Pyrazolylborate−Zinc−Nucleobase-Complexes, 3:¹ Base Pairing Studies

In solution, the pyrazolylborate−zinc−nucleobase complexes show self-association and base pairing with external nucleobases. The self-association was studied quantitatively for TpCum,MeZn−hypoxanthinate and TpCum,MeZn−thyminate; the dimerization constants KD are 63 ± 8 and 0.2 ± 0.1 M-1, respectively. Of the external nucleobases, 9-ethyladenine forms stable base pairs with the thyminate, uracilate, and xanthinate complexes, 9-isobutylguanine only with the cytosinate complex, 1-methylthymine with the adeninate and diaminopurinate complexes, and 1-methyluracil with the diaminopurinate complex. The association constant for the base pair TpCum,MeZn−thyminate:9-ethyladenine was determined by NMR methods as K = 66 ± 10 M-1. Structure determinations of the crystalline adducts have confirmed the base pairing for TpCum,MeZn−thyminate:9-ethyladenine, TpCum,MeZn−cytosinate:9-isobutylguanine, and TpCum,MeZn−xanthinate:9-ethyladenine. Both Watson−Crick and Hoogsteen base pairs have been observed. In the solid state, extended base pairing leads to quartet and polymer arrangements.