Structure of the radiation protection agent S-2-(3-aminopropylamino)ethylphosphorothioic acid (WR 2721)

C5H15N2O3PS.3H2O, Mr = 268.3, orthorhombic, P2(1)2(1)2(1), a = 6.762 (1), b = 8.458 (1), c = 21.564 (3) A, V = 1233.3 A3, Z = 4, Dx = 1.445 g cm-3, Cu K alpha, lambda = 1.54178 A, mu = 36.1 cm-1. F(000) = 576, room temperature, final R = 3.55% for 1075 reflections with magnitude of Fo greater than 3 sigma. The overall conformation of the molecule is folded with an intramolecular hydrogen bond between N(3) and O(1) of the SPO3 group. The folded conformation of the molecule has a chiral twist. The molecule is a double zwitterion with the two phosphate hydrogens moved to the two nitrogen atoms, and the three P-O bonds are of equal length at 1.52 A. The S-P bond is unusually long at 2.12 A. Each H atom on each N atom and in each water molecule participates in hydrogen bonding. One of the N(7) hydrogen atoms forms a bifurcated intermolecular hydrogen bond to two of the phosphate O atoms.