Purines vs pyrimidines

Discovery of Purine and Pyrimidines 

The German Albrecht Kossel (1853–1927), another physician much more interested in physiological chemistry than in medicine, who had as well served as assistant in Hoppe-Seylers’s Institute of Physical Chemistry in Strasbourg from 1877 to 1881, where Miescher had isolated nuclein 10 years earlier, explored the chemistry of nuclein further. Nuclein was still thought to be a phosphorus-rich protein. With Richard Altmann (1852–1900), Kossel isolated a protein-free nuclein showing that Miescher’s nuclein was actually composed of a protein contaminating portion and a non-protein portion. In 1889 Altmann, considering the protein a subcomponent of nuclein of yeast cells, proposed for the deproteinized material the more appropriate term nucleic acid that, by the 1930s, became deoxyribose nucleic acid and then deoxyribonucleic acid (DNA), thus serving to obscure Miescher’s discovery of DNA

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Purines vs Pyrimidines

.Over the next 20 years, Kossel and his research team made other groundbreaking discoveries about the structure of both the nucleic acids and cellular proteins. Using hydrolysis and other techniques to chemically analyze the nucleic acids, he discovered the components adenine, cytosine, guanine, thymine, and uracil, and the fundamental building blocks of nuclein, purine and pyrimidine bases, one sugar and phosphoric acid. He also determined their structures, showing that a pyrimidine has a single six-member ring, while a purine has a six-member ring that shares one side with a five-member ring. He isolated the two purines adenine and guanine and showed that they existed in both ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). He also isolated the three pyrimidines: thymine and cytosine, found in DNA, and uracil (present in RNA instead of thymine). Adenine was composed of carbon, hydrogen, and nitrogen; the others also contain oxygen. 

Purines vs Pyrimidines 
Shape:
 Purines and pyrimidines differ in their shape. The shape of the pyrimidine ring is planar, whereas the shape of the purine rings is nearly planar but exhibits some amount of puckering. 

 Solubility: 
Purine and pyrimidine molecules are hydrophobic in nature and have a relatively low solubility in water near neutral pH. However, at acidic or alkaline pH, the purines and pyrimidines become charged, and their solubility therefore increases. 

Chemical properties: 
They are conjugated molecules and weakly basic in nature. 

Tautomerism: 
Both purines and pyrimidines exhibit keto-enol tautomerism. The keto tautomer is known as a lactam ring, whereas the enol tautomer is known as a lactim ring. At neutral pH, the keto-tautomer remains the more predominanting form. Upon interaction with other molecules, ring nitrogens in the lactam serve as donors of hydrogen bond (H-bond), and the keto oxygens behave as H-bond acceptors. 

 Absorption: 
As a consequence of aromatic ring structure and associated resonance, pyrimidine and purine bases absorb ultraviolet light (UV light), with anabsorption maxima at a wavelength of 260 nm. The measurement of the concentration of DNA or RNA in a given sample is therefore performed by measuring the UV absorbance at this wavelength. 

Base pairing of Purines and Pyrimidines: 
Purines and pyrimidines, being complementary bases, can participate in base pairing, based on the specific shapes and hydrogen bond properties. 

Guanidine, being a complement of cytosine, pairs with cytosine through three hydrogen bonds. Adenine (A) is the complement of thymine (T)  in DNA and uracil (U) in RNA. Adenine base pairs with thymine and uracil through two hydrogen bonds. 

Chargaff’s Rule 
Erwin Chargaff (1905-2002), an Austrian-American biochemist gave the Chargaff's rule, according to which DNA always contains equal amounts of certain base pairs. He observed that the amount of adenine (A) always equaled with the amount of thymine (T), and the amount of guanine (G) always equaled the amount of cytosine (C), regardless of the DNA source.

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