The thermal stability of bovine pancreatic ribonuclease (RNase A) has been investigated in the presence of trimethylamine N-oxide (TMAO), a naturally occurring osmolyte, by means of differential scanning calorimetry (DSC) and circular dichroism (CD) measurements at neutral and acid pH conditions. It is well known that compatible osmolytes such as TMAO are effective in stabilizing protein structure and counteracting the denaturing effect of urea and GuHCl. Calorimetric results show that TMAO stabilizes RNAse A at pH 7.0 and does not stabilize the protein at pH 4.0. RNase A thermal denaturation in the presence of TMAO is a reversible two-state N ⇄ D process. We also show that TMAO counteracts the urea and guanidinium hydrochloride (GuHCl) denaturing effect at neutral pH, whereas the counteracting ability is lost at acid pH
