Adenine metal complexes

As shown in Figure la, adenine has five potential binding sites, all of which have been reported as having been used in coordination to transition metal ions in various adenine metal complexes. The anionic adenine in the complexes used in this study is deprotonated at the N-9 site (Figure la). Adenosine has the four potential binding sites shown in Figure lb. 

Base-pairing interactions have been realized in metal complexes by extending ligand functionalization to include two complementary nucleobases (136). Sequential alkylation of 1,2-dithioethane using 2-chloroethyl-9-adenine and 3-chloropropyl-l-thymine yields the AT-... 

The very same philosophy can also be applied to metal complexes. Indeed, metal compounds containing methylated nucleobases [e.g., 1,9-dimethylguanine (1,9-DiMeG) (258), 7,9-DiMeG (131a, 259), 6,9-DiMeG (258), 1,9-DiMeA (260), 6,9-DiMeA (52), or 6,6,9-trimethylguanine (6,6,9-TriMeA) (53)] have been reported, but only in one case has the effect of metal ion coordination on the tautomer equilibrium of a nucleobase (adenine) been measured (52). 

The mechanism of electron transfer reactions in metal complexes has been elucidated by -> Taube who received the Nobel Prize in Chemistry for these studies in 1983 [xiv]. Charge transfer reactions play an important role in living organisms [xv-xvii]. For instance, the initial chemical step in -> photosynthesis, as carried out by the purple bacterium R. sphaeroides, is the transfer of electrons from the excited state of a pair of chlorophyll molecules to a pheophytin molecule located 1.7 mm away. This electron transfer occurs very rapidly (2.8 ps) and with essentially 100% efficiency. Redox systems such as ubiquinone/dihydroubiquinone, - cytochrome (Fe3+/Fe2+), ferredoxin (Fe3+/Fe2+), - nicotine-adenine-dinucleotide (NAD+/NADH2) etc. have been widely studied also by electrochemical techniques, and their redox potentials have been determined [xviii-xix]. 

In general the pyrimidines show a much lower reactivity towards the metal ions. Apparently no reaction was observed with uracil while the stability constants of Cu-cytosine are even lower than the lgJCi and gK% values of Cu(NH3)62+ (79). The high stability of the purine metal complexes can be attributed to the binding site at the imidazole residue. There the imino proton competes with the metal ion. Fig. 1 presents a model of the 2 1 complex of Cu-(adenine)%. 

O Sullivan, W. J., and D. D. Perrin The stability constants of metal-adenine nucleotide complexes. Biochemistry 3, 18 (1964). 

Phillips, R. S. J. Adenosine and the adenine nucleotides, ionization, metal complex formation and conformation in solution. Chem. Rev. 66, 501 (1966). 

No. But that s OK because that kept on pushing us towards better and better experiments. I think that the last experiment that we published in Science this year may have made clear how exquisitely sensitive the electron transfer was to base-pair stacking. In this experiment we no longer appended metal complex intercalators but we simply looked at electron transfer from one modified base to another modified base. In this experiment we used two modified adenines as our fluorescent excited electron acceptor to oxidize guanines. The modified adenines were very similar in structure, very similar in redox characteristics, very similar in energetics. But when they were incorporated into DNA, one was well-stacked in the helix and one was... 

During 1966, a review of methods in nucleoside syntheses and a review of the ionization and metal complex formation of adenosine and adenine nucleotides were published. ... 

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