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Keywords:

  • DNA;
  • fluorescent probes;
  • G-quadruplexes;
  • purine;
  • transition metals

Abstract

A synthetic strategy that utilizes O6-protected 8-bromoguanosine gives broad access to C8-guanine derivatives with phenyl, pyridine, thiophene, and furan substituents. The resulting 8-substituted 2′-deoxyguanosines are push–pull fluorophores that can exhibit environmentally sensitive quantum yields (Φ=0.001–0.72) due to excited-state proton-transfer reactions with bulk solvent. Changes in nucleoside fluorescence were used to characterize metal-binding affinity and specificity of 8-substituted 2′-deoxyguanosines. One derivative, 8-(2-pyridyl)-2′-deoxyguanosine (2PyG), exhibits selective binding of CuII, NiII, CdII, and ZnII through a bidentate effect provided by the N7 position of guanine and the 2-pyridyl nitrogen atom. Upon incorporation into DNA, 2-pyridine-modified guanine residues selectively bind to CuII and NiII with equilibrium dissociation constants (Kd) that range from 25 to 850 nM; the affinities depend on the folded state of the oligonucleotide (duplex>G-quadruplex) as well as the identity of the metal ion (Cu>Ni≫Cd). These binding affinities are approximately 10 to 1 000 times higher than for unmodified metal binding sites in DNA, thereby providing site-specific control of metal localization in alternatively folded nucleic acids. Temperature-dependent circular-dichroism studies reveal metal-dependent stabilization of duplexes, but destabilization of G-quadruplex structures upon adding CuII to 2PyG-modified oligonucleotides. These results demonstrate how the addition of a single pyridine group to the C8 position of guanine provides a powerful new tool for studying the effects of N7 metalation on the structure, stability, and electronic properties of nucleic acids.