STRUCTURE AND THERMAL VIBRATIONS OF SPERMINE PHOSPHATE HEXAHYDRATE FROM NEUTRON-DIFFRACTION DATA AT 125K

Spermine phosphate hexahydrate crystallizes in space group P2(1)/a wit h unit-cell dimensions a = 7.931 (I), b = 23.158 (5), c = 6.856 (2) An gstrom, and beta = 113.44(2)(o) at 125 K with unit-cell contents [(C10 H30N4)(2)(4+)(HPO4)(4)(2-). 12H(2)O]. The packing of spermines and mon ohydrogen phosphates in this crystal structure has features which may be relevant to the binding of spermine to DNA. Another important struc tural feature is the presence of channels containing water that is hyd rogen bonded as in ice-Ih with disordered protons. The channels occur between sheets of spermine long chains and are also bordered by hydrog en-bonded monohydrogen phosphate chains. The hydrogen-bonding scheme o f these water chains proposed on the basis of an earlier X-ray study i s now confirmed. Nuclear positions, anisotropic mean-square (m.s.) dis placements, an overall scale factor and two extinction parameters (rho and g) were refined using full-matrix least-squares giving values of R(F-0(2)) = 0.09, Rw(F-0(2)) = 0.11 and S = 1.02. Thermal vibrational analysis revealed that the backbone of the spermine cation can be desc ribed as a single rigid segment with a substantial libration of 27 deg (2) around the spermine molecular long axis.