Structure and thermal vibrations of spermine phosphate hexahydrate from neutron diffraction data at 125 K

Spermine phosphate hexahydrate crystallizes in space group P2(1)/a with unit-cell dimensions a = 7.931 (1), b = 23.158 (5), c = 6.856 (2) A, and beta = 113.44 (2) degrees at 125 K with unit-cell contents [(C10H30N4)2(4+)(HPO4)4(2-).12H2O]. The packing of spermines and monohydrogen phosphates in this crystal structure has features which may be relevant to the binding of spermine to DNA. Another important structural feature is the presence of channels containing water that is hydrogen bonded as in ice-Ih with disordered protons. The channels occur between sheets of spermine long chains and are also bordered by hydrogen-bonded monohydrogen phosphate chains. The hydrogen-bonding scheme of these water chains proposed on the basis of an earlier X-ray study is now confirmed. Nuclear positions, anisotropic mean-square (m.s.) displacements, an overall scale factor and two extinction parameters (rho and g) were refined using full-matrix least-squares giving values of R(F0(2)) = 0.09, Rw(F0(2)) = 0.11 and S = 1.02. Thermal vibrational analysis revealed that the backbone of the spermine cation can be described as a single rigid segment with a substantial libration of 27 deg2 around the spermine molecular long axis.