| There is a great interest in the interaction of zinc with nucleobases, mainly due to the important role zinc plays in gene expression. Zinc-metalloenzymes are also involved in the metabolism of nucleobases, nucleosides, nucleotides and even of DNA and RNA. We have found the trispyrazolyl borato zinc hydroxide complex Tp*ZnOH (1) to be a good model for the typical His3Zn-OH site, as found in the active sites of many zinc enzymes.
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2 |
We have now investigated the coordination properties of nucleobases towards the Tp*Zn moiety. The nucleobases uracil, thymine, adenine and hypoxanthine react with complex 1 by condensation, binding to zinc with one endocyclic nitrogen atom (NS), c. f. the adeninate complex 2 (see figure). This nitrogen atom is normally attached to the sugar moiety in nucleosides. With uracil or hypoxanthine no interaction has been observed between zinc and the exocyclic oxygen atoms, despite a possibile chelate formation. In the solid state the Tp*Zn complexes of the nucleobases form dimeric structures due to association of the zinc bound nucleobases via two hydrogen bonds.
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3 |
With nucleobases substituted at NS (e. g. 1-methyluracil, inosine, uridine) the condensation reaction with 1 occurs at another endocyclic N-H group. This reaction is even favored if it competes with phosphate ester cleavage followed by building of a Tp*Zn-phosphate complex, as shown for a uridine monophosphate ester derivative. Phosphate complexes are obtained if there is no endocyclic N-H group left in the base moiety of nuclotides, e. g. with [(2’, 3’- isopropyliden- N-methyl-uridine) -bis-p-nitrophenyl]
phosphate, which forms complex 3 (see figure). |
