Pyrimidine base definition

Fig. 18.3. a Definition of backbone torsion angles a to C and standard numbering for ribose atoms. The ribose shown adopts a C(3 )-endo pucker purine or pyrimidine bases are designated with a capital B. Hydrogens at carbon atoms C(3 ) and C(4 ) have been omitted for clarity, b Conformational ranges of torsion angles. 

The three covalent biomacromolecules share some similarities in their global structures but differ in their cellnlar functions and structural details. DNA is the carrier of genetic information, which mnst be faithfully duplicated and selectively transcribed so that each cell can synthesize the proteins it needs. RNA functions mainly in the transcription and translation of this information. In order that the nucleic acids may convey genetic information, a pattern or code must be incorporated in their chemical structure. Purine and pyrimidine bases are arranged in a definite sequence and this sequence constitutes the genetic code. 

Without an accurate determination of the products of the electrolytic process, we cannot state definitively the structural features of a molecule required for electrodetection. However, we have expanded the work into the area of nucleotides, nucleosides and bases, and predicted that the bases would be inactive due to their lack of a ribose moiety, while either purine or pyrimidine nucleosides and nucleotides would be electro-active. It was observed that adenine and uracil (a purine and pyrimidine base respectively), were not detected by this method, while the nucleotides and nucloesides were responsive. Due to the inactivity of 2-deoxyribose we predicted that the 2 -deoxynucleotides would not respond, and this lack of response was observed with 2 -deoxyuridine. 

We have seen that the macromolecule of DNA is a double helix in the form of a spiral staircase in which the links between the different nucleic adds form the banisters 2uid the purine and pyrimidine bases form the steps. As we have seen, each step is formed hy one of the two combinations adraiine-thymine (or thymine-adenine) and guanine-cytosine (or cytosine-guanine). This means that a particular sequence of bases on one of the banisters is paired to one definite complementary sequence on the other. 

Characteristic Organic Structures, 9-116 PSD, definition, 12-1 to 4 psia and psig, definition, 1-40 Purine bases, properties of, 7-5 Pyrimidine bases, properties of, 7-5 Pyrophoric chemicals, safe handling, 16-1 to 12... 

Although no definite evidence is available, some suggestions have been made to support the case of electron transfer with radicals from some pyrimidines (Haysom et al., 1972 Zemel and Neta, 1973). This was based on the observation of a pH effect on the product distribution which was explained as an effect on the parallel reactions (48) and (49). The production of hydroquinone and... 

The bases are in syn or anti orientation. The orientation of the base relative to the sugar moiety is defined by torsion angle x which is constrained by steric interactions, and by the anomeric effect. The main conformations are referred to as syn and anti (see Fig. 17.5). In syn, j X is close to 0° (sp for torsion angle definition, see Box 13.3 Fig. 13.12), and the base is I oriented "above the ribose ring causing steric interactions which in the anti conformation with x close to 180° (ap) are avoided. The anti conformation is therefore preferred and is the j only form observed in double-helical DNA and RNA. An exception is the left-handed Z-DNA with alternating purine/pyrimidine nucleotide sequence where the purines are in the syn conformation. 

Phosphoproteins.—Proteins of this class when subjected to the action of pepsin and hydrochloric acid, undergo hydrolysis, and yield proteoses and an acid containing phosphorus, which is called paranucleic acid. This acid is sharply distinguished from the nucleic acids, as neither xanthine bases, pyrimidine derivatives, nor pentoses occur among its dissociation-products. Paranucleic acid has not been obtained in the pure condition, and its decomposition-products are not definitely known. The phosphoproteins are distinctly acid, are insoluble in water, and form soluble salts which contain the alkali metals and ammonium. 

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