Influence of block microstructure on the interaction of styrene-maleic acid copolymer aggregates and lipid nanodiscs

Investigation of the properties of membrane proteins (MPs) is essential to the successful development of medicines and biotechnology. However, their study is often complicated by denaturation caused by the use of detergents during conventional extraction methods. Copolymers of styrene and maleic acid (SMA) have shown promise in extracting MPs directly from cells while reconstituting lipid membranes into nanodiscs. Despite their potential, there remains a dearth of information on the precise interactions that take place between the copolymers and lipid membranes although they are known to be sensitive to small variations in copolymer composition or structure. We have used reversible addition-fragmentation chain transfer (RAFT) polymerisation to synthesise SMA copolymers with equivalent molar mass, but with inverted block sequences and end group termini. Through a range of experiments, including dynamic light scattering and small-angle neutron scattering (SANS) on SMA aggregates and nanodisc formation studies using UV-vis spectroscopy with both model DMPC lipids and E. coli membranes, the impact of both block distribution and end group chemistry on copolymer-membrane interactions was investigated. It was found that mismatched hydrophilic and hydrophobic end groups on the styrene block and alternating block, respectively, impeded membrane disruption and subsequent solubilisation. This highlights not only how the amphiphilic balance of these blocks is important for efficient nanodisc formation, but also how end groups influence these and may be optimised towards extraction of more challenging MPs. The work contributes to a better understanding of SMA behaviour and offers insight into how these nanomaterials may be better designed and tailored for specific applications.