Biaxial Ordering of Melted Chains Near the Gel Phase Transition: an NMR Study

Abstract

A new phase in the simple lipid bilayer system (potassium palmitate-water) in the vicinity of the L _α to gel phase transition is studied using deuterium magnetic resonance. In this phase the hydrocarbon chains are observed to remain melted but become biaxially ordered. Measurements of the motionally averaged quadrupole coupling constant and of the motionally induced asymmetry parameter were made as a function of temperature and are interpreted in terms of orientational order parameters. The analysis shows that the principal feature of the phase transition is the onset of a transverse long range orientational order of the chains in the plane of the bilayer while the order of the chains in the direction normal to the bilayer remains little affected. This order is suggestive of a cooperative tilted arrangement of the melted chains in the bilayer. The uniaxialbiaxial phase transition is observed to be of first order.