Molecular-level interactions of nanodisc-forming copolymers dissected by EPR spectroscopy

Abstract

This study focuses on analyzing the noncovalent interaction patterns between three lipid-nanodisc-forming polymers and nitroxide radicals which are used as small organic tracer molecules. Besides the negatively charged polymers diisobutylene/maleic acid (DIBMA) and styrene/maleic acid (SMA) (2:1), the solvation behavior of a newly synthesized zwitterionic styrene/maleic amide sulfobetaine copolymer named SMA-sulfobetaine (SB) is characterized. The applied spin probes vary in their respective chemical structure, allowing the report of different local micropolarities and nanoscopic regions by recording temperature-dependent continuous-wave electron paramagnetic resonance (CW EPR) spectra. In combination with light scattering experiments, a nanoscopic interpretation of the dominant polymer/guest molecule interaction patterns is provided. The results indicate that in SMA and DIBMA, ionic interactions dominate the interaction patterns with other molecules. In SMA-SB, the zwitterionic side chains mainly induce a dynamic assembly with guest molecules due to weaker noncovalent interactions. Depending on the applied spin probe, temperature-dependent CW EPR measurements reveal nanoscopic cloud points depending on the interaction patterns with SMA-SB which can occur more than 20 °C below its macroscopically observed upper critical solution temperature. Finally, the detailed dissection of interaction patterns may provide a better understanding that may even allow tuning the polymers’ properties for use in lipid nanodisc formation.