Pyrimidines bases

Nucleic acids are acidic substances present m the nuclei of cells and were known long before anyone suspected they were the primary substances involved m the storage transmission and processing of genetic information There are two kinds of nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) Both are complicated biopolymers based on three structural units a carbohydrate a phosphate ester linkage between carbohydrates and a heterocyclic aromatic compound The heterocyclic aro matic compounds are referred to as purine and pyrimidine bases We 11 begin with them and follow the structural thread... 

Purine and pyrimidine bases Nucleosides Nucleotides Nucleic acids... 

Each pair contains one purine and one pyrimidine base This makes the A T and G C pairs approximately the same size and ensures a consistent distance between the two DNA strands... 

FIGURE 28 5 (a) Tube and (b) space filling models of a DNA double helix The carbohydrate-phosphate backbone is on the out side and can be roughly traced in (b) by the red oxygen atoms The blue atoms belong to the purine and pyrimidine bases and he on the inside The base pairing is more clearly seen in (a)... 

Section 28 8 The most common form of DNA is B DNA which exists as a right handed double helix The carbohydrate-phosphate backbone lies on the outside the punne and pyrimidine bases on the inside The double helix IS stabilized by complementary hydrogen bonding (base pairing) between adenine (A) and thymine (T) and guanine (G) and cytosine (C)... 

The only other pyrimidine-based preparation of pyrido[3,2-d]pyrimidines involves reaction of 5-aminopyrimidine with crotonaldehyde to give (255) (70JHC1219). 

Nucleoside (Section 28.2) The combination of a purine or pyrimidine base and a carbohydrate, usually ribose or 2-deoxyribose. 

FIGURE 11.3 The common pyrimidine bases—cytosine, nracil, and thymine—in the tantomeric forms predominant at pH 7. 

RNA is relatively resistant to the effects of dilute acid, but gentle treatment of DNA with 1 mM HCl leads to hydrolysis of purine glycosidic bonds and the loss of purine bases from the DNA. The glycosidic bonds between pyrimidine bases and 2 -deoxyribose are not affected, and, in this case, the polynucleotide s sugar-phosphate backbone remains intact. The purine-free polynucleotide product is called apurinic acid. 

The names of these compounds as aza analogs were coined in the same way as those of the 6-aza analogs employing the frequently used numbering of uracil (1). This nomenclature is most often used for the principal aza analogs of pyrimidine bases (e.g., 5-azauracil) it is rarely used for further systematic derivatives. 

All these findings, as well as the similarity of UV spectra - caused dioxotetrahydrotriazine to be classified as the simplest member of the formerly known 6-substituted derivatives. These derivatives are not interesting in connection with the analogs of natural pyrimidine bases and have been reviewed elsewhere. The structure of allantoxaidine and its appurtenance to the triazine series have been recently demonstrated by its unequivocal synthesis. 

The chemistry of the 6-aza analogs of pyrimidine bases which has been developed from the biochemical aspect since about 1956 was based on work reported in relatively numerous older papers. In spite of the fact that 6-azauracil was prepared only in 1947 and suitable syntheses were described only quite recently, substances of this type and methods of their preparation had been known for a long time. The chemistry of 6-aza analogs of pyrimidine bases is therefore relatively closely linked with the chemistry of the 1,2,4-triazine derivatives. 

The first chemical synthesis of these substances, using a procedure which yields 1-ribofuranosyl derivatives by pyrimidine bases, was described by Hall. By using the mercuric salt of 6-azathymine and tribenzoate of D-ribofuranosyl chloride, he obtained a mixture of two monoribosyl derivatives and a diribosyl derivative. He determined the structure of the 3-substituted derivative by the similarity of spectra and other properties to those of 3-methyl-6-razauracil. The structure of the 1-ribosyl derivative was then determined from the similarity of the spectra with 6-azathymine deoxyriboside obtained enzymatically. 

Chemical and enzymatic ribosidization of the aza analogs of the pyrimidine bases thus take different routes. These results and independent earlier studies of the alkylation of 6-azauraciE led to the conclusion that, in order to achieve ribosidization in position 1 (i.e., position 2 of the triazine ring), the position 3 (4 of the triazine ring) must be protected. ... 

Just as proteins are biopolymers made of amino acids, nucleic acids are biopolv-mers made of nucleotides joined together to form a long chain. Each nucleotide is composed of a nucleoside bonded to a phosphate group, and each nucleoside is composed of an aldopentose sugar linked through its anomeric carbon to the nitrogen atom of a heterocyclic purine or pyrimidine base. 

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