Probing the critical unilamellar state of membranes

Aqueous dispersions of membrane phospholipids comprised of multilamellar vesicles (MLVs) will spontaneously transform to a stable unilamellar structure equivalent to the membrane bilayer, but only at a critical temperature T*. Since much of the evidence for this transformation derives from equilibrium thermodynamic studies, a description of the molecular and topological events occurring as the critical unilamellar state assembles has not previously been possible. Here we report experiments that provide evidence of a spontaneous topological change from MLVs towards unilamellar vesicles (ULVs) at T*. By applying a shearing stress to vesicle suspensions we have observed a decrease of approximately 25% in the force required to cause bilayers to leak; this decrease is confined to temperatures near T*. The T* values observed agree with those previously obtained by equilibrium methods. Using the method with total lipid extracts from normal biological membranes confirms that T* equals the physiological temperature of the original membranes.