Puric bases

Puric bases fluoresce at least three times more than pyrimidic bases. Guanidine is the most fluorescent basis. In the native state, DNA and RNA show a very weak fluorescence,... 

AP sites may be spontaneous or induced, and can block the replication. Instability of the glycosylic site constitutes the principal cause of AP sites formation. Cellular DNA can undergo puric base losses, estimated at between 250 and 500 per hour. AP sites can be regenerated during reparation processes by base excision and free radicals. 

Puric bases fluoresce at least three times more than pyrimidic bases The guanidine is the most fluorescent basis. 

To end this chapter, we ll note that other kinds of complexes exist, whose formation may induce modifications of some physicochemical properties of the compounds giving them. Hence, their analysis may be perturbed. It is the case of ion pairs that we decided to include in the group of complexes and also of charge transfer derivatives. A good example is that of puric bases derived from 7H-purines xanthine, caffeine, theophylline, and theobromine. Some abnormal physical properties are exhibited by these derivatives. 

The basic components of these macromolecules are the orthophosphor ic acid, a sugar (ribose or deoxyribose), and purines [adenine (A) and guanine ( )] and Pyrimidines [cytosine (C), thymine (T), and uracil (I/)]. The puric and pyrimidinic bases are the active chemical components, while the orthophosphoric acid and the sugar constitute the skeleton of these macromolecules. 

Tinoco, I. Hypochromism in polynucleotides. J. Am. Chem. Soc. 82, 4785 (1960). Valdemoro, C., and S. Fraga The eletronic structure of puric and pyrimidinic bases and the biological codes. Technical Report TC-6701, Department of Chemistry, University of Alberta (1967a). 

Puric and pyrymidic bases show a weak fluorescence in aqueous medium, which increases by a factor of 10 in a medium of pH 2 and by a factor of 100 at 77 K. The quantum yield of the nucleic acids depends largely on the temperature, and their fluorescence lifetime is weak. Fluorescence polarization is high, even at ambient temperature. At a low pH, the emission maximum is temperature-dependent. 

Among nitrogen compounds, puric and pyrim-idic bases play an important role in activating growth. In this case, the needs for adenine, guanine, uracile, thymine and thymidine are also dependent on the strain. They are not always essential. 

Similarly, analogues of puric or pyrimidic bases may be incorporated in the synthesis of nucleic adds and alter the physicochemical and biologicd properties of these acids (see p. 456). 

Alkylation of the parent calix[6]arene 14 with PicCl HCl (0.5-30 equiv.) was investigated in anhydrous DMF at 60°C (and in one case in refluxing CH3CN) in the presence of base [NaH, K2CO3, or BaO/Ba(OH)2]. Exhaustive alkylation reactions are clean, while reactions with limiting amounts of electrophile produce complex reaction mixtures, which can be eventually separated into the puric components by extensive chromatographic means. 

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