5 -Uridylic acid

Shaw synthesis, 3, 109 Uridine, 2 -deoxy-5-iodo-as pharmaeeutical, 1, 160 Uridine 5 -monophosphate biosynthesis, 1, 88 Uridylic acids occurrence, 3, 142 Urispas... 

Imidazolides of adenylic acid (ImpA) or uridylic acid (ImpU) are polycondensed to oligonucleotides by means of Zn2+ ions. 1673 The resulting phosphordiester bond was found to be of the 2, 5 type. In the reaction of nucleoside 5 -phosphoric acid methyl ester with ImpA in the presence of MgC, 2, 5 -dinucleotides are formed six to nine times more frequently than the corresponding 3, 5 compounds. 63 Polycondensations of ImpA in aqueous solution in the presence of various divalent metal ions lead to short oligo-adenylic acids (pA) (n = 1—5) mainly with 2, 5 -intemucleotide linkages. With Pb2+, for example, the total yield of oligomers was as high as 57%. 1683 1693... 

This reaction has also been shown to occur in cytidine, cytidylic acid, uracil, uridine, and uridylic acid (found in RNA) but reportedly not in thymine, thymidine, or thymidylic acid/55 The photohydration has been found to be partially reversible, dehydration being nearly complete at extremes of temperature and pH. 

Uracil Uridine Uridylic acid Uridine monophosphate (UMP) Uridine diphosphate (UDP) Uridine triphosphate (UTP)... 

A common intermediate for all the nucleotides is 5-phosphoribosyl-l-diphosphate (PRPP), produced by successive ATP-dependent phosphorylations of ribose. This has an a-diphosphate leaving group that can be displaced in Sn2 reactions. Similar Sn2 reactions have been seen in glycoside synthesis (see Section 12.4) and biosynthesis (see Box 12.4), and for the synthesis of aminosugars (see Section 12.9). For pyrimidine nucleotide biosynthesis, the nucleophile is the 1-nitrogen of uracil-6-carboxylic acid, usually called orotic acid. The product is the nucleotide orotidylic acid, which is subsequently decarboxylated to the now recognizable uridylic acid (UMP). 

Uridylic acid 262 1.0 20 Only hydrate reported Only dimer reported... 

The rate of dehydration of uridine hydrate (and the hydrates of various uridine phosphates) has been reported by Logan and Whitmore,52 at 86°C and a pH of 8.4. The concentration of hydrate was unspecified (it was prepared by photolysis of uridine solution and not isolated), and the fraction of complete recovery was not specified (about 97.5% recovery is shown). The main purpose was to compare the dehydration rates of uridine, various uridine phosphates and polyuri-dylic acid. The uncertainty about the nature of the products formed in the dehydration of irradiated uridylic acid (see below), however, makes interpretation of the observed rates quite difficult. It would be better to measure the rates of formation of a particular product than to rely too heavily on measurements of absorbance change. 

Schuster54 reported that uridylic acid, Up, irradiated in buffered solution with a resonance lamp, formed the monohydrate which was identified by heat instability. If this material is warmed in alkaline solution, only 65% of the Up is recovered the other 35% decomposes to a new unidentified product (compound A) with the pyrimidine ring opened. Irradiation of the Up in frozen buffered solution produced the dimer, identified by photolytic reversion to Up. 

An attempt has been made to apply the Cohen and Reiss theory to dimer and hydrate formation in RNA.158 The results were inconclusive, probably because of a poor choice of example. Application of the theory to RNA was complicated by the necessity of estimating the distribution of uracil residues on the chain. The results are made still more tentative by the fact that Tanaka ignored the probability of dimer formation between cytosine residues, mixed dimers between cytosine and uracil, and hydrate formation in cytosine as well as the resultant deamination phenomena. A better choice of example would have been poly-uridylic acid. 

It is evident that most future work with irradiated polynucleotides will have to employ techniques such as these. Many pertinent observations were made about the effect of irradiation of the poly U upon enzyme specificity and rate. Irradiation of the polynucleotide drastically reduced the rate of hydrolysis of poly U by RNase. It was observed that RNase could not split the phosphodiester bond on the 3 -OH end of uridylic acid dimer. It was also shown that dehydration of irradiated poly U was accompanied by marked phosphodiester bond breakage and degradation of the polynucleotide. 

The observed rates of photohydration for uridylic acid derivatives show constant quantum yields by this measure only at concentrations of 10 M , at higher concentrations7 (Fig. 38) more than one molecule of the uridylic acid is involved in the formation of the product. Even the... 

The deviation from first-order kinetics for the uridylic acids is also reflected in quantum yields which vary with concentration and which therefore vary during the course of a particular photolysis.7 These deviations from first-order kinetics have been discussed in terms of a collision-induced transition of an excited-singlet pyrimidine to a long-lived state.7 We shall say more about the probability of long-lived states later. 

Uridylic acid (di-Na salt) [27821-45-0] M 368.2, m 198.5 . Crystd from MeOH. 

Uridylate kinase 655 5 -Uridylic acid. See UMP Urocanase reaction 778 Urocanic acid 755,756s Urokinase 634 Uronic acid 164 Uroporphyrin(s) 843 Uroporphyrin I, 845s Urothione 804s, 891 Urticaria 385 Usher protein 364... 

Carboxylation followed by a later decarboxylation is an important pattern in other biosynthetic pathways, too. Sometimes the decarboxylation follows the carboxylation by many steps. For example, pyruvate (or PEP) is converted to uridylic acid (Eq. 17-41 details are shown in Fig. 25-14) ... 

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