Lipid ordered domains (LODs) may provide the scaffold that membrane proteins need for concerted functioning. Accordingly, the LODs must be <200 nm. Thus far, the size-regulating mechanism remained enigmatic.
Two hypotheses have been put forward. The first envisions spontaneous monolayer curvature to be the size determining factor. According to the theory, the individual leaflet's bending propensity is counteracted by the opposing leaflet's equally large disposition
to bend the bilayer in the other direction. The equilibrium line tensions at the edge of the resulting small and flat domains are equal to zero. In contrast, the second hypothesis asserts that the hydrophobic mismatch between LOD and liquid disordered domains (LDD)
gives rise to elastic lipid deformations, which minimize the height difference. Domain fusion would decrease the total length of all domain borders and thus also the boundary energy. Yet, line active substances kinetically hamper small domains' coalescence by
imposing a local energy barrier to domain fusion. Here we exploit photo-switchable lipids capable of rather large spontaneous monolayer curvature changes to distinguish between both theories. We used domain tracing and confocal fluorescence microscopy to observe domain
size distribution and domain fusion kinetics in free-standing planar lipid bilayers.1 The results are only compatible with the second hypothesis. Theoretical calculations show a local minimum of elastic energy at a distance between domain boundaries of~ 8 nm. At closer
quarters, both neighboring domains affect the intervening lipids. As a result, the elastic energy reaches its peak at a distance of ~4-5 nm, which rafts must overcome to merge. We conclude that biological membranes may regulate raft size through line active substance
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