Presentation and accessibility of surface bound ligands on amphiphilic graft copolymer films

Abstract

Amphiphilic comb-type graft copolymers comprising a poly(methyl methacrylate) (PMMA) backbone and short, polyethylene oxide (PEO) side chains, PMMA-g-PEO, are proposed to self-organize at the polymer/water interface, resulting in quasi-2D confinement of the backbone at the immediate surface. The branched architecture and amphiphilic chemistry of these polymers results in a dense PEO brush that resists cell adhesion. To facilitate specific cell-surface interactions, small biological molecules such as adhesion peptides can be selectively tethered to PEO chain ends. Quasi-2D confinement of the polymer backbone results in clustering of tethered epitopes on a length scale dictated by the backbone. The present work investigates two aspects of this polymer architecture on organization of tethered ligands: nanometer length-scale clustering through backbone 2D confinement, and tether length effects on the availability of tethered peptides for cell adhesion.

(cont.) To directly probe 2D confined polymer conformations, combs at the film/water interface were labeled with gold nanoparticles and observed by transmission electron microscopy. A 2D radius of gyration (Rg) was calculated by reconstructing nanoparticle-decorated chain trajectories, and compared with Monte Carlo simulations of a 2D melt of similarly broad length distribution. The 2D Rg calculated from observed conformations scaled with the number of backbone segments (N) as Rg - N.69-0.02 Monte Carlo simulations yielded a scaling exponent v = 0.67 + 0.03, suggesting that the deviation from classical 2D melt behavior (v= 0.5) arose from polydispersity. Tether length effects on cell adhesion to comb copolymer films functionalized with the adhesion peptide PHSRNGGGK(GGC)GGRGDSPY were further investigated by observing cell attachment and spreading on combs with long (22 EO unit) and short (10 EO unit) tethers. Lofiger tethers increased the rate of spreading and reduced the time required to form focal adhesions. Fluorescence resonance energy transfer (FRET) measurements suggest that the added mobility afforded by longer tethers allowed cells to reorganize tethered peptides.

(cont.) In addition, adhesion peptides were selectively coupled to short or long PEO tethers within a bimodal brush. Short peptide tethers in a bed of long inert chains did not promote cell attachment. Long peptide tethers with short inert chains resulted in cell attachment comparable to a monomodal brush of long chains. These findings may be of value in designing protein-resistant bioactive surfaces, where nanometer length-scale organization of ligands plays an important role in cell-surface interactions.