Pyrazolylborate-Zinc Complexes of RNA Precursors and Analogues Thereof.

The Tp ligand tris(3-cumenyl-5-methylpyrazolyl)borate was found to stabilize zinc complexes of nucleobases, of their natural precursors, and of nucleoside and nucleotide derivatives. Dihydroorotic acid and orotic acid are bound as monodentate carboxylate ligands. Uracil is coordinated via its deprotonated N1; 6-methylthiouracil acts as a bidentate ligand via N1 and S. Analogously, xanthine is a monodentate ligand bound by its deprotonated N7, while 6-mercaptopurine seems to bind in a bidentate fashion via N7 and S. Spectroscopic evidence indicates coordination of uridine and 2',3'-O-isopropylideneuridine via their deprotonated N3, as well as of xanthosine via N7. The hydrolytic cleavage of 2',3'-O-isopropylideneuridine 5'-(bis(p-nitrophenyl) phosphate) by TpZn-OH is preceded by an attachment of one TpZn unit to the deprotonated uracil base, presumably via N3.