06-Methylguanine

Most eukaryotic mRNAs have a 7-methylguanine cap at the S end, which promotes efficient translation of the message and protects it from degradation by S to S exonucleases. 

Methylguanine [938-85-2] M 165.2, m >300 (dec). Crystd from 50% aqueous acetic acid. 7-Methylguanine [578-76-7] M 165.2. Crystd from water. 

Becker. R.A.. Barrows, L.R. Shank. R.C. (1981) Methylation of liver DNA guanine in hydrazine hepatotoxicity dose-response and kinetic characteristics of 7-methylguanine and O -methylguanine formation and persistence in rats. Carcinogenesis, 2, 1181-1188... 

Figure 2. Self-complementary base pair between zwitterionic neutral (left) and cationic (right) forms of 7-methylguanine derivatives.

A further example (the oldest recorded) of isomerization, which differs from the preceding two examples, is credited to Fischer," who attempted the hydrolysis of the chlorine atom of 6-amino-2-chloro-7-methylpurine (54, R = H) with alkali but obtained 7-methylguanine (55, R = H) instead of 7-methylisoguanine. Some insight into the intramolecular rearrangement may be gained from the fact that the... 

DNA and RNA also contain minor amounts of other bases. Among the minor purines are 1-methylguanine, 6-methylaminopurine (primarily amino group of adenine is methylated), 6-dimethylaminopurine, and 7-methylguanine. Among the minor pyrimidines are 3- or 5-methylcytosine and 5-hydroxymethylcytosine. 

Substituted purines may arise in mixtures with 9-substituted derivatives from direct alkylation of purine anions. They are best prepared however by Traube syntheses. Thus formylation of 4,5-diamino-6-benzylthiopyrimidine and ethylation of the formyl derivatives gave a formamidopyrimidine which readily cyclized to 6-benzylthio-7-ethylpurine in the presence of potassium carbonate (B-68MI40901, p. 31). The derivative is clearly a ready source of 7-substituted adenines by reaction with ammonia or amines, or of 7-substituted purines by dethiation with Raney nickel. 7-Methylguanine has also been obtained from 7-methyl-guanosine, sodium borohydride and aniline at pH 4.5 (B-78MI40903, p. 615). 

Cyclization of 5-(l-methylureido)pyrimidine by refluxing with 3.5 M hydrochloric acid gives 8-hydroxy-7-methylguanine (14). ... 

The first ring-degenerate transformation in heterocyclic chemistry was discovered in 1898 by E. Fischer, who found that treatment of 2-chloro-7-methyladenine with base does not yield the expected 7-methylisoguanine but 7-methylguanine (6). This can undoubtedly be considered as a ring-degenerate transformation by a nucleophilic displacement according to a S ,(ANRORC) mechanism. 

A different problem occurs with 7-methylguanosine. The purine base of this nucleoside is hydrolyzed at basic pH, and very significant degradation occurs in an hour at pH 9.5 at room temperature. The mixture of oxidized nucleoside and protein, therefore, is kept at pH 9.1 and at 0-4°, at which only very limited hydrolysis of the base occurs in 1 hr. A different reducing agent, tert-butylamine borane (Aldrich Chemical Co.), is used the reduction is done for only 30-60 min at 4°, and the product is separated from free nucleoside on a Sephadex G-25 column at 4°. The nucleotide of the 7-methylguanosine is more stable than the nucleoside, and it also has been used to induce antibody to the 7-methylguanine structure. ... 

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