Solution and solid state 13C NMR and X-ray studies of genistein complexes with amines. Potential biological function of the C-7, C-5, and C-4′-OH groups

Abstract

Parent genistein and its new amine complexes with morpholine and piperazine were studied comparatively in the solid and liquid states by X-ray crystallography and ¹³C and ¹⁵N NMR spectroscopy. Biochanine A and its complexes were used as reference. Secondary deuterium isotope effects on ¹³C chemical shifts in solution were studied in parent isoflavones and their morpholine and piperazine complexes to aid in evaluation of the electronic distribution in both systems. In addition, to quantify the extent of proton transfer as well as to establish strong hydrogen bonding of the 7-OH group in a morpholine complex, proton transfer from the 7-OH group to the piperazine nitrogen atom was also confirmed by ¹³C NMR in the solid state and by X-ray studies. The effect of 7-OH deprotonation yields a high frequency shift of 7–8 ppm on the C-7 carbon atom of the piperazine complex whereas it is as large as 12 ppm in the morpholine complex in the solid. The former trend is confirmed from solution state concentration studies which also show that the isoflavones have a strong tendency to form complexes with bases. Depending on the pK_a difference between the isoflavones and the base this leads either to proton transfer and ion-pair formation or, in the case of a larger pK_a difference, to a hydrogen bonded ion pair. The concentration studies show formation of a 1 : 1 genistein–piperazine complex in DMSO. Addition of water leads to formation of solvent separated ions. The C-5 OH group is involved in strong intramolecular hydrogen bonding leading to a pseudo aromatic ring extending the aromatic part of the drug pharmacophore. The analysis also suggests the way that both the C-7 and C-4′ hydroxyl group of genistein may participate in stabilising the ternary inhibitor complexes of tyrosine-specific kinases or DNA topoisomerase II.