Isolation, Crystal Structure and Na+/K+-ATPase Inhibitory Activity of 1β-Hydroxydigitoxigenin

The title compound 1β-hydroxydigitoxigenin(1) was isolated from the ethanol extract of the roots of Streptocaulon juventas. The crystal structure of 1, C23H34O5·H2O, was determined by Synchrotron X-ray diffraction analysis due to small crystal size(0.14 mm × 0.04 mm × 0.01 mm). The crystal belongs to monoclinic, space group P21, with a = 7.6624(15), b= 13.460(3), c = 10.370(2) ?, b = 92.40(3)°, V = 1068.6(4) ?3, Z = 2, Mr = 406.50, Dx = 1.263 g/cm3, λ(synchrotron) = 1.2399 ?, μ(synchrotron 1.23990) = 0.333 cm-1, F(000) = 550, S = 1.059, R = 0.0625 and w R = 0.1687 for 4247 unique reflections, of which 3687 were observed(I > 2σ(I)). The asymmetric unit contains one independent molecule of 1 and one water molecule which are connected through hydrogen bonds. The conformation of 1 in crystalline state is in good agreement with the solution structure in methanol as indicated by1H-NMR analysis. The absolute configuration of 1 could be assigned by referring to the known configuration of the lactone ring at C(17b). In the solid state, intermolecular hydrogen bonds involving carbonyl group in the lactone moiety and the hydroxyl groups in the steroid moiety ester linked adjacent molecules into a three-dimensional network. Compound 1 showed significant inhibition on Na+/K+-ATPase with an IC50 of 2.46 mM, which is stronger thiocarbonylbufalin but weaker than a close analog digitoxigenin, suggesting that a lactone ring is important and the substitution of a hydroxyl group at C(1) is not favored for the inhibition of Na+/K+-ATPase.