Conformational transitions of maleic acid segment drive pH induced changes in SMA polymer structure and solubility

[Display omitted] •The molecular mechanism of the pH-mediated conformational transition in the SMA polymer has been investigated.•This work used a combination of NMR, IR and fluorescence spectroscopy together with DLS and quantum chemical calculations.•With decreasing pH, the polymer undergoes dihedral angle changes and hydrogen bonds form between maleic fragments. The popularity of styrene-maleic acid (SMA) copolymers is based on their ability to solubilize biological membranes into nanoscale disks facilitating the extraction and purification of membrane proteins. Unfortunately, these disks are unstable in acidic environments which is due to the conformational transition of the SMA polymer. In this work, using a combination of NMR, IR and fluorescence spectroscopy together with DLS and quantum chemical calculations, we have investigated the pH-mediated conformational transition in the SMA polymer. With decreasing pH, polymer undergoes the following changes: 1) the dihedral angle in the maleic moieties of the polymer changes from 180° to 60° and then reaches 50°; 2) the carboxyl groups of the maleic fragment form hydrogen bonds with other maleic fragments, leading to aggregation of the SMA molecules; 3) the phenyl rings of the styrene fragments are brought closer together and the hydrophobicity of the environment inside the SMA polymer globule increases.