Departments of Physical Chemistry and of Biochemistry, University of Umei, S-901 87 Umei, Sweden Received March 31, 1986; Revised Manuscript Received July 22, 1986 abstract: A basis for the reorganization of the bilayer structure in biological membranes is the different aggregate structures formed by lipids in water. The phase equilibria of all individual lipids and several in vivo polar lipid mixtures from acyl chain modified membranes of Acholeplasma laidlawii were investigated with different NMR techniques. All dioleoyl (DO) polar lipids, except monoglucosyldiglyceride (MGDG), form lamellar liquid crystalline (La) phases only. The phase diagram of DOMGDG reveals reversed cubic (In), reversed hexagonal (Hn), and La phases. In mixtures of DOMGDG and dioleoyldiglycosyldiglyceride (DODGDG), the formation of an In (or Hn) phase is enhancedby DOMGDG and low hydration or high temperatures. For in vivo mixtures of all polar DO lipids, a transition from an La to an In phase is promoted by low hydration or high temperatures (50 C). Thephospholipids are incorporated in this Iu phase. Likewise, In and Hn phases are formed at similar temperatures in a series of in vivo mixtureswith different extents of acyl chain unsaturation. However, their melting temperatures (Tm) vary in an expected manner. All cubic and hexagonal phases, except the In phase with DOMGDG, exist in equilibrium with excess water. The maximum hydration of MGDG and DGDG is similar and increases with acyl chain unsaturation but is substantially lower than that for, eg, phosphatidylcholine. The translational diffusion of the lipids in the cubic phases is rapid, implying bicontinuous structures. However, their appearances in freeze-fracture electron microscope pictures are different. The In phase of DOMGDG belongs to the laid space group. It is concluded that the formation of nonlamellar phases by A. laidlawii lipids depends critically upon the MGDG concentration.

Biological membranes consist of a large number of different lipids which together with proteins constitute the bilayer structure. The physicochemical properties of the membrane lipids, in their role as structure builders and their participation in membrane-associated physiological processes, have been only partly studied. In this respect, the contributions from all the individual components of the in vivo lipid mixtures are very little known. For an understanding of the processes involving,