Guanidinium

Guanidinium is the positively charged cation [C(NH2)3]+, the conjugate acid of guanidine. It is formed by protonation of guanidine and is stabilized by resonance across the three nitrogen atoms, giving a planar, highly delocalized charge distribution. In aqueous solution the cation remains largely protonated under physiological conditions.

In biology, the guanidinium group is a defining feature of the amino acid arginine. At physiological pH it carries a strong positive charge, contributing to electrostatic interactions, hydrogen bonding, and the stabilization of protein structures and enzyme active sites. The group’s high hydrogen-bond donor ability makes it important in molecular recognition and binding.

Guanidinium forms salts with various anions, such as guanidinium chloride, sulfate, and acetate. Guanidinium chloride, in particular, is widely used as a chaotropic agent to denature proteins by disrupting hydrogen-bond networks. Guanidinium salts are water-soluble and typically highly ionic, reflecting the positive charge of the guanidinium moiety.

Chemically, guanidinium salts are prepared by protonation of guanidine or by metathesis reactions that generate the guanidinium cation. The guanidinium group is a versatile functional motif in organic and medicinal chemistry, used to introduce guanidine functionalities into molecules and to modulate basicity, solubility, and binding properties.

Overall, guanidinium is a small, highly polar cation carrying a localized but resonance-stabilized positive charge, playing a central role in biochemistry and a variety of chemical applications.