Physical Chemistry of Foods

7.2.1 Thermodynamic Considerations

We will first consider fairly ideal cases, implying globular protein molecules
containing one domain, and conditions that do not cause appreciable
changes other than unfolding. Figure 7.4 gives examples of the change from
the native to the unfolded conformation (and vice versa), induced in various
ways. The change can be monitored from the change in hydrodynamic
properties (e.g., viscosity) or in spectroscopic properties (e.g., optical
rotation) and expressed as the fraction changed; it is generally observed that
various methods give (almost) identical results. It is also observed that the
changes are reversible and occur quickly (time scale of the order of a
second). This all means a dynamic equilibrium of two reactions: unfolding,
N?U, reaction constantkU; and (re)folding, N?D, reaction constantkF.
We may rewrite Eq. [4.6] as

DN?UG

RT

¼

DN?UH

RT

DN?US

R

¼ln

½NŠ

½UŠ



¼lnK ð 7 : 3 Þ

where K¼kF=kUis the equilibrium constant. An example ofDGas a
function of temperature is shown in Figure 7.5. It is seen that there are two
temperatures at which DG¼0, implying½NŠ¼½UŠ; these are called the
denaturation temperatures. DG may be considered a measure of the

FIGURE7.4 Transition of proteins from the native to the unfolded state or vice

versa. (a) Ribonuclease at pH 3.15, as a function of temperature. (b) Lysozyme as a

function of guanidinium chloride concentration. (c) Nuclease A as a function of pH.