Purine structure

The chemical name of caffeine is 1,3,7-trimethylxanthine, and it is part of the purine family of derivatives of methylxanthines (Figure 4-2). Caffeine s basic chemical structure is similar to the purine structure found in DNA (see below). This similarity in structure generated speculation that caffeine may somehow cause... 

Compounds based on the purine structure are classified as purines. Adenine is one of the two purines found in DNA and RNA. The other is guanine. Adenine and guanine are called bases in reference to DNA and RNA. A nucleic acid base attached to ribose forms a ribonucleoside. Adenine combined with ribose produces the nucleoside adenosine. When an oxygen atom is removed from the second carbon of ribose, the sugar unit formed is... 

The purine bases are formed from the parent compound purine, which in turn is formed by the fusion of a pyrimidine and an imidazole ring. The parent purine structure and systematic numbering are shown in Fig. 2. Like the pyrimidine class of compounds, purine ring systems are it electron deficient however, unlike pyrimidines, there is no position in the ring... 

Molecular spectra have always played an important part in investigations of purine structure and reactions, and recent improvements, especially in the development of H,... 

In coenzymes that contain these ring systems the intramolecular interaction is weaker than in the above-mentioned coenzyme analogues that contain other purine structures. Energy transfer was observed in the case of dihydro-... 

This important aspect of purine structure is still of active interest. Theoretical and practical studies have been carried out to ascertain the site of... 

Nebularine has long been known as a metabolite of Clitocybe nebularis (383), revealing tuberculostatic and antimitotic activity (383-385). Few syntheses of this compound having a 9-(D-ribosyl)purine structure have been reported, namely, the Brown (386) approach, starting from a chloro-mercuri-6-purine and 2,3,5-tri-O-acetyl-D-ribofuranosyl chloride, the Fox (387) procedure, based on the transformation of inosine to nebularine via a desulfurization by Raney nickel of the thioinosine intermediate, and the Iwamura-Hashizume (384,388) method, in which nebularine and its N-7 isomer were synthesized simultaneously by the fusion of purine and tetra-O-acetyl-D-ribofuranose using bis(p-nitrophenyl)hydrogen phosphate as the catalyst. 

As was discussed for imidazole, tautomerism makes it difficult to predict the position of the proton in this component of the purine structure. In adenine, the proton is largely found at the 9-position, whereas in guanine a mixture of the two forms is present in solution. In nucleosides, it is the 9-position that is attached to the sugar moiety as noted in section 3.2.3. The numbering in purines is exceptional to the mles of nomenclature but is accepted by the International Union of Pure and Applied Chemistry (lUPAC). It is illustrated in Figure 9.2. 

The purine nucleotides GTP and ATP are very important in intermediary metabolism and the regulation of metabolism. Adenine is also a component of cyclic AMP, FAD, NAD, NADP and coenzyme A. Moreover, GTP, ATP and their deoxy derivatives dGTP and dATP are important precursors for the synthesis of RNA and DNA respectively, which are essential for cell growth and division. Purine biosynthesis (Fig. 59.1) needs the amino acids giutamine, giycine and aspartate. Also, tryptophan is needed to supply formate which reacts with tet-rahydrofolate (THF) to produce A "-formyl THF, which donates the formyl group to the purine structure. A molecule of CO2 is also needed. 

Because of the conflicting results, it seems necessary tentatively to accept the possibility that nucleic acid purines may be synthesized under normal metabolic conditions from preformed purines as well as from elementary precursors. The problem of ascertaining the role in normal metabolism of preformed purine compounds is difficult, since it is known that the mammalian organism is able to synthesize its own purines from simple metabolites and does not require dietary purines. Recent studies indicate that incompleted purine structures joined to ribose and phosphate are intermediates in purine synthetic reactions in vitro, and that... 

Marek et al. have reviewed applications of NMR spectroscopy in the investigation of the structure and the intra- and intermolecular interactions of purine derivatives. NMR methods suitable for studying the purine structure and their application to exploring samples at natural levels of the C and N isotopes using CP MAS were briefly reviewed. In addition, quantum-chemical calculations of the NMR parameters on the DFT level have also been considered. Typical examples of applications were also presented. 

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