Tethering Efficiency of Reversible Addition-Fragmentation Chain Transfer-Synthesized Styrene Maleic Acid Polymers and Associated Styrene Maleic Acid Lipid Nanoparticles on Gold Surfaces.

Styrene maleic acid lipid nanoparticles (SMALPs) arise from amphipathic styrene maleic acid (SMA) copolymer encapsulation of membranes into polymer-lipid nanodiscs, structures applied in the native extraction of membrane proteins (MPs). Strategies to immobilize SMALPs via their polymer belt onto surfaces allow the biophysical study of MPs without direct protein-surface anchoring. In this work, reversible addition-fragmentation chain transfer (RAFT) polymerization is used to synthesize a library of diblock SMA copolymers to determine the optimal sequence for SMALP assembly. The further ability of trithiocarbonates (T) and attached (Z)-end-groups, generated by RAFT polymerization, to tether SMALPs to gold surfaces via sulfur-gold bonds is evaluated. Improved DMPC liposome solubilization is achieved with a hydrophilic (Z)-end-group, shorter polystyrene block and lower molecular weight for diblock R-(Sty)-b-(Sty-alt-MA)-T-Z polymers. Quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) revealed that diblock SMA polymers bound to gold as a micellular film, irrespective of the presence of the trithiocarbonate group. SMALPs, however, showed an enhanced gold affinity when terminated by a trithiocarbonate and hydrophilic RAFT (Z)-end-group compared to end-group removed SMALPs, the latter exhibiting nonspecific gold adhesion. These findings offer a new approach in utilizing RAFT end-groups of nanodisc assembling polymers for label-free analysis of MPs.