liquiddisordered

Liquid disordered, often abbreviated as Ld, is a phase observed in lipid bilayer membranes in which the fatty acyl chains are highly disordered and loosely packed. This results in a fluid, dynamically evolving environment with little long-range structural order. In Ld, acyl chains adopt numerous gauche conformations, the area per lipid is relatively large, and lateral diffusion of lipids and membrane components is rapid.

Ld typically arises in membranes enriched with unsaturated phospholipids at physiological temperatures. The presence of cholesterol and saturated lipids can alter the phase behavior, promoting more ordered phases such as liquid-ordered (Lo) or gel-like states, depending on composition and temperature. Nonetheless, pure or predominantly unsaturated lipid matrices often favor the Ld state, providing a highly fluid milieu conducive to rapid molecular mobility.

Detection and study of the Ld phase rely on a combination of techniques. Differential scanning calorimetry often shows broad, weak transitions rather than sharp melting peaks. Scattering methods (SAXS/WAXS) reveal reduced long-range order, while fluorescence anisotropy and time-resolved spectroscopy indicate lower molecular order and higher mobility. Nuclear magnetic resonance order parameters and diffusion measurements further characterize the disordered nature. In biological contexts, Ld regions are implicated in modulating membrane protein function, signaling, and interactions with cholesterol-rich Lo domains, contributing to the overall lateral heterogeneity of membranes.

In sum, the liquid disordered phase describes a fluid, disordered lipid environment central to membrane dynamics and function, especially in membranes rich in unsaturated lipids.