Lipid Composition Modulates Secondary Structure at the Biocondensate-Membrane Interface

Biomolecular condensates (BMCs) formed through liquid–liquid phase separation (LLPS) play key roles in cellular organization, yet their molecular-scale interactions with membranes remain poorly understood. Here, we use surface-enhanced infrared absorption spectroscopy (SEIRAS) to quantify the secondary structure of poly-GR (glycine–arginine) inside condensates in contact supported lipid bilayers. Amide I SEIRAS provides a surface-sensitive measure of the peptide backbone conformation at the membrane-condensate interface. Results show that neutral POPC bilayers preserve the β-sheet rich organization of the bulk condensate, whereas negatively charged POPC/POPS bilayers reduce β-sheet content, and enhanced β-turn formation, accompanied by perturbations of the lipid headgroups. These results demonstrate that membrane charge modulates condensate secondary structure, hydration, and interfacial behavior, providing molecular-level insight into electrostatic regulation of condensate organization at the membrane-water interface.