Adsorbed α-helical diblock copolypeptides: Molecular organization, structural properties, and interactions

Abstract

In this work, we have developed 11-mercaptoundecanoic acid (MUA)-polypeptide "bilayer" systems by adsorbing poly(diethylene glycol-L-lysine)-poly(L-lysine) (PEGLL-PLL) diblock copolypeptide molecules of various architectures onto MUA-functionalized gold substrates. An objective of our present work is to use the PEGLL-PLL/MUA bilayer as a model system for studying the interfacial phenomena that occur when PEGLL-PLL molecules interact with carboxylic acid (COOH) moieties of nanoparticle ligands. Specifically, we have elucidated the nature of the interactions between the PEGLL-PLL and COOH moieties as well as the resulting polypeptide conformation and organization, using a combination of surface techniques-grazing-incidence IR spectroscopy, ellipsometry, and contact angle. We have also thoroughly characterized other film properties such as the packing and graft density of the polypeptide molecules as a function of the PEGLL-PLL architecture. From the IR data, the adsorption process occurs primarily by means of electrostatic interaction between the protonated PLL residues (pKa ≈ 10.6) and carboxylate moieties of the MUA self-assembled monolayer (SAM) (pK3 ≈ 6) that is enhanced by H-bonding. The PLL block is thought to adopt a random-coil (extended) conformation, while the PEGLL block that is not interacting with the MUA molecules is found to adopt an α-helical conformation with an average tilt angle of ∼60°. The PEGLL-PLL molecules have also been deduced to form a heterogeneous film and adopt liquidlike/disordered packing on the surface. The average contact angle of the MUA-polypeptide bilayer systems is ∼40°, which implies that the diethylene glycol (EG2) side chains of the PEGLL residues may be oriented somewhat toward the surface normal. From ellipsometry measurements, it is found that PEGLLx-PLL y molecules with a longer α-helical block are associated with a lower graft density on the MUA surface compared to those with a shorter α-helical block. This observation may be attributed to the greater repulsion-steric and H-bonding effects-that is imposed by the EG2 side chains found on and projected area occupied by the longer PEGLL block. The bilayer systems have been found to be extremely stable over a 2-week period with no changes in the contact angle, thickness, polypeptide tilt angle, or conformation. Beyond that, there is a gradual decrease in the thickness and increase in the contact angle of the bilayer that could be attributed to the oxidation of the MUA SAM molecules. © 2009 American Chemical Society.