Guanidine is a strong chaotropic agent and a strong denaturant of proteins.

Guanidine at low concentrations refolds acid-unfolded apomyoglobin and cytochrome c, stabilizing the molten globule state, i.e. a compact denatured state with a significant amount of secondary structure, but substantially disordered tertiary structure. Guanidine (> 1 M) causes co-operative unfolding of the molten globule state. [1]
Guanidine at millimolar concentrations, is able to causes efficient loss of the normally stable [PSI+] element from yeast cells. 5 mM Guanidine in growth media cures [PSI+] and other prions of yeast. 5 mM Guanidine significantly reduces Hsp104-mediated basal and acquired thermotolerance by 30-fold and 50 fold. [2]

Two injections of Guanidine at 145 mg/kg per injection is significantly reducing in the death rate of infant mice infected with ten LD50 of coxsackievirus A16 is observed when they are treated 58 h after infection. [3]