Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 152

Fig. 3.78 Dependence of the
effective activation energy
on the extent of conversion
determined by an isoconver-
sional method. (Reproduced
from Vyazovkin et al. [ 185 ]
with permission of Wiley)

400

350

300

250

200

150

0.0 .2 0.4 0.6 0.8 1.0

α

Clearly, E ʱ at ʱ ₒ1 (Fig. 3.78 ) should provide an estimate for the activation en-
ergy of the irreversible step. In that region, the respective E ʱ values appear to ap-
proach asymptotically ~ 160 kJ mol −1 . Nevertheless, as stated earlier, most of the
literature E values [ 187 , 189 ] for the thermal denaturation of mammalian tissues are
quite large. For example, a thermal denaturation study [ 192 ] of collagen in water
in the temperature range 57-60 ﾰC reports an activation energy 518 kJ mol −1 . This
value is closer to the E ʱ at ʱ ₒ0 (Fig. 3.78 ) that is associated with the lowest tem-
perature range of the DSC data (Fig. 3.76 ).
E ʱ at ʱ ₒ0 is approximately 370 kJ mol −1 that ideally should serve as an esti-
mate for E + Δ H 0 . The difference between this value and 160 kJ mol −1 , which is an
estimate for the activation energy of the irreversible step, is ~ 210 kJ mol −1 . This
should give an estimate for Δ H 0 . It is difficult to judge how accurate this estimate is
because E ʱ at ʱ ₒ0 does not demonstrate any tendency to plateau at 370 kJ mol −1 .
Recall that the existence of the low-temperature plateau is predicted from the Lum-
ry-Eyring model (Eq. 3.99 and Fig. 3.75 ). Therefore, the actual value of Δ H 0 can be
larger than 210 kJ mol −1 . Nevertheless, this crude estimate fits fairly well within the
range of the literature [ 190 ] Δ H 0 values: 190-430 kJ mol −1 .
The inceptive application [ 185 ] of an isoconversional method to denaturation
of collagen has been followed by a series of isoconversional studies on several
proteins. Several studies [ 193 - 196 ] have been conducted on thermal denaturation
of collagen, including denaturation of dry [ 195 ] and fish [ 196 ] collagen as well as
vitrified collagen gels [ 194 ]. All these studies demonstrated decreasing E ʱ depen-
dencies similar to that shown in Fig. 3.78 . None of these dependencies has shown
a tendency to plateau at small values of ʱ . On the other hand, decreasing E ʱ depen-
dencies with a well-defined plateau at small ʱ values has been reported [ 197 , 198 ]
for the thermal denaturation of the globular protein lysozyme. Also, a plateau in E ʱ
followed by a decreasing dependence has been found [ 199 ] for thermal denaturation
of keratin. On the other hand, for the thermal denaturation of bovine serum albumin,
the isoconversional activation energy remains practically unchanged throughout the
process [ 200 ].