Two-distinct polymer ubiquitin conjugates by photochemical grafting-from.

Protein-polymer bioconjugates present a way to make enzymes more efficient and robust for industrial and medicinal applications. While much work has focused on mono-functional conjugates, i.e. conjugates with one type of polymer attached such as poly(ethylene glycol) or poly(N-isopropylacrylamide), there is a practical interest in gaining additional functionality by synthesizing well-defined bifunctional conjugates in a hetero-arm star copolymer architecture with protein as the core. Using ubiquitin as a model protein, a synthetic scheme was developed to attach two different polymers (OEOMA and DMAm) directly to the protein surface, using orthogonal conjugation chemistries and grafting-from by photochemical living radical polymerization techniques. The additional complexity arising from attempts to selectively modify multiple sites led to decreased polymerization performance and indicates that ICAR-ATRP and RAFT are not well-suited to bifunctional bioconjugates applications. Nonetheless, the polymerization conditions preserve the native fold of the ubiquitin and enable production of a hetero-arm star protein-polymer bioconjugate.

Mapping protein-polymer conformations in bioconjugates with atomic precision.

Rational design of protein-polymer bioconjugates is hindered by limited experimental data and mechanistic understanding on interactions between the two. In this communication, nuclear magnetic resonance (NMR) paramagnetic relaxation enhancement (PRE) reports on distances between paramagnetic spin labels and NMR active nuclei, informing on the conformation of conjugated polymers. 1H/15N-heteronuclear single quantum coherence (HSQC) NMR spectra were collected for ubiquitin (Ub) modified with block copolymers incorporating spin labels at different positions along their backbone. The resultant PRE data show that the conjugated polymers have conformations biased towards the nonpolar ß-sheet face of Ub, rather than behaving as if in solution. The bioconjugates are stabilized against denaturation by guanidine-hydrochloride, as measured by circular dichroism (CD), and this stabilization is attributed to the interaction between the protein and conjugated polymer.