Cytosine pyrimidine structure

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]...

Each nucleoside is constructed from two components—a deoxyribose sugar and a base. The sugar is the same in all four nucleosides and only the base is different. The four possible bases are two bicyclic purines (adenine and guanine), and two smaller pyrimidine structures (cytosine and thymine) (Fig. 6.2). 

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. 

The ability of o-QM to form several metastable adducts with pyrimidine (at cytosine N3) and purine bases (at guanine N7 and adenine Nl) in water suggested that the above adducts may be exploited as o-QM carriers under mild conditions, anticipating that o-QM could actually migrate along the structure of an oligonucleotide.35... 

Nucleotides can be linked together into oligonucleotides through a phosphate bridge at the 5 position of one ribose unit and the 3 position of another. The purine bases, adenine and guanine, have two heterocyclic rings, while the pyrimidines cytosine, thymine, and uracil have one. The structure of adenosine monophosphate is shown in Figure 11. 

An introduction to the method of in vitro evolution is given by Wilson and Stoszak (1999). The RNA lipase ribozyme, with about 140 nucleotides (but without the pyrimidine base cytosine), folded in a defined structure and was able to reach a reaction rate 105 times higher than in the uncatalysed reaction. This result certainly surprised those biogenesis researchers who were critical of the RNA world but we do not know whether the result changed their attitude to it ... 

Figure 1.43 indicates major sites of reactivity within the ring structures for nucleophilic displacement reactions. Cytosine, thymine, and uracil all react toward nucleophilic attack at the same two sites, the C-4 and C-6 positions. The presence of powerful nucleophiles, even at neutral pH, can lead to significant base modification or cleavage with pyrimidine residues (Debye, 1947). For instance, hydrazine spontaneously adds to the 5,6-double bond, initiating further ring reactions,... 

As in the case of pyrimidine bases discussed previously, adenine and guanine are subject to nucleophilic displacement reactions at particular sites on their ring structures (Figure 1.50). Both compounds are reactive with nucleophiles at C-2, C-6, and C-8, with C-8 being the most common target for modification. However, the purines are much less reactive to nucleophiles than the pyrimidines. Hydrazine, hydroxylamine, and bisulfite—all important reactive species with cytosine, thymine, and uracil—are almost unreactive with guanine and adenine. 

There are five common bases found in nucleic acids. Adenine (A), guanine (G) and cytosine (C) are found in both DNA and RNA. Uracil (U) is found only in RNA and thymine (T) only in DNA. The structures of these bases are shown in Figure 13.2. Adenine and guanine are purine bases while uracil, thymine and cytosine are the pyrimidine bases. 

Pyrimidines A family of 6-membered heterocyclic compounds occurring in nature in a wide variety of forms. They include several nudeic add constituents (cytosine, thymine, and uracil) and form the basic structure of the barbiturates. [NIH]... 

The nitrogen-containing bases that occur in DNA and RNA fall into two structural categories the purines and the pyrimidines. The former contain a five-membered ring fused to a six-membered ring, while the latter contain a six-membered ring only. The two purines are common to both DNA and RNA adenine (A) and guanine (G). The pyrimidine cytosine (C) occurs in both DNA and RNA. The other pyrimidine is thymine (T) in DNA but uracil (U) in RNA. 

Additionally, nucleic acid bases have been used in the dynamic assembly of mixed-metal, mixed-pyrimidine metallacalix[n]arenes [47]. In this approach, Lippert and coworkers investigated the dynamic assembly of metallacalixarenes based on platinum (Pt ), palladium (Pd°), uracil, and cytosine assemblies with mixed amines. These combinations form cyclic metallacalix[n]arenes structures with n = A and = 8. Of the metallacalix[4]arenes, compounds were formed with five, six, and eight bonded metals, and a variety of nucleobase coimecfivities (UCUC and UCCU). The dynamic nature of this assembly allows access to novel and structurally diverse set of nucleobase metallacalixarenes. 

The first evidence of the special structure of DNA was the observation that the amounts of adenine and thymine are almost equal in every type of DNA. The same applies to guanine and cytosine. The model of DNA structure formulated in 1953 explains these constant base ratios intact DNA consists of two polydeoxynucleotide molecules ( strands ). Each base in one strand is linked to a complementary base in the other strand by H-bonds. Adenine is complementary to thymine, and guanine is complementary to cytosine. One purine base and one pyrimidine base are thus involved in each base pair. 

Cytosine, thymine, and uracil are pyrimidines along with adenine and guanine they account for the five nucleic acid bases. Pyrimidines are heterocyclic single-ringed compounds based on the structure of pyrimidine. Cytosine, thymine, and uracil, like adenine and guanine, form nucleosides and nucleotides in RNA and DNA. When the bases combine with ribose, a ribo-nucleoside forms and when it attaches to deoxyribose, a deoxyribosenucleoside is formed. Names of the nucleoside are summarized in Table 29.1. These in turn combine with phospho-ryl groups, in a process called phosphorylation, to form their respective nucleotides that form nucleic acids. The nucleotides can be tri, di, and mono phosphate nucleotides similar to the way in which adenine forms ATP, ADP, and AMP. 

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