| AVS 56th International Symposium & Exhibition | |
| Biomaterial Interfaces | Tuesday Sessions |
| Session BI-TuM |
| Session: | Proteins and Cell Interactions at Interfaces II |
| Presenter: | B. Atmaja, Stanford University |
| Authors: | B. Atmaja, Stanford University J.N. Cha, University of California, San Diego C.W. Frank, Stanford University |
| Correspondent: | Click to Email |
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. Previously, we reported the self-assembly of PEGLL-PLL with nanoparticles that were functionalized with carboxylic acid (COOH) moieties to form a variety of supramolecular structures. In relation to this previous work, we have used the PEGLL-PLL/MUA bilayer as a model system for studying the interfacial phenomena that occur when the PEGLL-PLL molecules interact with the 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 infrared (IR) spectroscopy, ellipsometry, and contact angle. Because our PEGLL-PLL/MUA bilayer system can potentially be applied as a nonfouling surface, we have thoroughly characterized other film properties such as the packing and graft density of the polypeptide molecules as a function of the PEGLL-PLL architecture. A complete understanding of the film’s molecular structure would then allow us to elucidate the relationship between the bilayer’s nonfouling characteristics and its underlying structure in our future work. Using IR spectroscopy, the adsorption process is determined to occur primarily by means of electrostatic interaction between the protonated PLL residues (pKa ~ 10.6) and carboxylate moieties of the MUA SAM (pKa ~ 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 a liquid-like/disordered packing on the surface. The average contact angle of the polypeptide/MUA 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 PEGLL-PLL 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.