1- cytosine

The structures and energies of the tautomers of the bases of the nucleic acids have been subject to numerous computational studies. Since DNA is found in the aqueous environment of the cell interior, it is essential that the base tautomers, and the base pairs, be computed in the aqueous phase. In this section, we discuss the structures of the tautomers of each of the four DNA bases and uracil, found in RNA, focusing on the differences in the gas and solution phases. We then take on the structure of the base pairs in solution. 

In this section, we present the results of computational studies of the five nucleic acid bases cytosine 13, guanine 14, adenine 15, thymine 16, and uracil 17. The canonical structures, those that are involved in the Watson-Crick base pairing within DNA, are drawn below. Other tautomers for each base can be energetically competitive with the canonical structure, and these other tautomers are invoked in some models of DNA mutations and anomalous DNA structures. The ensuing discussion focuses on the relative energies of the tautomers, in both the gas and solution phases. Structural changes that accompany this phase change are also noted. 

The amino groups of 13a-c are not planar. The inversion barrier at RI-MP2/TZVPP is 0.2, 0.2, and 0.4 kcal mol for 13a-c, respectively. For each tautomer, the sum of the angles about the amine nitrogen is about 350°. 

All of the nucleic acid bases have multiple sites for hydrogen bond formation. Explicit microsolvation may be appropriate for describing the important interactions between the base and its first solvation shell. The complicating factor, as is always true when attempting to carry out microsolvation studies, is how many solvent molecules to place about the substrate and what configurations should be sampled. For the nucleic acid bases, the many tautomeric forms available further add to the number of potential geometry optimizations that need to be performed. 

TABLE 7.12 Relative Energies (in kcal mol of Cytosine Tautomers (13a-h) 

Excitation energies and ionization potentials are also tautomer-dependent properties [51]. This property can be seen in the observation of different photoionization curves for guanine resulting from different types of molecular beams, presumably reflecting different tautomer distributions [52]. 

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It is the parent substance of a group of compounds which includes cytosine, thymine and uracil, which are constituents of nucleic acids and barbituric acid and its derivatives, which are important medicinally. 

The DNA base pairs guanine (G), cytosine (C), adenine (A) and thymine (T). The uracil-2,6-diaminopyridine pair can also form three hydrogen bonds but has a much lower association constant than G-C. 

A,G,CT, (U) Adenine, guanine, cytosine, thymine - the four bases present in DNA. Urac replaces thymine in RNA... 

Write a structural formula for the enol tautomer of cytosine... 

Three of the bases (cytosine adenine and guanine) occur m both RNA and DNA... 

Avery s paper prompted other biochemists to rethink their ideas about DNA One of them Erwin Chargaff of Columbia University soon discovered that the distribution of adenine thymine cytosine and guanine differed from species to species but was the same within a species and within all the cells of a species Perhaps DNA did have the capacity to carry genetic information after all Chargaff also found that regardless of the source of the DNA half the bases were purines and the other half were pyrimidines Significantly the ratio of the purine adenine (A) to the pyrimidine thymine (T) was always close to 1 1 Likewise the ratio of the purine guanine (G) to the pyrimidine cyto sine (C) was also close to 1 1 For human DNA the values are... 

Base pair (Section 28 7) Term given to the punne of a nu cleotide and its complementary pyrimidine Adenine (A) is complementary to thymine (T) and guanine (G) is comple mentary to cytosine (C)... 

The primary stmcture of DNA is based on repeating nucleotide units, where each nucleotide is made up of the sugar, ie, 2 -deoxyribose, a phosphate, and a heterocycHc base, N. The most common DNA bases are the purines, adenine (A) and guanine (G), and the pyrimidines, thymine (T) and cytosine (C) (Fig. 1). The base, N, is bound at the I -position of the ribose unit through a heterocycHc nitrogen. 

The exocychc nitrogens on cytosine and the purine bases must be protected during the synthesis. The search is ongoing for protecting groups that are subject to fewer side reactions and that can be removed more easily in the final deprotection step (34). 

Chemotherapeutic agents are grouped by cytotoxic mechanism. The alkylating agents, such as cyclophosphamide [50-18-0] and melphalan [148-82-3] interfere with normal cellular activity by alkylation deoxyribonucleic acid (DNA). Antimetabohtes, interfering with complex metaboHc pathways in the cell, include methotrexate [59-05-2] 5-fluorouracil [51-21-8] and cytosine arabinoside hydrochloride [69-74-9]. Antibiotics such as bleomycin [11056-06-7] and doxombicin [23214-92-8] h.a.ve been used, as have the plant alkaloids vincristine [57-22-7] and vinblastine [865-21-4]. 

Fosfadecin (186) and fosfocytocin (187) are adenine and cytosine nucleotide antibiotics isolated from the culture filtrates of Pseudomonas viridiflava PK-5 and P. fluorescens PK-52, respectively (283). Hydrolysis produces fosfoxacin which is also isolated from the culture filtrates. Compounds (186) and (187) inhibit gram-positive and gram-negative bacteria. 

With the aid of cytosine permease, flucytosine reaches the fungal cell where it is converted by cytosine deaminase into 5-fluorouracil [51-21-8]. Cytosine deaminase is not present in the host, which explains the low toxicity of 5-FC. 5-Fluorouracil is then phosphorylated and incorporated into RNA and may also be converted into 5-fluorodeoxyuridine monophosphate, which is a potent and specific inhibitor of thymidylate synthetase. As a result, no more thymidine nucleotides are formed, which in turn leads to a disturbance of the DNA-synthesis. These effects produce an inhibition of the protein synthesis and cell repHcation (1,23,24). 5-Fluorouracil caimot be used as an antimycotic. It is poorly absorbed by the fungus to begin with and is also toxic for mammalian cells. 

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