Nucleic acid bases , pyrimidine ring

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. 

These relationships are general Hydroxyl substituted purines and pyrimidines exist in their keto forms ammo substituted ones retain structures with an ammo group on the ring The pyrimidine and punne bases m DNA and RNA listed m Table 28 1 follow this general rule Beginning m Section 28 7 we 11 see how critical it is that we know the cor rect tautomeric forms of the nucleic acid bases... 

Relatively few simple pyrimidine monomers have been incorporated into heterocyclic polymers (70MI11100). By far the greatest efforts with this ring system have involved polymerizable derivatives of nucleic acid bases and related compounds (81MH1102). The majority of this work has involved the synthesis and free radical polymerization of suitable vinyl- and acrylic-functional monomers, e.g. (134)-(136), although epoxy and other derivatives have also been studied. A number of the polymers exhibit base-paired complex formation with natural nucleic acid polymers or synthetic analogues, have found use in... 

Pyrimidines are pyridines with two nitrogen atoms having a 1,3-relationship 66. They occur in nucleic acids as pyrimidine bases such as cytosine 67 and thymine 68 and you will notice that these are pyrimidones. An important new type of anti-cancer drug Glivec 69 has a pyrimidine core with a linked pyridine ring. 

Conformational Flexibility of Pyrimidine Ring in Nucleic Acid Bases... 

An analysis of character and frequencies of normal vibrations and scan of relaxed potential energy surface represents two complementary approaches to investigation of conformational flexibility of pyrimidine rings in nucleic acid bases. A combined application of these approaches allows estimating population of conformations with nonplanar rings for each molecule. 

The most complete picture of conformational flexibility of pyrimidine rings in nucleic acid bases has been provided by molecular dynamics study of isolated molecules using ab initio Carr-Parinello method [45]. According to these studies, the population of planar conformation of heterocycle does not exceed 20% for thymine, cytosine, and guanine and amounts to about 30% for adenine (Table 21.4). These values are considerably smaller as compared to estimations based on vibrational frequencies mentioned above. Such difference is quite natural because in the case of vibrational analysis, only the lowest ring out-of-plane normal mode is considered. However, there are also smaller contributions of the other ring out-of-plane vibrations not included in this analysis. Therefore, such estimation should be considered as an upper limit for assessment of population of planar conformation of ring. 

As discussed in Chapter 3, the nucleic acid bases cytosine, thymine, and uracil contain a pyrimidine ring. Adenine and guanine are based on the purine ring, which includes a pyrimidine ring. Because the five nucleic acid bases contain the pyrimidine ring, perhaps we should not be surprised that pyrimidines are prominent in the pharmaceutically active ingredients used in a variety of therapies including antipsychotic, cholesterol reduction, cancer, erectile dysfunction, antivirals, and human immunodeficiency virus (HIV). 

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