The Mononucleotides

RNA can be hydrolyzed by alkali to 2 3 cyclic diesters of the mononucleotides, compounds that cannot be formed from alkah-treated DNA because of the absence of a 2 -hydroxyl group. The alkali lability of RNA is useful both diagnostically and analytically. 

The formation of the transient reduced complex can also be observed by flash photolysis in the presence of CT-DNA [1(X)]. This shows clearly the existence of a photoinduced electron transfer from a base of the polynucleotide to the excited complex. However, the relative amount of reduced complex which is photoproduced, is smaller in the presence of CT-DNA than in the presence of GMP this may be attributed to a more important back electron transfer process in the ion pair produced on the polynucleotide compared to that in solution with the mononucleotide. 

Interestingly, the correlation of the luminescence quenching by the mononucleotides and polynucleotides with the occurrence of a photoelectron transfer process examined by flash photolysis, has been found [73,95] for the whole series of Ru(II) complexes with TAP and HAT ligands discussed in Sect. 4.1. As will be developed in Sect. 5., this photoelectron transfer with polynucleotides is also connected to enhanced yield of strand breaks and the appearence of adducts on DNA. 

Example 34 bis(iV,iSr-diisopropylamino)trimethylsiloxyphosphine was prepared from readily available chlorobis(diisopropylamino)phosphine in almost quantitative yield [67]. Michalski et al. have observed that this trimethylsilyloxy-bis-(diisopropylamino)phosphine has the ability to undergo highly selective nucleophilic substitution at the atom without affecting the silicon centre. A number of 3, 5 -dinucleoside trimethylsilylphos-phites have been prepared in this way in very high yield without isolation of the mononucleotide intermediate. 

The major intermediates in the biosynthesis of nucleic acid components are the mononucleotides uridine monophosphate (UMP) in the pyrimidine series and inosine monophosphate (IMP, base hypoxanthine) in the purines. The synthetic pathways for pyrimidines and purines are fundamentally different. For the pyrimidines, the pyrimidine ring is first constructed and then linked to ribose 5 -phosphate to form a nucleotide. By contrast, synthesis of the purines starts directly from ribose 5 -phosphate. The ring is then built up step by step on this carrier molecule. 

The cytostatic drugs administered (indicated by a syringe in the illustration) are often not active themselves but are only converted into the actual active agent in the metabolism. This also applies to the adenine analogue 6-mercaptopurine, which is initially converted to the mononucleotide tIMP (thioinosine monophosphate). Via several intermediate steps, tIMP gives rise to tdGTP, which is incorporated into the DNA and leads to crosslinks and other anomalies in it. The second effective metabolite of 6-mercaptopurine is S-methylated tIMP, an inhibitor of amidophos-phoribosyl transferase (see p. 188). 

The absorption coefficients of polynucleotides are different from those calculated from the sum of the mononucleotides in part this reflects the secondary structure. The abrupt increase in the absorption of DNA at the melting point, where the secondary structure changes from the double helix to a random coil, is well known. It is therefore... 

Fig. 37. Course of the photolysis of UpU in 0.02A7 phosphate buffer the concentration of the solution was 10 iM with respect to the mononucleotides. The results are the same at a concentration of 10 3M if the initial optical density is the same (McLaren and Shugar7).

It should be kept in mind that these prebiotic syntheses concern only the bases, and not the mononucleotides of the nucleic acids. The mononucleotide consists of three moieties attached to each other - the base, the phosphate, and the sugar - and we do not yet know how this prebiotic synthesis may have taken place. 

Purine Antimetabolites. Purine synthesis can be blocked by 6-mercaptopurine (7.77) and 6-thioguanine (7.78). Both require conversion to the mononucleotide in a lethal synthesis —a mechanism distinguished from the formation of suicide substrates in that the enzyme that transforms the inactive pro-dmg to the active inhibitor is different from the enzyme that is being blocked. inhibitors are formed and bound by the same... 

The close correspondence of the DNA absorption spectrum with that of a mixture of mononucleotides of the same composition illustrates the weak nature of the interactions between neighboring purine and pyrimidine bases guanine (G), cytosine (C), adenine (A), and thymine (T) at an interplanar separation of 3.36 A in the unexcited double-helical configuration. On the other hand the structureless fluorescence band of (calf-thymus) DNA is red-shifted by 3500 cm-1 from the fluorescence spectral origin of the mononucleotides it closely resembles the fluorescence spectrum of the dinocleotide ApT (and of poly dAT) and is accordingly identified131 with the fluorescence... 

In the small intestine, ribonuclease and deoxyribonuclease I, which are secreted in the pancreatic juice, hydrolyze nucleic acids mainly to oligonucleotides. The oligonucleotides are further hydrolyzed by phosphodiesterases, also secreted by the pancreas, to yield 5 - and 3 -mononucleotides. Most of the mononucleotides are then hydrolyzed to nucleosides by various group-specific nucleotidases or by a variety of nonspecific phosphatases. The resulting nucleosides may be absorbed intact by the intestinal mucosa, or they may un-... 

These early contributions were later supplemented by a comprehensive article on the mononucleotides by Ueda and Fox.4 The present article is an extension of these articles to chemical synthesis of oligo-and poly-nucleotides, and it is hoped that, in conjunction with the earlier articles, it will provide the organic chemist with a reasonable appreciation of some of the challenges, both past and present, of the synthetic chemistry of the nucleic acids. 

It would seem that, to date, no sufficient evidence has been furnished in support of the view that any of the mononucleotides are united through phosphorus-nitrogen linkages. Since uracil has no amino group, uridylic... 

Levene mentioned in 1931 that he had been unable to hydrolyze the polynucleotide to the mononucleotides with this enzyme, and in 1937 Dubos continued the study of the enzyme. Schmidt and Levene came to the conclusion that the phenomenon is one of depolymerization of polytetranucleotides to the tetranucleotide state. They were unable to detect any reaction-product of small enough molecular size to diffuse through cellophane membranes, and found the freezing point of the solution remained unchanged during action of the enzyme. However, a mixture of the four mononucleotides dialyzed rapidly through cellophane and caused a considerable depression of the freezing point. 

Faced with the problem of reconciling these findings with the fact that the nucleotides actually isolated on chemical hydrolysis are not 5-phosphonucleosides, Gulland has envisioned a system of phosphate transfer from position (2) or (3) to position (5) prior to dephosphorylation by 5-nucleotidase, or from position (5) to position (3) on chemical fission to the mononucleotides. The results of experiments designed to test the validity of one or other of these theories are awaited with interest. 

The binding of the nonfunctional part of the coenzyme can be studied by measuring the absorption spectra of the complex against those of the isolated components. The formation of enzyme—coenzyme complexes changes the intramolecular interactions the intensity of these interactions can be measured by comparing the spectra of dihydro coenzyme analogues and the mononucleotides. Against NADH the spectrum of an equimolar mixture of dihydronicotinamide mononucleotide and adenosine monophosphate shows rises at 335 nm and 260 nm and a minimum at 280 nm. The peaks at 320 nm and... 

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