preventing return to the native state. This is far less likely at low
temperature. Kinetic aspects are discussed in Section 7.2.3.
- Extreme pH. Most proteins are the most stable near their
isoelectric pH. Figure 7.5 gives an example of the stability curve (DGversus
T) at two different pH values. The decreased stability at extreme pH values
must be ascribed to electrostatic repulsion between groups of like charge
and to the impossibility of forming internal salt bridges.
Figure 7.7a shows some relations between pH and denaturation
temperature, which give a clear example of the way in which destabilizing
agents enhance each other’s effects. Varying pH is thus a method of causing
denaturation at a fairly low temperature, where irreversible changes are less
likely to occur, especially at low pH. At high pH, sulfur bridges tend to
break, as mentioned. - Solvent quality. Various solutes added in high concentrations
affect solvent quality and thereby solubility and conformation of macro-
molecules; see Sections 3.2 and 6.2.1. Solutes may thus affect conforma-
tional stability. Relations are not straightforward for proteins, because they
have polar as well as apolar groups, that may be affected in opposite
manner. For salts(ions), the Hofmeister series (Section 3.2) is mostly
obeyed. Examples are in Figure 7.8a. It is seen that the very hydrophilic ions
at the beginning of the series, i.e., ammonium and sulfate, stabilize the
conformation, whereas those at the other end, guanidinium and thiocya-
FIGURE7.7 Combined effects of two variables on conformational stability of
globular proteins. (a) Denaturation (unfolding) temperature as a function of pH for
papain (P), lysozyme (L), cytochrome C (C), parvalbumin (A), and myoglobin (M).
(b) Effect of concentration of guanidinium chloride concentration and temperature
on conformation of lysozyme at pH 1.7. (c) Effect of pressure (1 kbar¼ 108 Pa) and
temperature on conformation of chymotrypsinogen. (d) Effect of pressure and pH on
conformation of myoglobin (20 8 C).