Six–membered heterocycles with one heteroatom – group of pyridines
The group of aromatic six-membered heterocycles with two nitrogen atoms are called diazines. The three isomeric diazines are pyridazine, pyrimidine, and pyrazine.
Pyrimidine.
Pyrimidine nucleus is found in many biologically important compounds. Pyrimidine is a six-membered aromatic heterocycle. Its electronic structure is similar to that of benzene and pyridine. The distinguishing feature of this structure is -electron sextet. Both nitrogen atoms are sp2 hybridized.
Pyrimidine derivatives.
Pyrimidine and purine, described later, are heterocyclic fundamental structural components of a major class of biomolecules - the nucleic acids. The nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are the chemical carriers of a cell’s genetic information.
If the polymer contains identical monomeric units, it is a homopolymer.
When the polymer is constructed by various monomeric units, it is a heteropolymer.
The pyrimidine bases are cytosine, uracil, and thymine.
Cytosine is a pyrimidine substituted with an amino group at position C(4) and a keto group at position C(2) (at neutral pH). Cytidine triphosphate (CTP), it can act as a co -factor to enzymes and can transfer a phosphate in conversion of adenosine diphosphate (ADP) into adenosine triphosphate (ATP). In DNA and RNA, cytosine is paired with guanine using three hydrogen bonds.
Uracil has pyrimidine core substituted with two keto groups at C (2) and C (4). All three of the tautomeric structures shown are aromatic. Uracil is found in RNA only. Its base pairs with adenine using two hydrogen bonds (dashed lines) and replaces thymine during DNA transcription
Thymine is one of the four nucleobases in DNA. Thymine is like uracil having one additional methyl group at C (5), therefore sometimes referred to as a 5 -methyluracil. The most stable in physiological conditions is the bis -lactam tautomer with pyrimidine core substituted with two keto groups at C(2) and C(4).All three tautomeric forms are Aromatic systems. Thymine pairs with adenine through two hydrogen bonds (dashed lines) which stabilize the nucleic acid structure
Pyrimidine nucleosides and nucleotides.
Nucleosides are glycosylamines consisting of a nucleobase (often referred to as simply base) bound to a ribose or deoxyribose sugar via a beta -glycosidic linkage. Pyrimidine nucleosides are cytidine, uridine, and thymidine.A nucleotide is composed of a nucleobase, a five-carbon sugar (either ribose or 2'-deoxyribose), and one to three phosphate groups.