Abstract. C8N2C12F4, FW 271.0; monoclinic, P2 I, a = 11-925 (2), b= 5.523 (1), c= 6.785 (1) A, fl=95.09 (2) ° (20°C). Z = 2, Dx= 2.02 g cm -3. The final R index was 5.2% (weighted R 4.0%). No absorption correction was applied (/~= 7.84 cm~). Substantial deformation of the parent 2,7-naphthyridine skeleton results because of the short CI-C1 distance (2.307 A).

The/3-phase crystal structures of unsaturated triacylglycerols (TAGs) have been analyzed. Long spacings and melting points of monounsaturated TAGs such as 16" O. 16 and 16.0" 18 indicate a fi-3C packing mode with a methyl terrace that differs from that of the saturated /3-3 TAGs. In addition to the hydrocarbon chain subcell packing and the methyl terrace structure, the conformation of the oleoyl chain also has to be considered. This conformation is analyzed in connection with the symmetry relation between the two half-layers on either side of the plane through the double bonds. Geometric analysis shows four possibilities, which have been explored further by means of energy minimization computations. In these calculations the structure of the oleoyl chain, in its crystalline environment, has been optimized while taking all relevant intramolecular and intermolecular forces into account. Though the calculations reveal relatively small energy differences between the four possibilities, the most likely structure can still be identified. On the basis of the results obtained for the monounsaturated TAGs, proposals for the crystal structures of diunsaturated (e.g., 0 9 18. O) and triunsaturated (e.g., 0" 0" O) TAGs are briefly outlined.

To reveal the structure ofβ′ triacylglycerols in detail, LML (C12C14C12) was purified by a zone‐melting procedure, and twinned crystals ofβ′ stable LML were obtained from a melt,β′ LML crystallizes in the monoclinic space group C2, with eight molecules in the unit cell. A powder X‐ray diffraction study of solid compounds of 1:1 mixtures of selected triacylglycerols led to the conclusion that the triacylglycerol molecules in theβ modification have a 1,2 chair‐conformation (i.e., the fatty acid chains on glycerol positions 1 and 2 are adjacent, with the chain on the 3‐position forming the back rest of the chair). Packing studies and the positions of two‐fold axes and two‐fold screw axes in the unit cell require that the molecules are bent at the glycerol site. The fatty acid chains make an angle of 25° with the long axis of the unit cell. Electron micrographs and precession photographs indicate that the twinning results from the stacking of a large number of thin crystalline platelets in two distinct orientations.

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A comprehensive packing analysis is presented of the crystal structures of a large number of saturated triacylglycerols in the/3-3 phase. The triacylglycerols p.q.r have been grouped into four classes: r=p, p-t-2, p+4 and p+6. The length of the middle chain, q, dictates whether a ~-2 or/3-3 packing occurs. The latter packing arrangement is adopted when q differs at least 4 from por r.The model-building approach starts from the known molecular conformation of {3-2 10.10.10 and the T// subcell packing mode of the hydrocarbon chains. Purely geometrical model-building allows a preliminary assignment of crystal structures. Crystal lattice energy calculations using the atom-atom potential method support these tentative assignments and clarify some details regarding the stacking of layers. In common with the ~-2 phase crystal structures, the structure of the terrace-like arrangement of the terminal methyl groups plays a crucial role. Triacylglycerols with chains 1 and 3 differing 2 or 4 carbon atoms have a common arrangement of the end-methyl groups (type ~-3A). Those with a difference of 0 (i.e. symmetrical) or 6 carbon atoms have a different, less favorable methyl terrace (type/3-3B). The available experimental evidence (x-ray powder diffraction, unit-cell data and melting points) is entirely compatible with these proposals.The growth of good single crystals of acylglycerols is beset with extremely great difficulties. A complete solution of the crystal structure is hard to obtain, even if single crystals are available. There is therefore a need for alternative approaches toward the solution of acylglycerol crystal structures.For an up-to-date review of crystal structure analyses of lipid systems see (1} and (2); for a more general treatment of the polymorphism of fatty systems see (3) and (4).In a previous study (5) we derived detailed packing modes of a large number of saturated triacylglycerols in the/~-2 phase, extrapolating from one known crystal structure by using a model-building approach. Particularly important features of this approach were the systematic packing of the hydrocarbon chains (T//subceU) and the terrace-like structure of the end methyl groups at the layer boundaries. For each triacylglycerol, two or three distinct solutions could be advanced, only one of which, in general, was compatible with powder diffraction and melting-point data. In the present work, a similar approach is applied to the ~-3 phase of triacylglycerols.Roughly, the ~-3 phase is observed for those triacylglycerols having a middle chain which differs considerably in length from (one of) the outer chains. This

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