ESR studies on the ripple phase in multilamellar phospholipid bilayers

The structure of the ripple phase in multilamellar DPPC liposomes was studied by measuring electron spin resonance (ESR) spectra of the stearic acid spin probe (16-SASL). The spectrum observed in the P beta' phase is explained in terms of the superposition of ordered- and disordered-type spectra. This fact suggests the existence of a nonhomogeneous structure in the P beta' phase, i.e., the coexistence of the ordered region present in the L beta' phase and the disordered region present in the L alpha phase. The fluidity of acyl chains in both regions and the intensity ratio of the disordered-type spectra to the total intensity were estimated from the ESR spectra at various temperatures. The fluidity of both regions did not exhibit remarkable temperature dependence. On the other hand, the intensity ratio showed almost similar temperature dependence of the partitioning of the TEMPO observed by ESR measurement and of the enthalpy curve by DSC measurement, that is, the intensity ratio value increases slowly near the pretransition temperature, is almost constant in the P beta' phase, and increases abruptly at the main transition temperature with temperature. The proportion of the disordered region is estimated to be about one-fifth to the total region in the P beta' phase. Based upon the above results, a model for the ripple structure is proposed.