)mm(-2)) and PLL layers (3.2 +/- 0.2 ng(.)mm(-2)). Interestingly, the fully biotinylated dendrimer monolayers gave rise to efficient avidin-biotin interactions, resulting in about 80% of the maximum avidin binding level, even under the condition that a serious steric hindrance would occur due to densely packed biotin ligands. These results strongly imply that efficient avidin-biotin interaction originates from a structural feature of dendrimer monolayers such as a surface exposure of derivatized biotin ligands and a corrugated surface.