Hydroxamic acid
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The general structure of a hydroxamic acid

A hydroxamic acid is a class of chemical compounds sharing the same functional group in which an amine is inserted into an carboxylic acid. Its general structure is R-CO-NH-OH, with an R as an organic residue, a CO as a carbonyl group, and a hydroxylamine as NH₂-OH. They are used as metal chelators.

Hydroxamic acids can be synthesized from aldehydes via the Angeli-Rimini reaction. A well-known hydroxamic acid reaction is the Lossen rearrangement.

Hydroxamates are essential growth factors, or vitamins, for some microbes. They function as iron-binding compounds (siderophores) that solubilise iron and transport it into the cell.

Iron is a key component of cytochromes and iron-sulphur proteins (involved in electron transport) and is thus important in cellular respiration. In an environment, absent from oxygen (anoxic); iron will be present in the ferrous +2 oxidation state (Fe²⁺), which is water-soluble. Under oxic conditions; iron will be in the ferric +3 oxidation state (Fe³⁺), in insoluble mineral form. The powerful chelating properties of hydroxamic acid and its derivatives is exploited by bacteria to obtain ferric iron. Once the iron-hydroxamate complex has entered the cell, the iron is liberated and the hydroxamic acid can be excreted and reused for iron transport.