Heterocyclic in the context of Pyridine

Cytosine (symbol C or Cyt) is one of the four nucleotide bases found in DNA and RNA, along with adenine, guanine, and thymine (uracil in RNA). It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached (an amine group at position 4 and a keto group at position 2). The nucleoside of cytosine is cytidine. In Watson–Crick base pairing, it forms three hydrogen bonds with guanine.

Pyrazine is a heterocyclic aromatic organic compound with the chemical formula C₄H₄N₂. It is a symmetrical molecule with point group D_2h. Pyrazine is less basic than pyridine, pyridazine and pyrimidine. It is a "deliquescent crystal or wax-like solid with a pungent, sweet, corn-like, nutty odour".

Pyrazine and a variety of alkylpyrazines are flavor and aroma compounds found in baked and roasted goods. Tetramethylpyrazine (also known as ligustrazine) is reported to scavenge superoxide anions and decrease nitric oxide production in human granulocytes.

Pyridazine is an aromatic, heterocyclic, organic compound with the molecular formula C₄H₄N₂. It contains a six-membered ring with two adjacent nitrogen atoms. It is a colorless liquid with a boiling point of 208 °C. It is isomeric with two other diazine (C₄H₄N₂) rings, pyrimidine and pyrazine.

Chelation (/kiːˈleɪʃən/) is a type of bonding and sequestration of metal atoms. It involves two or more separate dative covalent bonds between a ligand and a single metal atom, thereby forming a ring structure. The ligand is called a chelant, chelator, chelating agent, or sequestering agent. It is usually an organic compound, but this is not a requirement.

The word chelation is derived from Greek χηλή, chēlē, meaning "claw", because the ligand molecule or molecules hold the metal atom like the claws of a crab. The term chelate (/ˈkiːleɪt/) was first applied in 1920 by Sir Gilbert T. Morgan and H. D. K. Drew, who stated: "The adjective chelate, derived from the great claw or chele (Greek) of the crab or other crustaceans, is suggested for the caliperlike groups which function as two associating units and fasten to the central atom so as to produce heterocyclic rings."

Porphine or porphin is an organic compound of empirical formula C₂₀H₁₄N₄. It is heterocyclic and aromatic. The molecule is a flat macrocycle, consisting of four pyrrole-like rings joined by four methine bridges, which makes it the simplest of the tetrapyrroles.

The nonpolar tetrapyrrolic ring structure of porphine means it is poorly soluble in most organic solvents and hardly water soluble. As a result, porphine is mostly of theoretical interest. It has been detected in GC-MS of certain fractions of Piper betle.

Pyrrole is a heterocyclic, aromatic, organic compound, a five-membered ring with the formula C₄H₄NH. It is a colorless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., N-methylpyrrole, C₄H₄NCH₃. Porphobilinogen, a trisubstituted pyrrole, is the biosynthetic precursor to many natural products such as heme.

Pyrroles are components of more complex macrocycles, including the porphyrinogens and products derived therefrom, including porphyrins of heme, the chlorins, bacteriochlorins, and chlorophylls.

Diazepines are a group of seven-membered heterocyclic compounds with two nitrogen atoms.