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J Biol Chem, Vol. 275, Issue 10, 6857-6867, March 10, 2000

Physical Properties of the Transmembrane Signal Molecule, sn-1-Stearoyl 2-Arachidonoylglycerol
ACYL CHAIN SEGREGATION AND ITS BIOCHEMICAL IMPLICATIONS^*

Jan-Ove Hindenes^§, Willy Nerdal^¶, Wen Guo^**, Li Di^**, Donald M. Small^**, and Holm Holmsen

From the ^** Department of Biophysics, Boston University School of Medicine, Boston, Massachusetts 02118, the  Department of Biochemistry and Molecular Biology, University of Bergen, Bergen, and ^¶ Department of Chemistry, University of Bergen, N-5007 Bergen, Norway

sn-1,2-Diacylglycerol (DAG), a key intermediate in lipid metabolism, activates protein kinase C and is a fusogen. Phosphoinositides, the main sources of DAG in cell signaling, contain mostly stearoyl and arachidonoyl in the sn-1 and -2 positions, respectively. The polymorphic behavior of sn-1-stearoyl-2-arachidonoylglycerol (SAG) was studied by differential scanning calorimetry, x-ray powder diffraction, and solid state magic angle spinning (MAS) ¹³C NMR. Three  phases were found in the dry state. X-ray diffraction indicated that the acyl chains packed in a hexagonal array in the  phase, and the two sub- phases packed with pseudo-hexagonal symmetry. In the narrow angle range strong diffractions of ~31 and ~62 Å were present. High power proton-decoupled MAS ¹³C NMR of isotropic SAG gave 16 distinct resonances of the 20 arachidonoyl carbons and 5 distinct resonances of the 18 stearoyl carbons. Upon cooling, all resonances of stearoyl weakened and vanished in the sub-₂ phase, whereas arachidonoyl carbons from 8/9 to 20 gave distinct resonances in the frozen phases. Remarkably, the -carbon of the two acyl chains had different chemical shifts in , sub-₁, and sub-₂ phases. Large differences in spin lattice relaxation of the stearoyl and arachidonoyl methene and methyl groups were demonstrated by contact time (cross-polarization) MAS ¹³C NMR experiments in the solid phases , sub-₁, and sub-₂. This shows that stearoyl and arachidonoyl in SAG have different environments in the solid states (, sub-₁, and sub-₂ phases) and may segregate during cooling. The NMR and long spacing x-ray diffraction results suggest that SAG does not pack in a conventional double layer with the two acyls in a hairpin fashion. Our findings thus provide a physicochemical basis for DAG hexagonal phase domain separation within membrane bilayers.

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