A low-temperature structural phase transition of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine bilayers in the gel phase

A new thermotropic phase transition, at ?30°C and atmospheric pressure, was found to occur in the gel phase of aqueous DPPC dispersions. The Raman spectral changes at this phase transition are similar to those observed in the gel phase of DMPC dispersions at ?60°C. The thermotropic phase transition at ?30°C is equivalent to the barotropic GII to GIII phase transition observed in DPPC at 1.7 kbar and 30°C. It is shown that the rate of the large angle interchain reorientational fluctuations decreases gradually with decreasing temperature, and that the orientationally disordered acyl chain structure of the GII phase is extended into the GIII phase of DPPC. The interchain interaction, arising from the damping of the reorientational fluctuations, increases with decreasing temperature in the GII gel phase as well as in the GIII gel phase.