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Diacylglycerols (DAG) play an important role in metabolism, signal transduction and protein kinase activation. Naturally occurring DAGs usually contain a saturated chain in the 1-position and an unsaturated chain in the 2-position. We have investigated the physical behavior of 1-stearoyl-2-oleoyl-sn-glycerol (sn-SODG) both in the dry and hydrated states by means of differential scanning calorimetry, X-ray diffraction, and NMR. In the dry state the saturated stearate and unsaturated oleate chains have difficulties in packing. As a result marked polymorphism occurs as the chains try to find a suitable packing. Eight phases were found in the dry state: alpha (transition temperature = 16.4 degrees C; delta H = 6.8 kcal/mol); beta 4 (20.7 degrees C; 13.8); beta 3 (21.5 degrees C; 13.8); beta 2 (22.2 degrees C; 14.4); beta 1 (23.1 degrees C; 12.3); beta' (25.7 degrees C; 11.9); gamma 1 (-2.9 degrees C; 0.5), and gamma 2 (-5.9 degrees C; 1.2), all of relatively low stability compared to 1,2 distearoyl-sn-glycerol (beta', 77.2 degrees C; 30.6). gamma 1 and gamma 2 are metastable low temperature phases. beta 1-beta 4 are bilayers (d001 = 3.4, 43.4, 44.7, 46.1 A, respectively) with elements of triclinic parallel chain packing, while beta' is a bilayer (d001 = 47.1 A) with orthorhombic perpendicular chain packing. The metastable alpha phase has hexagonal chain packing and an unusual eight-layer structure (d001 = 174 A). Hydrated sn-SODG contains about one-half of a water molecule per diacylglycerol. Three phases can be distinguished gamma w, alpha w (15.1 degrees C; 6.7) and beta w (19.9 degrees C, 14.3). Both alpha w and beta w are bilayers but alpha w has hexagonal chain packing and beta w is predominantly triclinic parallel packing. Thus, when saturated and unsaturated chains must pack side by side, complex chain conformation, disorder, and instability result giving rise to marked polymorphism. Hydration appears to partly stabilize the interactions.