Uracil, structure

Unsaturation, degree of. 174 Upfield, (NMR), 445 Uracil, structure of, 1101 Urea, from ammonium cyanate, 2 Urethane, 1214 Uric acid, pKa of, 778... 

Figure 11-6. Structures of representative conical intersections Sj/Sq in the pyrimidine bases, uracil, thymine, and cytosine. Uracil structures (a,d) are taken from Ref. [147, 210]. Thymine structures (b,e) are taken from Ref. [152], Cytosine structures (c,f) are taken from Ref. [157]...

Alloxan 29 is formed in organisms by the oxidative degradation of uric acid (see p 414). Willardiin 30 is a nonproteinogenic a-amino acid with a uracil structure which is present in the seeds of some types of acacia. A few pyrimidine antibiotics, especially those isolated from streptomyces, possess potent antitumour properties, e.g. the structurally complex bleomycins 31. The commercial product is a mixture of various bleomycins, main components being bleomycins A2 (55-70%) and B2 (25-32%). 

Yu et. al, found that molecules containing two 2,6-diacetamido-4-pyridone groups and a nonlinear optical chromophore will self-assemble with diimides or bis(uracil) structures, to yield polymer-like hydrogen-bonded assemblies. 

It is naturally occurring as one of the four base pairs in ribonu-cleosides. The uracil structure is shown in Figure 16.7. 

N-aqflated derivatives of 2-oxoamides (ketoimides) 9, and subsequently 2-oxoamides (ketoamides) 10, are formed these are the key derivatives and lower the concentration of the components of the initiation system. Here, each condensation step consumes two molecules of classical growth centers during the formation of one ketoimide structure. As its acidity is distinctly higher than that of the amidic group, this leads also to a decrease in the concentration of lactam anions, and hence in the concentration of the initiator. The ketoamides at higher temperatures decompose to yield isocyanates, to form uracil structures and water (the latter being an inhibitor of the polymerization process), and so on [9, 12]. 

In the case of the 2-dimethylamino- (or 2-amino-)methylene derivatives, the products were at first thought to be pyrimido[4,5-c]isoquinolines (267), but later work with 6-(N-substituted amino)uracils assigned the structures of the products (268) as belonging to the isomeric pyrimido[4,5-f>]quinoline system (74CB2537), in agreement with the regioselection rules above. 

Another property of pyrimidines and purines is their strong absorbance of ultraviolet (UV) light, which is also a consequence of the aromaticity of their heterocyclic ring structures. Figure 11.8 shows characteristic absorption spectra of several of the common bases of nucleic acids—adenine, uracil, cytosine, and guanine—in their nucleotide forms AMP, UMP, CMP, and GMP (see Section 11.4). This property is particularly useful in quantitative and qualitative analysis of nucleotides and nucleic acids. 

Lethal drug interactions of new antiviral, sorivudin [l-(3-D-arabinofuranosyl-( )-5-(2-bromvinyl)uracil], with anticancer prodrugs of 5-fluorouracil structure 97YZ910. 

Uracil dimers were studied at the MP2 level using the 6-3IG basis with modified polarization functions. Eleven low-energy minima were located Seven of them are H-bonded, one is T-shaped, and three correspond to various stacked arrangements. The most stable structure was found to be the H-bonded dimer with two Ni-H O2-H bonds (Scheme 85) [98JPC(A)6921]. 

As noted previously, RNA is structurally similar to DNA but contains ribose rather than deoxyribose and uracil rather than thymine. There are three major kinds of RNA, each of which serves a specific function. All three are much smaller molecules than DNA, and all remain single-stranded rather than double-stranded. 

The uracil molecule is one of foe bases in DNA. Estimate foe approximate values of foe indicated bond angles. Its skeleton (not its Lewis structure) is gfven below. 

Attached by a covalent bond to carbon atom 1 of the deoxyribose ring is an amine (and therefore a base), which may be adenine, A (22) guanine, G (23) cytosine, C (24) or thymine, T (25). In RNA, uracil, U (26), replaces thymine. The base bonds to carbon atom 1 of deoxyribose through the nitrogen of the —NH— group (printed in red) and the compound so formed is called a nucleoside. All nucleosides have a similar structure, which we can summarize as the shape shown in (27) the lens-shaped object represents the attached amine. 

The UMP nucleotide contains uracil, ribose, and one phosphate group. The structure of uracil and the hydrogen eliminated during the condensation appear in Figure 13-22. Here are the component parts, drawn in position to eliminate water molecules and link ... 

After the components have been placed next to one another, drawing the final structure requires removal of the H2 O units. UMP has three components. From right to left in the drawing, they are uracil, a single-ring base ribose, the sugar and one phosphate group. 

Although most of an RNA molecule is single-stranded, there often are some double-stranded regions. Intramolecular base pairing between guanine and cytosine and between adenine and uracil creates loops and kinks in the RNA molecule. The structure of one kind of RNA molecule is shown in Figure 13-30. 

Further STM and SXS smdies [Wu et al., 1998] concerning this phenomenon indicated that the presence of specifically and nonspecifically adsorbing anions as well as organic molecules (e.g., pyridine, bipyridine, and uracil) may also lift the reconstructed surface by exhibiting a structural transition, and it has been extensively studied and reviewed in [Kolb, 1996]. 

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