Irregular bilayer structure in vesicles prepared from Halobactbrium cutirubrum lipids

The behaviour of the fluorescent probes, perylene and 8-anilinonaphthalene-sulfonic acid, was studied by determining fluorescence polarization in vesicles prepared from Halobacterium cutirubrum polar lipids and unfractionated lipids. In the latter case, when the non-polar lipids of this organism are included (carotenoids and squalenes, comprising 8% of the total), the environment of perylene is more fluid than in polar lipids alone. Studies of the fluorescent emission spectra of ANS and the effect of chaotropic perturbants on the motion of perylene suggest that the bilayer structure in vesicles of unfractionated lipids is distorted in such a way as to allow for the penetration of more water molecules near the hydrophobic region or to induce the probes to be nearer to the aqueous phase than is the case for the polar lipids alone. In buffers containing 100 mM MgCl₂, and especially in the presence of high concentrations of NaCl as well, an irreversible thermal transition of the liquid crystalline matrix was observed in the region occupied by perylene for vesicles of unfractionated lipids. Vesicles prepared from polar lipids alone do not show such transition, and the temperature at which the transition occurs depends on the amount of non-polar lipids included. It is likely that the irregularity of the bilayer structure and the thermal breakdown are both caused by the disruptive effect of the non-polar lipids.Cell envelopes of H. cutirubrum do not show the above transition, which occurs in the lipid vesicles in ionic environments and at temperatures which are physiological for these organisms. This finding is consistent with our previous suggestion, based on spin label studies, that in H. cutirubrum the membrane proteins immobilize most or all of the lipid phase.