Guanidine hydrochloride (Aminoformamidine Hydrochloride) is a strong organic base existing primarily as guanidium ions at physiological pH. It is found in the urine as a normal product of protein metabolism. It is also used in laboratory research as a protein denaturant.

Guanidine HCl is the most popular protein denaturant. Analysis of unfolding transitions by Guanidine HCl provides several important parameters regarding the mechanism of conformational stability of proteins. [1] Guanidine HCl at low concentrations refolds acid-unfolds apomyoglobin and cytochrome c, stabilizing the molten globule state. Guanidine HCl (> 1 M) causes co-operative unfolding of the molten globule state. [2] Guanidine HCl is 2.8 times more effective than urea in unfolding ribonuclease but only 1.7 times more effective for lysozyme. △GWaterapp. values of Guanidine HCl are 9.7 Cal/mole for ribonuclease at pH 6.6, 6.1 for lysozyme at pH 2.9, 8.3 for α-chymotrypsin at pH 4.3, and 11.7 for β-lactoglobulin at pH 3.2. [3] Guanidine HCl at millimolar concentrations, is able to causes efficient loss of the normally stable [PSI+] element from yeast cells. 5 mM Guanidine HCl in growth media cures [PSI+] and other prions of yeast. [4] 5 mM Guanidine HCl significantly reduces Hsp104-mediated basal and acquired thermotolerance by 30-fold and 50 fold, respectively. Guanidine HCl also reduces the ability of Hsp104 to restore activity of thermally denatured luciferase. [5]