Synthesis and Polymerization of Substituted Stilbenes with Maleic Anhydride for Membrane Solubilization and Protein Extraction

Integral membrane proteins represent nearly 25% of protein encoding genes yet account for less than 2 percent of solved structures in protein databases. This underrepresentation is due to the difficulty in membrane protein isolation with the use of detergents. The difficulty in structural characterization following isolation has been alleviated by use of amphipathic polymers that stabilize proteins by maintaining the native protein environment. Of these amphipathic copolymers, SMA2000—a commercially available styrene-maleic anhydride copolymer with a 2:1 styrene to maleic anhydride ratio—has demonstrated broad utility in the isolation and subsequent characterization of a wide range of integral membrane proteins. However, there are some limitations to the use of SMA2000 that may arise from a lack of control of polymer parameters such as sequence, composition, and dispersity. Methyl substituted stilbene and maleic anhydride copolymers with controlled sequences and compositions demonstrated membrane solubilization and protein isolation activity at levels comparable to SMA2000, with several advantages that include an increase in the useable pH range and the size and homogeneity of polymer-lipid particles. Regrettably, the synthesis of methyl substituted stilbene-maleic anhydride copolymers is not without challenges that would make laboratory scale-up difficult. The use of anisole as a polymerization solvent mitigates gelation during poly((E)-4-methylstilbene-alt-maleic anhydride) synthesis. Additionally, unsymmetrical dimethyl-substituted stilbenes and monosubstituted stilbenes ((E)-4-tert-butylstilbene, (E)-4-methoxystilbene, (E)-2-methoxystilbene, methyl (E)-4-carboxylatestilbene, and (E)-4-trifluoromethylstilbene) copolymerize with maleic anhydride without gelation. Rates of conversion reveal that stilbenes with donor substituents copolymerize with maleic anhydride faster than stilbenes with acceptor substituents. The reversible addition–fragmentation transfer (RAFT) controlled radical polymerization technique afforded poly((E)-4-methylstilbene-alt-maleic anhydride) copolymers with controlled sequence, composition, molecular weight, and dispersity. Amphipathic copolymers with fluorescent activity are also desired for membrane protein solubilization. A series of donor-acceptor di-substituted stilbene and maleic anhydride copolymers with fluorescence activity have been synthesized and characterized. These copolymers possess electron-donating and electron accepting substituents on each phenyl ring of the stilbene monomer. These copolymers exhibit red-shifts that vary in position and width with changes in substituent.