Quantitative characterization of the compensating effects of trimethylamine-N-oxide and guanidine hydrochloride on the dissociation of human cyanmethmoglobin.

Dynamic light scattering was used to measure the extent of dissociation of human cyanmethemoglobin (HbCN) α2β2 tetramers into αβ dimers as a function of HbCN concentration in the presence of varying concentrations of guanidine hydrochloride (GuHCl) and trimethylamine-N-oxide (TMAO). It was found that increasing concentrations of GuHCl enhance the dissociation of HbCN, and that GuHCl-induced dissociation is progressively inhibited with increasing concentrations of TMAO. The effects of both cosolutes upon the free energy of HbCN dissociation are shown to be additive. The effect of TMAO on Hb dissociation is largely attributed to steric volume exclusion but is partially compensated by a small attractive interaction between TMAO and the protein.