Guanidine

Guanidine is a crystalline compound of strong alkalinity formed by the oxidation of guanine. It is used in the manufacture of plastics and explosives. It is found in urine as a normal product of protein metabolism. The molecule was first synthesized in 1861 by the oxidative degradation of an aromatic natural product, guanine, isolated from Peruvian guano.^[2] Despite the simplicity of the molecule, the crystal structure was first described 148 years later.^[3]

Guanidine is probably most easily recognized as the functional group of arginine and its structure can be thought of as a nitrogenous analogue of the carbonate functional group.

Guanidinium cation

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 pK_a is 13.6^[1] 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.

Guanidine derivatives

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.

See also

• Guanidinium thiocyanate
• Guanidine nitrate

References

1. ^ ^{a b} Perrin, D.D., Dissociation Constants of Organic Bases in Aqueous Solution, Butterworths, London, 1965; Supplement, 1972.
2. ^ A. Strecker, Liebigs Ann. Chem. 1861, 118, 151.
3. ^ T. Yamada, X. Liu, U. Englert, H. Yamane, R. Dronskowski, Chem. Eur. J. 2009, 15, 5651.