guanidinium

Meaning: The guanidino group, –NH–C(=NH₂⁺)–NH₂ in its protonated form, is the terminal group of arginine's side chain. The positive charge is delocalized by resonance across the central carbon and all three nitrogens, which makes the cation unusually stable and the conjugate base a very poor one. The side-chain pKa is about 12.5, the highest of the standard amino acids. The group is planar and has several N–H hydrogen-bond donors, so it pairs well with negatively charged carboxylates (Asp, Glu) and phosphates.

On the MCAT: Expect arginine to be classed as a basic amino acid, with lysine and histidine. Typical questions ask for its net charge at a given pH, or its pI. The pI is roughly the average of the two highest pKa values (α-amino group and guanidinium), which is about 10.8. Passages also show arginine and lysine residues in histones or DNA-binding proteins, where the positive side chains bind the negative phosphate backbone, or in salt bridges that stabilize tertiary structure. Guanidinium chloride at high concentration is a chaotropic denaturant, like urea.

Common confusion: Students often place the charge on one nitrogen, or think a side chain this basic could be neutral at pH 7. Resonance shares the charge, and with a pKa near 12.5 the group stays protonated well above physiological pH.