Biophys J, April 2001, p. 1873-1890, Vol. 80, No. 4

New Ordered Metastable Phases between the Gel and Subgel Phases in Hydrated Phospholipids

Boris Tenchov,^* Rumiana Koynova,^* and Gert Rapp

 ^*Institute of Biophysics, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria, and  Max-Planck-Institute for Colloids and Interface Science, c/o HASYLAB, DESY, D-22603 Hamburg, Germany

Formation of low-temperature ordered gel phases in several fully hydrated phosphatidylethanolamines (PEs) and phosphatidylcholines (PCs) with saturated chains as well as in dipalmitoylphosphatidylglycerol (DPPG) was observed by synchrotron x-ray diffraction, microcalorimetry, and densitometry. The diffraction patterns recorded during slow cooling show that the gel-phase chain reflection cooperatively splits into two reflections, signaling a transformation of the usual gel phase into a more ordered phase, with an orthorhombic chain packing (the Y-transition). This transition is associated with a small decrease (2-4 µl/g) or inflection of the partial specific volume. It is fully reversible with the temperature and displays in heating direction as a small (0.1-0.7 kcal/mol) endothermic event. We recorded a Y-transition in distearoyl PE, dipalmitoyl PE (DPPE), mono and dimethylated DPPE, distearoyl PC, dipalmitoyl PC, diC₁₅PC, and DPPG. No such transition exists in dimyristoyl PE and dilauroyl PE where the gel L phase transforms directly into subgel L_c phase, as well as in the unsaturated dielaidoyl PE. The PE and PC low-temperature phases denoted L_R1 and SGII, respectively, have different hydrocarbon chain packing. The SGII phase is with tilted chains, arranged in an orthorhombic lattice of two-nearest-neighbor type. Except for the PCs, it was also registered in ionized DPPG. In the L_R1 phase, the chains are perpendicular to the bilayer plane and arranged in an orthorhombic lattice of four-nearest-neighbor type. It was observed in PEs and in protonated DPPG. The L_R1 and SGII phases are metastable phases, which may only be formed by cooling the respective gel L and L_' phases, and not by heating the subgel L_c phase. Whenever present, they appear to represent an indispensable intermediate step in the formation of the latter phase.

Biophys J, April 2001, p. 1873-1890, Vol. 80, No. 4
© 2001 by the Biophysical Society   0006-3495/01/04/1873/18  $2.00

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