Carbamate
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Overview
Carbamates or urethanes are a group of organic compounds sharing a common functional group with the general structure -NH(CO)O-. Carbamates are esters of carbamic acid, NH2COOH, an unstable compound. Since carbamic acid contains a nitrogen attached to a carboxyl group it is also an amide. Therefore, carbamate esters may have alkyl or aryl groups substituted on the nitrogen, or the amide function. For example, urethane or ethyl carbamate, is unsubstituted, while ethyl N–methylcarbamate has a methyl group attached to the nitrogen (see methyl isocyanate for formation of N-methylcarbamates).
Carbamates in biochemistry
A nitrogen substituted carbamic acid is formed when a carbon dioxide molecule reacts with the amino terminus of a peptide chain or an amino group of an amino acid, adding a COO− group to it and releasing a cation (H+ ion) to form a carbamate ion.
"R" stands for the atoms attached to the other end of the nitrogen molecule of the amino group. Note that the COO− group is a resonance structure, so the single bonds both show a degree of double bond character, and the charge is delocalised over the two oxygen atoms. This reaction is reversible (with equilibrium constant K << 1 in the above reaction), as the N–C bond is highly labile.
Some occurrences of carbamate groups in nature
In hemoglobin, carbamate groups are formed when carbon dioxide molecules bond with the amino termini of the globin chains. This helps to stabilise the protein when it becomes deoxyhaemoglobin and increases the likelihood of the release of remaining oxygen molecules bound to the protein.
Ribulose 1,5-bisphosphate carboxylase/oxygenase (the enzyme required to fix a carbon dioxide molecule at the start of the Calvin cycle) also requires the formation of a carbamate to function. At the active site of the enzyme, a Mg2+ ion is bound to a glutamate residue, an aspartate residue and a lysine carbamate, which hold the ion in place. The carbamate is formed when an uncharged lysine side chain near the ion reacts with a carbon dioxide molecule from the air (not the substrate carbon dioxide molecule), which then renders it charged, and so therefore able to bind the Mg2+ ion.
Commercial carbamate compounds
A group of insecticides also contain the carbamate functional group for example Aldicarb, Carbofuran, Furadan, Fenoxycarb, Carbaryl, Sevin, Ethienocarb and 2-(1-Methylpropyl)phenyl N-methylcarbamate. These insecticides can cause cholinesterase inhibition poisoning by reversibly inactivating the enzyme acetylcholinesterase. The organophosphate pesticides also inhibit this enzyme, though irreversibly, and cause a more severe form of cholinergic poisoning.
Polyurethanes contain multiple carbamate groups as part of their structure, but urethane is not a component of polyurethanes. These polymers have a wide range of properties and are commercially available as foams, elastomers, and solids.
Urethane or ethyl carbamate is occasionally used as a veterinary medicine.
In addition, some carbamates are used in human pharmacotherapy, for example the cholinesterase inhibitors neostigmine and rivastigmine, whose chemical structure is based on the natural alkaloid physostigmine.
See also