Tethering Efficiency of RAFT-Synthesised SMA Polymers and Associated SMALPs on Gold Surfaces.

Abstract

Styrene maleic acid lipid nanoparticles (SMALPs) arise from amphipathic SMA copolymer encapsulation of membranes into polymer-lipid nanodiscs, structures applied in the native extraction of membrane proteins (MPs). Strategies to immobilise 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) polymerisation was used to synthesise a library of diblock SMA copolymers to determine the optimal sequence for SMALP assembly. The further ability of trithiocarbonate (T) attached (Z)-end-groups, generated by RAFT polymerisation, to tether SMALPs to gold surfaces via sulfur-gold bonds was evaluated. Improved DMPC liposome solubilisation was 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 non-specific gold adhesion. These findings offer a new approach in utilising RAFT end-groups of nanodisc assembling polymers for label-free analysis of MPs.