Liquid-Liquid Phase Separation (LLPS)

Liquid–liquid phase separation (LLPS) is a biophysical process in which a homogeneous solution of biomolecules, typically proteins, nucleic acids, or their complexes, spontaneously separates into two coexisting liquid phases. One phase becomes enriched in specific biomolecules and forms condensates, while the other remains a dilute surrounding phase.

This separation occurs without membranes and gives rise to dynamic, reversible, and functionally specialized compartments known as biomolecular condensates or membraneless organelles. LLPS can be triggered by changes in concentration, temperature, pH, or ionic strength, leading to the formation of cellular structures such as stress granules, P-bodies, and nucleoli that selectively concentrate proteins and RNAs while maintaining liquid-like dynamics.

The formation, dissolution, and material state transitions of these condensates are governed by a finely tuned balance of multivalent molecular interactions, intrinsically disordered proteins (IDPs), intrinsically disordered regions (IDRs), and environmental conditions.

Fluorescence-based techniques are particularly well suited to studying liquid–liquid phase separation, as they enable quantitative analysis of molecular dynamics, interactions, and local environments in both dilute and condensed phases. Fluorescence correlation spectroscopy (FCS) provides access to molecular diffusion and exchange between phases, revealing differences in mobility inside and outside biomolecular condensates. Förster resonance energy transfer (FRET) and fluorescence anisotropy probe molecular interactions and local crowding within condensates, while fluorescence lifetime measurements and fluorescence lifetime imaging microscopy (FLIM) add sensitivity to environmental changes and heterogeneity. Together, these complementary approaches allow LLPS to be characterized across multiple length and time scales under physiologically relevant conditions.