guanidino

Guanidino refers to the functional group derived from guanidine, typically represented as R-NHC(=NH)NH2, in which the guanidino unit is attached to a carbon skeleton through a nitrogen. The guanidino group is characterized by a central carbon bonded to two amino groups and an imino group, with resonance that distributes positive charge when protonated. The conjugate acid (guanidinium) is highly basic, with a pKa around 12–13, and exists as a positively charged group under physiological conditions.

In biology, guanidino groups occur prominently in the side chain of the amino acid arginine, and in metabolites such as guanidinoacetate and creatine. The group enables strong hydrogen bonding and salt-bridge interactions, contributing to protein structure, enzyme catalysis, and molecular recognition. Its high basicity also affects membrane transport and binding properties of guanidine-containing compounds.

Synthesis and applications: In organic synthesis, guanidinylation converts amines into guanidino groups using reagents such as cyanamide, carbodiimides, or S-methylisothiourea. Guanidine and guanidino derivatives are widespread in pharmaceuticals, natural products, and biochemical probes, with arginine-rich sequences used in nucleic acid recognition and metabolic pathways involving creatine.

Etymology: The name guanidine, and by extension guanidino, derives from the compound guanidine, first isolated from guano; the term reflects its historical origin.