Guanine reactions

The Ru(bpy)3 /guanine reaction can be carried out in solution, as well as many different types of electrode surfaces. It was originally developed using DNA physically adsorbed to ITO, but has also been applied to DNA attached to carbon nanotubes and polymer-modified electrodes. As long as the DNA-electrode linkage can tolerate potentials up to -1.3 V (the oxidation potential of Ru(bpy)3 ) the assay should be feasible at virtually any interface. 

We must now consider how the various "aquo species" interact with the DNA bases. So far, we have examined interactions of thymine, uracil and cytosine in detail and to a lesser extent with guanine. Reaction products from the aqueous solution produced by silver nitrate treatment of the chloride-complexes are complex mixtures, which is hardly surprising based on the known conq)lexity of the aqueous solution. It is possible to prepare monomeric (NHj)2 PtCbase " species similar to those obtained from the direct" reaction ot cis-PtCk (NH ) and the bases we assume they come from monomerxc aquo-complexes. 

Joy A, Ghosh AK, Schuster GB. One-electron oxidation of DNA oligomers that lack guanine reaction and strand cleavage at remote thymines by long-distance radical cation hopping.JAm Chem Soc. 2006 128 5346-5347. 

Cleavage at A or G If the DNA is first treated with acid, dimethyl sulfate methylates adenine at the 3-position as well as guanine at the 7-position (not shown). Subsequent reaction with OH and piperidine triggers degradation and displacement of the methylated A or G purine base and strand scission, essentially as indicated here for reaction of dimethyl sulfate with guanine. 

A mixture of 2chloro5-(2-hydroxyethoxymethyl)+iypoxanthine (0.375 g) and methanol (80 ml) saturated with anhydrous ammonia was heated in a bomb at 125°C for 5 hours. The bomb was cooled in an ice bath and the reaction mixture removed. Solvent and excess ammonia were removed under reduced pressure at 50°C. After the residue was triturated with cold water to remove the ammonium chloride formed, the remaining solid was dried and then recrystallized from methanol to give pure 9-(2-hydroxyethoxymethyl)guanine (0.24 g), MP 256.5°-257°C. 

Amine bases in nucleic acids can react with alkylating agents in typical Sjsj2 reactions. Look at the following electrostatic potential maps, and tell which is the better nucleophile, guanine or adenine. The reactive positions in each are indicated. 

The different possible adducts formed between mitomycin C and DNA have been isolated by degradation of DNA after in vitro alkylation/crosslinking reactions and structurally characterized. Monoadduct 21 (Scheme 11.3), derived from alkylation at C-l only [53], and monoadducts 22 [54] and 23 [55, 56] (derived from C-10 alkylation by 16 at N-7 or N-2 of guanine, respectively) have been isolated, together with bisadducts 24 [57] and 25 [58], derived from interstrand and intrastrand crosslinks, respectively, and adduct 26 [59], formed by addition of a molecule of water to C-10 instead of the second guanine. All of these adducts have also been isolated from DNA after in vivo crosslinking [60, 61]. 

Saito et al. achieved the first direct confirmation of double alkylation of purine bases by azinomycin B [140]. They incubated azinomycin B with the self-comple-mentary DNA duplex d(TAGCTA)2 and monitored the reaction by HPLC and ion spray MS. They observed initial formation of a monoadduct that was then converted into a crosslinked bisadduct. The crosslink position was identified as between the guanine of one strand and the 5 -adenine on the other strand by thermo-lytic depurination. Further decomposition prevented structural analysis of the azi-... 

It was concluded that the crosslinking reaction proceeded through initial alkylation of N-7 of adenine by the aziridine ring, followed by guanine N-7 alkylation by the epoxide (Figure 11.12). This seems inconsistent with Armstrong s earlier conclusions, but the very short oligonucleotides used by Saito et al. may influence the DNA alkylation chemistry. 

Crosslinks result from the reaction of a bifunctional electrophilic species with DNA bases and imply a covalent link between two adjacent DNA strands which inhibits DNA replication. Primary targets within bases are N7 and 06 in guanine and N3 in cytosine. The initial lesions are removed by the suicide enzyme alkyltrans-ferase, whereas nucleotide excision repair is needed for frilly established crosslinks. 

The reactions of the oxadiazolium ions with nucleophiles such as thiophenolSy guanine, and guanosine resulted in methylation of the nucleophiles (53). Thus, the formation of the oxadiazolium ion creates an electrophilic species not... 

Since the results of our experiments with isolated rat liver fractions supported a reaction sequence Initiated by microsomal oxidation of the nitrosamine leading to formation of a carbonium ion, the results of the animal experiment suggested that in the intact hepatocyte, one of the earlier electrophilic intermediates (II, III or V, Figure 1) is intercepted by nucleophilic sites in DNA (exemplified here by the N7 position of guanine) before a carbocation is formed. 

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