Atomistic study of zwitterionic peptoid antifouling brushes

Using molecular dynamics (MD) simulations the molecular behavior and hydration properties of a set of antifouling brushes formed by “peptoid” chains grafted on a rutile surface have been investigated. Peptoids are novel poly(N-substituted glycine) peptide mimics with sidechains attached to amide nitrogens. They constitute a unique model polymer system because hundreds of sidechains have been demonstrated, and the exact chain length and sequence order of the residues/monomers may be specified in experiments. In this report we study the behaviour of a zwitterionic sequence with demonstrated excellent resistance against protein adsorption and cell attachment. We vary the brush grafting density as well as the sidechain/polymer molecular volume. We include in our study polysarcosine, the simplest polypeptoid with only methyl groups as sidechains and which is reported to also exhibit high antifouling performance, as an uncharged comparison with a small polymer chain cross-section. Overall, we show in detail how molecular volume and hydration effects are intertwined in a zwitterionic polymer brush. For example, the zwitterionic design significantly promotes extended chain conformations and could actually lower the overall electrostatic potential. Some properties promoted by the balanced charges, such as chain flexibility and hydration, increase more prominently at “low” to “intermediate” chain densities. These and other observations should provide insight on the molecular behavior of peptoids and inform the design of zwitterionic antifouling polymer brushes.