urethanes, are a group of organic compounds sharing a common functional groupwith the general structure -NH(CO)O-. Carbamates are esters of carbamic acid, NH2COOH, an unstable compound. Since carbamic acid contains a nitrogenattached to a carboxylgroup, it is also an amide. Therefore, carbamate esters may have alkylor arylgroups substituted on the nitrogen, or the amide function. For example, urethaneor ethyl carbamate, is unsubstituted, whereas ethyl N–methylcarbamate has a methylgroup attached to the nitrogen (see methyl isocyanatefor formation of N-methylcarbamates).
Carbamates in biochemistry
A nitrogen-substituted carbamic acid is formed when a
carbon dioxidemolecule reacts with the amino terminusof a peptidechain 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
nitrogenatom 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.
ome occurrences of carbamate groups in nature
hemoglobin, carbamate groups are formed when carbon dioxide molecules bond with the amino termini of the globinchains. This helps to stabilize the protein when it becomes deoxyhemoglobin and increases the likelihood of the release of remaining oxygenmolecules 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 lysinecarbamate, which hold the ion in place. The carbamate is formed when an uncharged lysine side-chainnear the ion reacts with a carbon dioxide molecule from the air (not the substrate carbon dioxide molecule), which then renders it charged, and, therefore, able to bind the Mg2+ ion.
Commercial carbamate compounds
A group of
insecticides also contains the carbamate functional group, for example, Aldicarb, Carbofuran, Furadan, Fenoxycarb, Carbaryl, Sevin, Ethienocarb, and 2-(1-Methylpropyl)phenyl N-methylcarbamate. These insecticides can cause cholinesteraseinhibition poisoning by reversibly inactivating the enzyme acetylcholinesterase. The organophosphatepesticides 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 urethaneis not a component of polyurethanes. These polymers have a wide range of properties and are commercially available as foams, elastomers, and solids. Urethaneor ethyl carbamateis occasionally used as a veterinary medicine.
In addition, some carbamates are used in human
pharmacotherapy, for example, the cholinesteraseinhibitors neostigmineand rivastigmine, whose chemical structureis based on the natural alkaloid physostigmine. Other examples are meprobamateand its derivatives, a class of anxiolytic drugs widely used in the 60s before the rise of benzodiazepines, and still used nowadays in some cases.
insect repellent icaridinis a substituted carbamate.
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