Polycyclic aromatic are hydrocarbons that are composed of multiple aromatic rings. An aromatic ring is an organic ring in which the electrons are delocalized. The simplest such chemicals are naphthalene, having two aromatic rings, and the three-ring compounds anthracene and phenanthrene. A heterocycle is a cyclic compound that contains a heteroatom in the ring. A heteroatom is any atom other than carbon that is present in the ring structure, such as nitrogen, oxygen or sulfur. About two-thirds of organic compounds belong to this class, and they number among them some of the most significant compounds for human beings.
Greek letters are used to describe the carbon atom position which is situated the closest to heteroatom. Alpha (α) would represent the closest, while for the further ones it would be beta (β) and gamma (γ) respectively. Some examples of heterocycles are haemoglobin, bilirubin and porphyrins.
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