Protein Adsorption on Grafted Zwitterionic Polymers Depends on Chain Density and Molecular Weight

This study demonstrates that protein adsorption on end-grafted, zwitterionic poly(sulfobetaine) (pSBMA) thin films depends on the grafting density, molecular weight, and ionic strength. Zwitterionic polymers exhibit ultralow nonspecific fouling (protein adsorption) and excellent biocompatibility. This picture contrasts with a recent report that soluble pSBMA chains bind proteins and alter the protein folding stability. To address this apparent contradiction, the dependence of protein adsorption on the chain grafting parameters is investigated: namely, the grafting density, molecular weight, and ionic strength. Studies compared the adsorption of phosphoglycerate kinase and positively charged lysozyme versus the scaled grafting parameter s/2R_F, where s is the distance between grafting sites and R_F is the Flory radius. Plots of the adsorbed protein amount versus s/2R_F exhibit a bell-shaped curve, with a maximum near s/2R_F ≈ 1 and an amplitude that decreases with ionic strength. This behavior is qualitatively consistent with theoretical models for colloid interactions with weakly attractive, grafted chains. The results confirm that proteins do adsorb to pSBMA thin films, and they suggest an underlying mechanism. Comparisons with polymer models further identify design rules for pSBMA films that effectively repel protein.