BPS2026 – pH-dependent mesophase transitions as key determinants of lipid nanoparticle function

Lipid nanoparticles (LNPs) are the leading platform for nucleic acid delivery and mRNA vaccination, yet the structural determinants of their activity remain incompletely understood. Here, we show that cationic ionizable lipids (CILs) undergo pH-driven structural transitions—from inverse cubic (Fd3m) to inverse hexagonal (HII) phases—in model systems, and that these transitions correlate with endosomal fusion and the timing of gene delivery. Using high-resolution small-angle X-ray scattering and live-cell imaging on single-cell arrays (LISCA), we directly link these phase changes to the onset and efficiency of mRNA expression. We present an integrated approach combining small-angle X-ray scattering (SAXS) experiments, molecular dynamics (MD) simulations, and a continuum model to elucidate lipid distribution and water content within HII phases. The different approaches consistently yield water contents that seem to correlate with the lipids’ transfection efficiencies. Together, our findings identify structural transitions as a key mechanism in LNP-mediated delivery that offer a powerful approach for rational design and optimization of lipid nanoparticles.