Purine base, 33

This enzyme, sometimes also called the Schardinger enzyme, occurs in milk. It is capable of " oxidising" acetaldehyde to acetic acid, and also the purine bases xanthine and hypoxanthine to uric acid. The former reaction is not a simple direct oxidation and is assumed to take place as follows. The enzyme activates the hydrated form of the aldehyde so that it readily parts w ith two hydrogen atoms in the presence of a suitable hydrogen acceptor such as methylene-blue the latter being reduced to the colourless leuco-compound. The oxidation of certain substrates will not take place in the absence of such a hydrogen acceptor. 

Imidazole can be A -allylated. The A -glycosylimidazole 299 is prepared by regiospecific amination at the anomeric center with retention of configuration. Phenoxy is a good leaving group in this reaction[181]. Heterocyclic amines such as the purine base 300 are easily allylatedfl 82]. 

Regioselective 1,4-azidohydroxylation to give 309 takes place by the reaction of the vinyloxirane 308 with sodium azide[188]. The reaction of the cyclopen-tadiene monoepoxide 310 with sodium azide or purine base offers a good synthetic method for the carbocyclic nucleoside 311(189-191]... 

Sometimes the abbreviation applies to the pyrimidine or purine base sometimes to the nucleoside Though this may seem confusing it is normally clear from the context what is intended and causes no confusion in practice... 

The pyrimidine and purine bases are cis to the —CH2OH group of the furanose ring (p stereochemistry)... 

As the strands unwind the pyrimidine and purine bases become exposed Notice that the bases are exposed m the 3 — 5 direction in one strand and m the 5 — 3 direction m the other... 

Section 28 2 Nucleosides are carbohydrate derivatives of pyrimidine and purine bases The most important nucleosides are derived from d ribose and 2 deoxy D ribose... 

The exocychc nitrogens on cytosine and the purine bases must be protected during the synthesis. The search is ongoing for protecting groups that are subject to fewer side reactions and that can be removed more easily in the final deprotection step (34). 

The pur pose of work is to develop the technique of separ ation of purine bases (caffeine, theophylline, theobromine) and the technique of detection of purine bases in biological fluid by TLC using micellar mobile phases containing of different surfactants. 

These relationships are general. Hydroxyl-substituted purines and pyrimidines exist in their keto forms fflnino-substituted ones retain structures with an anino group on the ring. The pyrimidine and purine bases in DNA and RNA listed in Table 28.1 follow this general rule. Beginning in Section 28.7 we ll see how critical it is that we know the correct tautomeric forms of the nucleic acid bases. 

Section 28.2 Nucleosides are car bohydrate derivatives of pyrimidine and purine bases. 

FIGURE 11.4 The common purine bases—adenine and guanine—in die tautomeric 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. 

Cleavage at A or G If the DNA is first treated with acid, dimethyl sulfate methylates adenine at the 3-position as well as guanine at the 7-position (not shown). Subsequent reaction with OH and piperidine triggers degradation and displacement of the methylated A or G purine base and strand scission, essentially as indicated here for reaction of dimethyl sulfate with guanine. 

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 analogs of pyrimidine and purine bases can be derived by purely formal structural modifications or, more rationally, from the results of biochemical investigation. 

An important group of antimetabolites are the aza analogs of pyrimidine and purine bases which are theoretically derived by a replacement of the methine group of a pyrimidine or purine nucleus with a nitrogen atom. This replacement represents a relatively minor alteration of the structure of these substances as it does not change the functional groups, practically preserves the molecular weight, and produces almost isosteric compounds. The replacement of the methine... 

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