Spermidine phosphate trihydrate, C7H19N3.~H3PO4.3H20, is found to have a monoclinic unit cell with dimensions, a= 9.22, b= 26.03, c = 6.58/~, fl= 96.0 °. The space group is P21 and the asymmetric unit contains two formula units. The structure was solved by a sharpened Patterson function and refined by the block-matrix least-squares method with anisotropic temperature parameters to an R value of 0.09 for 2868 observed reflexions. The molecules take a normal extended zigzag chain conformation. The chemical formula of this compound has been shown to be [NH+(CH2)3NH+(CH2)4NH+]2.3[HPO4] 2. 6H20. The structure consists of two kinds of bands, the molecular band and the phosphate--water band. They are extended along the a axis and joined together alternately side by side in the b direction to form a sheet structure with stripes of the bands. Within the molecular band, the spermidine molecules are arranged in parallel along the b axis as is generally seen in aliphatic long chain compounds. The angle of tilt is 90 °. The phosphatewater band consists of dimer and monomer ions of monohydrogen phosphate which are separated by water molecules, forming a network of hydrogen bonds. The striped sheets are piled up alternately along the c axis through N-H. • • O hydrogen bonds in such a way that the phosphate-water band comes next to the molecular band of spermidine. The structure is discussed in comparison with that of spermine phosphate hexahydrate.
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Huse et al. (1969) studied this question.