Concise Physical Chemistry

c04 JWBS043-Rogers September 13, 2010 11:24 Printer Name: Yet to Come

DIFFERENTIAL SCANNING CALORIMETRY 67

and it will be little affected by small amounts of dissolved salts. Electrical circuitry

exists that permits one to supply heat to a dilute solution in an adiabatic (insulated)

calorimeter in very small pulses which may be regarded as infinitesimalsdq. We nor-

mally carry out the experiment at constant pressure, so the definition of heat capacity

at constant pressureCp=dqp/dTis satisfied. The gradual temperature rise over

many small pulses can be followed by means of a thermistor circuit or its equivalent.

If, instead of a dilute solution of simple salts, the calorimeter contains a solute

that is capable of undergoing athermal reaction, which is a reaction brought about

by heat, the heating curve is more complicated. Thermal reactions are important

in many areas, especially in biochemistry. Proteins undergo heat denaturation. Heat

denaturation involves unfolding of the native protein and requires breaking of some

or many of the bonds holding it in its native structure. Heat denaturation may be

quite specific as to the temperature at which it occurs, and it may bring about subtle

changes in the protein, like changes in physiological activity, or it may bring about

gross changes in the form of the protein as in the cooking of an egg.

Because heat denaturation involves breaking of internal bonds in the protein, it

requires an enthalpy input at constant pressure. The reaction isendoenthalpic.A

dilute solution of salt and protein takes more heat to bring about a small temperature

change than would the solution without the protein. The difference is observed only

at or near the temperature of denaturation. Thus we have a normal temperature rise

until denaturation begins, after which the heat capacity of the solution is abnormally

large until we achieve complete thermal denaturation whereupon the temperature rise

drops back to the normal baseline of a salt solution. PlottingCpas a function ofT,

we see a peak at the denaturation temperature. This is the upper line in Fig. 4.3. It

is a simple matter to interface a computer to the scanning calorimeter output and to

integrate under the experimental curve:

denH=

∫Tf

Ti

CpdT

Cp

T

FIGURE 4.3 Schematic diagram of the thermal denaturation of a water-soluble protein. The

straight line is the baseline of salt solution without protein. The peak is due to endoenthalpic

denaturation of the protein.