Purine and Pyrimidine Structures

The pyrimidine bases have a 6-membered ring with two nitrogens and four carbons. 

The purine bases have a 9-membered double-ring system with four nitrogens and five carbons. 

Although both purine and pyrimidine rings have one 6-mem-bered component with two nitrogens and four carbons, the purines and pyrimidnes are not related metabolically. Distinct pathways for purine biosynthesis and degradation and for pyrimidine biosynthesis and degradation, exist in all organisms. 

The combination of a 5-membered carbohydrate ring and a purine or pyrimidine is called a nucleoside. The rings are numbered as shown in the following figure. The two rings of a nucleoside or nucleotide must be distinguished from each other, so the positions of the sugar carbons are denoted with a (prime) notation. If one or more phosphates exist on the carbohydrate, the combination is called a nucleotide. For example, ATP is a nucleotide. 

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In other words, a synthetic route to polypeptides which requires only HCN and water is presented preformed a-amino acids are not necessary. According to Matthews, the pyrolysis of cyanide polymers can give nitrogen-containing heterocycles with purine and pyrimidine structures in other words, the HCN world ... 

Nucleic acids are acidic substances present m the nuclei of cells and were known long before anyone suspected they were the primary substances involved m the storage transmission and processing of genetic information There are two kinds of nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) Both are complicated biopolymers based on three structural units a carbohydrate a phosphate ester linkage between carbohydrates and a heterocyclic aromatic compound The heterocyclic aro matic compounds are referred to as purine and pyrimidine bases We 11 begin with them and follow the structural thread... 

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... 

The DNA double heUx illustrates the contribution of multiple forces to the structure of biomolecules. While each individual DNA strand is held together by covalent bonds, the two strands of the helix are held together exclusively by noncovalent interactions. These noncovalent interactions include hydrogen bonds between nucleotide bases (Watson-Crick base pairing) and van der Waals interactions between the stacked purine and pyrimidine bases. The hehx presents the charged phosphate groups and polar ribose sugars of... 

The double-stranded structure of DNA can be separated into two component strands (melted) in solution by increasing the temperature or decreasing the salt concentration. Not only do the two stacks of bases puU apart but the bases themselves unstack while still connected in the polymer by the phosphodiester backbone. Concomitant with this denaturation of the DNA molecule is an increase in the optical absorbance of the purine and pyrimidine bases—a phenomenon referred to as hyperchromicity of denaturation. Because of the... 

P5N has two isozymes, P5N-I (pyrimidine nucleotidase) and P5N-II (deoxyri-bonucleotidase) (H6, P2). P5N-I is active principally with pyrimidine substrates at an optimal neutral pH P5N-II activity occurs with both purine and pyrimidine substrates and was maximal with deoxy analogues at an acidic pH optimum. This enzyme was partially purified from human red blood cells and had a molecular weight of 28,000 (T19). The primary structures of both isozymes have not been... 

Acid and base extractions from this material have been shown to form spontaneous structures in solution termed coercevates that could easily form the basis for protypical membranes (more of this in Chapter 9). Hydrocarbons with chain lengths C15-C30 (both straight and branched chains) and of course PAHs, predominantly pyrene and fluoranthrene, polar hydrocarbons such as aromatic ketones, alkyl and aryl ketones, nitrogen and sulphur heterocycles and most intriguingly purine and pyrimidine analogues have all been observed from this rich carbonaceous cocktail of compounds. Why ... 

Basic to molecular biology is the concept that DNA and RNA are macromolecules that convey information. The sequence of purine and pyrimidine bases in DNA encodes all the information needed to form and direct the chemical reactions within cells. This information thus encodes all catalytic, regulatory, and structural proteins contained within an organism. The flow of information from DNA to RNA to proteins is... 

More than 50% of all known organic compounds are heterocyclic compounds. They play important roles in medicine and biological systems. A great majority of important drugs and natural products, e.g. caffeine, nicotine, morphine, penicillins and cephalosporins, are heterocyclic compounds. The purine and pyrimidine bases, two nitrogenous heterocyclic compounds, are structural units of RNA and DNA. Serotonin, a neurotransmitter found in our body, is responsible for various bodily functions. 

Anti metabolites are structural analogues of normal cellular constituents that interfere with the synthesis of purine and pyrimidine, which are essential for DNA synthesis and cell division. They are used in the treatment of gastrointestinal and pulmonary carcinomas as well as sarcomas and some leukaemias. The major toxic effects are bone marrow suppression, mucositis, severe diarrhoea, nausea and vomiting. They have been associated with acute and chronic hepatotoxicity. 

Free pyrimidines and purines are weakly basic compounds and are thus called bases. They have a variety of chemical properties that affect the structure, and ultimately the function, of nucleic acids. The purines and pyrimidines common in DNA and RNA are highly conjugated molecules (Fig. 8-2), a property with important consequences for the structure, electron distribution, and fight absorption of nucleic acids. Resonance among atoms in the ring gives most of the bonds partial double-bond character. One result is that pyrimidines are planar molecules purines are very nearly... 

The purine and pyrimidine bases are hydrophobic and relatively insoluble in water at the near-neutral pH of the cell. At acidic or alkaline pH the bases become charged and their solubility in water increases. Hydrophobic stacking interactions in which two or more bases are positioned with the planes of their rings parallel (like a stack of coins) are one of two important modes of interaction between bases in nucleic acids. The stacking also involves a combination of van der Waals and dipole-dipole interactions between the bases. Base stacking helps to minimize contact of the bases with water, and base-stacking interactions are very important in stabilizing the three-dimensional structure of nucleic acids, as described later. 

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