nate, have a strong destabilizing effect, acting even at room temperature
(Figure 7.7b).
Manyneutralsolutes also have a distinct effect: e.g., Figure 7.8b. Well
known is the destabilizing effect of urea at high concentration, similar to
that of guanidinium chloride. Urea makes strong H-bonds with water,
undoubtedly altering water structure, but the explanation of its denaturing
effect is not quite clear. It appears to bind to peptide bonds, thereby
dehydrating them. Solutes like ethanol, that are far less polar than water but
are nevertheless readily soluble in water, tend to be destabilizing. On the one
hand they strengthen H-bonds and salt bridges (because of the lower
dielectric constant), but on the other they strongly weaken hydrophobic
FIGURE7.8 Effect of concentration of (a) various salts (Gu¼guanidinium) and
(b) various organic solutes on the denaturation (unfolding) temperature of
ribonuclease A. In (b) water activityawis used as the independent variable, rather
than molar concentration; (1aw) is about proportional to the mole fraction of
solute. (After various sources, mainly von Hippel and Wong, J. Biol. Chem. 240
(1965) 3909.)