
heat capacity of the denatured protein and the calculated one
for the unfolded polypeptide chain is a strong argument that
heat denatured proteins are indistinguishable thermodynam-
ically from fully unfolded (D U).
8.2 PARTIAL VOLUME
The partial specific volume of proteins can be measured
experimentally [11,12] or can be estimated from the amino
acid composition of the protein using equation
V
2
¼ (N 1) V
2
(-----CHCONH-----) þ
X
N
i¼1
V
2
( ----- R
i
), (8:2)
where N is number of amino acid residues in the protein
sequence, V
2
(-----CHCONH-----) is the contribution of a peptide
unit, and V
2
( ----- R
i
) is the contribution of the ith amino acid
side chain. These contributions are listed in Table 8.6. For
more details, compilation of the experimental data on the
partial volume of proteins is recommended [12].
8.3 THERMODYNAMIC FUNCTIONS
Because the absolute value of the heat capacity of the
native state is always larger than the heat capacity of the
unfolded state, and because they have different temperature
dependencies, the heat capacity change upon unfolding is
positive and temperature dependent itself:
D
U
N
C
p
(T) ¼ C
U
p
(T) C
N
p
(T), (8:3)
where D
U
N
C
p
(T) is a nonlinear function of temperature.
It has maximum at around 50 8C, decreases at high temper-
atures and appears to reach zero at 140 8C). The heat
capacities of proteins in the native and unfolded states and
the heat capacity change upon unfolding are presented in
Table 8.7.
TABLE 8.1b. Continued.
T/K C
p
T/K C
p
T/K C
p
L-Tyrosine (181.2 Da) [27]
10 1.389 90 75.06 220 162.72
15 3.908 100 82.05 230 169.45
20 7.724 110 88.83 240 176.23
25 12.57 120 95.56 250 183.05
30 18.07 130 102.26 260 189.91
35 23.91 140 108.87 270 196.86
40 29.62 150 115.52 280 203.84
45 35.24 160 122.26 290 210.79
50 40.59 170 128.99 300 217.74
55 45.56 180 135.77 310 224.60
60 50.29 190 142.38 273.15 199.08
70 59.20 200 149.12 298.15 216.44
80 67.49 210 155.81 310.15 224.72
L-Valine (117.2 Da) [24]
10 1.100 90 64.73 220 133.01
15 3.192 100 71.34 230 137.57
20 6.615 110 77.57 240 142.05
25 10.69 120 83.55 250 146.61
30 15.18 130 89.16 260 151.17
35 19.86 140 94.56 270 155.85
40 24.46 150 99.83 280 160.50
45 29.04 160 104.98 290 165.06
50 33.50 170 109.91 300 169.70
55 37.87 180 114.73 310 174.31
60 42.09 190 119.37 273.15 157.28
70 50.04 200 123.89 298.15 168.82
80 57.61 210 128.41 310.15 174.39
THERMODYNAMIC PROPERTIES OF PROTEINS / 111