Cytidine dimer

Only a confused picture is revealed by these essentially qualitative studies. Many questions are left unanswered. The probable formation of uracil hydrate in cytosine photolysis has not been reported and the question of dimer formation, either in solution or in ice, is still unresolved. There are thus large gaps in our knowledge about the photolysis of cytosine and cytidine. Much more detailed work has been carried out on cytidylic acid and on dinucleotides of cytosine (Sect. XI-C). 

In acid medium (pH 4.5) electrochemical reduction led to formation of two products. One of these was characterized as the 6,6 -dimer of the riboside of pyrimidone-2. It was photochemically converted to the riboside of pyrimidone-2. The second, on the basis of its chromatographic behaviour, UV spectrum, and reaction with the Fink reagent, was identified as the riboside of 3,6-dihydropyrimidone-2. The mechanism of electrochemical reduction of cytidine in acid medium is consequently analogous to that for 1-methylcytosine. The products of reduction of cytidine at pH 7 were shown chromatographically to contain 5,6-dihydrocytidine 1 84). A comparison of the electrochemical and catalytic reduction products under analogous conditions at pH 7 demonstrated that both led to the same products, one of them 5,6-dihydrocytidine 84). 

Irradiation of cytosine, cytidine, and 2 -deoxycytidine has recently been reported to produce 4,5 -linked dimers (154 R = H, j3-D-ribose, 2-deoxy-/3-D-ribose, respectively) 210 this reaction is accompanied by deamination. Photodimerization (in aqueous solution) of uracil211 and thymine212 affords the 4,6 -linked dimer (155 R = R = H and Me, respectively) the oxetane (156) (probably formed by 1,2-cycloaddition of a triplet carbonyl to an olefinic bond) is proposed as the precursor.212... 

Silylated 3-nitro-1,2,4-triazole derivative 242 is readily aminated by ethylenediamine to give dimeric cytidine 243, as well by other oi-diamines. 

Schematic representation of the subunit structure of aspartate transcarbamoylase and its dissociation into catalytic and regulatory subunits by mercurials, which can be further converted to inactive monomeric subunits by strong denaturing agents (e.g., sodium dodecyl sulfate). The native enzyme consists of two catalytic trimers placed one above the other, along with three dimeric regulatory subunits surrounding the catalytic trimers in an equatorial plane (a). Substrate maintains the enzyme in the catalytically more active relaxed (R) conformation, while cytidine tripho.sphate maintains it in the catalytically less active taut (T) conformation (b). [Reproduced, with permission from E. L. Smith, R. L. Hill,...

Treatment of the D-arabinofuranosyloxazoline (24) with 1-chloro-l-cyano-ethene gave the 2,2-0-c clonucleosides (25) and (26) (Scheme 6)P 3,5-Di-C)-acyl derivatives of 2,2 -0-cyclo-]3-D-arabinofuranosyl-cytidine have been prepared by reaction of cytidine with the acid chloride or anhydride derivatives of 3-heptyloxypropanoic, 3-dodecyloxypropanoic, and 11-butyrylamino-undecanoic acid the dimeric derivative (27) was also prepared. 

Figure 5. Dimer 3 or trimer 4 of porphyrins formed via H bonds between guanosine and cytidine nucleic acids.

When 5 -0-Dmtr-cytidines, protected at N-4, were treated with alkyl iodides in the presence of silver(I) oxide, the 2 -0-alkylated compounds were the major product. N -t-Butylphenoxyacetyl protection gave the best selectivity, and the products were deaminated to 2 -C -alkyluridines. An alternative approach to 2 -0-alkylated ribonucleosides involves the use of 181, produced by selective debenzylation of the tris-(2,4-dichlorobenzyl) derivative (see Vol.21, p.49-50), in condensations with silylated bases. 3, 5 -Di-0-trityluridine has been alkylated at 0-2 using 2-chloromethylanthracene detritylation was followed by incorporation into oligonucleotides which exhibited enhanced fluorescence on binding to complementary RNA segments. The synthesis of 3 -D-(2-aminoethyl)-2 -deoxyuridines has been described these materials were used for the synthesis of dimers of type 175. ... 

FIGURE 141.1 (a) The tautomer bypass and the deamination mechanism of CC-dimer mutagenicity, used to explain C —> T transitions and CC —> TT tandem transition mutations, (b) A cytidylyl-cytidine sequence and a situation in which both cytidins are present in their E-imino tautomeric forms. 

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