Nucleic acids pyrimidine bases

The coloured parts of the molecules below emphasize the characteristic features of the bases, purine bases in nucleic acids pyrimidine bases in nucleic acids... 

Occurs in milk and other biol. systems. Key compd. involved in biosynth. of nucleic acid pyrimidine bases. Shows bacteriostatic and cytostatic props. Forms metal complexes. Uricosuric agent. Used in photometric detn. of Zn. Cryst. (H2O). Mp 322-325°, Mp 345-346°. 

The bases are either monocyclic pyrimidines or bicyclic purines (see Section 14.1). Three pyrimidine bases are encountered in DNA and RNA, cytosine (C), thymine (T) and uracil (U). Cytosine is common to both DNA and RNA, but uracil is found only in RNA and thymine is found only in DNA. In the nucleic acid, the bases are linked through an A-glycoside bond to a sugar, either ribose or deoxyribose the combination base plus sugar is termed a nucleoside. The nitrogen bonded to the sugar is that shown. 

A. R. Katritzky and M. Karelson,/. Am. Chem. Soc., 113, 1561 (1991). AMI Calculations of Reaction Field Effects on the Tautomeric Equilibria of Nucleic Acid Pyrimidine and Purine Bases and Their 1-Methyl Analogues. 

The most important naturally occuring diazines are the pyrimidine bases uracil, thymine and cytosine, which are constituents of the nucleic acids (see 32.4). The nucleic acid pyrimidines are often drawn horizontally transposed from the representations used in this chapter, i.e. with N-3 to the north-west , mainly to draw attention to their structural similarity to the pyrimidine ring of the nucleic acid purines, which are traditionally drawn with the pyrimidine ring on the left. There are relatively few naturally occurring pyr-azines or pyridazines. 

Nucleic acids contain bases of two different types, pyrimidines and purines. The catabolism of the purines, adenine and guanine, produces uric acid. At physiological hydrogen ion concentration. uric acid is mostly ioni/ed and present in plasma as sodium urate (Fig. I). An elevated. serum urate concentration is known as hyperuricaemia. Uric acid and urate are relatively insoluble molecules which readily precipitate out of aqueous solutions such as urine or synovial fluid (Fig. 2). The consequence of this is the medical condition, gout. 

Pyrimidine 1,3-diazine, a heterocyclic compound, consisting of a six-membered ring with 2 nitrogen atoms (Fig.l), M, 80.1, m.p. 20-22°C, b.p. 124°C. The P. ring system is present in many natural compounds, e. g. antibiotics (nucleoside antibiotics), pterins, purines and vitamins, it is especially important in the pyrimidine bases. Cytosine (see). Uracil (see) and Thymine (see), which are constituents of nucleic acids. Pyrimidine itself does not occur naturally. Pyrimidine analogs (see) can also be incorporated into nucleic acids. 

Finally, as mentioned above, with the advent of a technology for manipulation of nucleotides, the free uridine nucleotides were recognized as intermediates in the incorporation of orotate into the nucleic acid pyrimidines 2), and it became apparent that the first product of this pathway to possess a nucleic acid base was uridylate. 

As early as 1949, it was demonstrated that injected or " C-labeled orotic acid was readily incorporated into DNA and RNA of mammalian tissue, indicating that orotic acid is a precursor of nucleic acid pyrimidine. The next step in pyrimidine biosynthesis is the formation of the first nucleotide in the sequence. It involves the reaction between ribosyl pyrophosphate and orotic acid to yield 5 -orotidylic acid the reaction is catalyzed by orotidylic pyrophosphorylase. Thus, the first steps of pyrimidine biosynthesis differ from the early steps of purine biosynthesis in at least two ways. Orotic acid, instead of being synthesized atom by atom as is the case for the purine ring, is made from the condensation of rather large molecules, namely, carbamyl phosphate and aspartic acid. Furthermore, all the steps of purine biosynthesis occur at the level of the nucleotide, but the the pyrimidine ring is closed at the level of the base. 

Since Hammarsten s results showed that not only the pyrimidines of PNA but also those of DNA were labeled after administration of labeled cytidine, it was concluded that this was indirect evidence for the conversion of a pyrimidine riboside to a pyrimidine desoxyriboside. The reason for this was that cytidine could not have been split to cytosine and then reincorporated into desoxycytidine, since it had been shown earlier that the free base, cytosine, could not be utilized for the synthesis of nucleic acid pyrimidines. The possibility should not be overlooked, however, that the above conversion may possibly occur at the nucleotide level thus, cytidylic acid, in which the phosphate is attached to the nucleoside, may be the intermediate in the transformation of PNA to DNA pyrimidines. 

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... 

Two nitrogen containing heterocyclic aromatic compounds—pyrimidine and purine— are the parents of the bases that constitute a key structural unit of nucleic acids... 

Both pynmidme and purine are planar You will see how important this flat shape is when we consider the structure of nucleic acids In terms of their chemistry pyrimidine and purine resemble pyndme They are weak bases and relatively unreactive toward elec trophilic aromatic substitution... 

These relationships are general Hydroxyl substituted purines and pyrimidines exist in their keto forms ammo substituted ones retain structures with an ammo group on the ring The pyrimidine and punne bases m DNA and RNA listed m Table 28 1 follow this general rule Beginning m Section 28 7 we 11 see how critical it is that we know the cor rect tautomeric forms of the nucleic acid bases... 

Another property of pyrimidines and purines is their strong absorbance of ultraviolet (UV) light, which is also a consequence of the aromaticity of their heterocyclic ring structures. Figure 11.8 shows characteristic absorption spectra of several of the common bases of nucleic acids—adenine, uracil, cytosine, and guanine—in their nucleotide forms AMP, UMP, CMP, and GMP (see Section 11.4). This property is particularly useful in quantitative and qualitative analysis of nucleotides and nucleic acids. 

Aza Analogs of Pyrimidine and Purine Bases of Nucleic Acids... 

As is well-known, nucleic acids consist of a polymeric chain of monotonously reiterating molecules of phosphoric acid and a sugar. In ribonucleic acid, the sugar component is represented by n-ribose, in deoxyribonucleic acid by D-2-deoxyribose. To this chain pyrimidine and purine derivatives are bound at the sugar moieties, these derivatives being conventionally, even if inaccurately, termed as pyrimidine and purine bases. The bases in question are uracil (in ribonucleic acids) or thymine (in deoxyribonucleic acids), cytosine, adenine, guanine, in some cases 5-methylcytosine and 5-hydroxymethylcyto-sine. In addition to these, a number of the so-called odd bases occurring in small amounts in some ribonucleic acid fractions have been isolated. 

The 8-aza analogs of purine bases were the first to be studied among all the aza analogs of nucleic acid bases (as early as 1945). Before that time the chemistry of these substances had not been treated in detail from any aspect. Thus the entire chemistry of the u-triazolo [4,5-d]pyrimidines was developed only in connection with the study of antimetabolites of nucleic acid components. Therefore all the papers involved are largely of preparative character and only rarely discuss. theoretical points. 

Solute-solvent interactions in aqueous solutions of pyrimidine nucleic acid bases 99PAC1286. 

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