|Jmol-3D images||Image 1
|Molar mass||59.07 g mol−1|
|Melting point||50 °C (122 °F; 323 K)|
Std enthalpy of
|−57–−55 kJ mol−1|
Std enthalpy of
|−1.0511–−1.0531 MJ mol−1|
|LD50||475 mg/kg (oral, rat)|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Guanidine is the compound with the formula HNC(NH2)2. It is a colourless solid that dissolves in polar solvents. It is a strong base that is used in the production of plastics and explosives. It is found in urine as a normal product of protein metabolism. Guanidine is the functional group on the side chain of arginine.
Guanidine can be thought of as a nitrogenous analogue of carbonic acid functional group. That is, the C=O group in carbonic acid is replaced by a C=NH group, and each OH is replaced by a NH2 group. A detailed crystallographic analysis of guanidine was elucidated 148 years after its first synthesis, despite the simplicity of the molecule. In the year 2013, the positions of the hydrogen atoms and their displacement parameters were accurately determined using single-crystal neutron diffraction.
The molecule was first synthesized in 1861 by the oxidative degradation of an aromatic natural product, guanine, isolated from Peruvian guano. The commercial route involves a two step process starting with the reaction of dicyandiamide with ammonium salts. Via the intermediacy of biguanidine, this ammonolysis step affords salts of the guanidinium cation. In the second step, the salt is treated with base, such as sodium methoxide.
Guanidine is protonated in physiological conditions. This conjugate acid is called the guanidinium cation, [CH6N3]+. The guanidinium cation has a charge of +1. It is a highly stable cation in aqueous solution due to the efficient resonance stabilization of the charge and efficient solvation by water molecules. As a result, its pKa is 13.6 meaning that guanidine is a very strong base in water.
Notable guanidinium salts include guanidinium chloride (GndCl), which has chaotropic properties and is used to denature proteins. Guanidine hydrochloride is known to denature proteins with a linear relationship between concentration and free energy of unfolding. Another such salt is guanidinium thiocyanate; creatine is a derivative of the guanidinium cation.
Guanidines are a group of organic compounds sharing a common functional group with the general structure (R1R2N)(R3R4N)C=N-R5. The central bond within this group is that of an imine, and the group is related structurally to amidines and ureas. Examples of guanidines are arginine, triazabicyclodecene, and saxitoxin. Another derivative is guanidinium hydroxide, the active ingredient in some non-lye hair relaxers. Guanidinium salts are well known for their denaturing action on proteins; guanidinium chloride is one of the most effective denaturants. In 6 M aqueous GndHCl, almost all proteins lose their ordered "secondary structure" (that results from intramolecular noncovalent interactions) and become "randomly coiled"; that is, their secondary structure interactions are disrupted by the dissolved guanidinium, leaving only the primary covalent structure of their polyamide backbones. Guanidine hydrochloride is used as an adjuvant in treatment of botulism, introduced in 1968, but now its role is considered controversial - because in some patients there was no improvement after this drug administration.
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