Biomedical Engineering Reference
In-Depth Information
Thermodynamic functions of protein denaturation were determined by least-squares fits of the
excess heat capacity data, using EXAM software as developed by Kirchoff [38].
The thermodynamic parameters of the one component transition for the thermal
unfolding process of BSA as obtained from DSC analysis are presented in Table 1. Each
value presents the average of three experiments and uncertainly represents the standard error
of the mean. The T
m
values have an experimental error of 0.57 and calorimetric entalpy H
cal
values have a maximum expected error of 5% including errors in sample preparation,
calibration constant and reproducibility. The van´t Hoff enthalpy ( ) and the
cooperativity , are also listed in Table 1. T
m
is the thermal midpoint; T
1/2
- the width of the
peak at half-height; H- the area under the peak, which represents the enthalpy of transition
for reversible processes; - the van´t Hoff enthalpy; - cooperativity which is the ratio
of calorimetric enthalpy to van´t Hoff enthalpy; S
cal
-denaturation entropy. The values of T
m
for such system are about 68.5 +/- 0.3
o
C, and H equals 1000-1100 kJ mol
-1
, According to
Michnil ( ) such behaviour is typical for BSA sample containing trace of fatty acids. It has
been shown [39] that the single endotherm is well approximated as the sum of three
independent two-state transitions.It is interesting to note that the endotherm peak for the BSA
solution, and for the BSA(1.0%)-gelatin(1.0%) system appears at temperatures higher than
50
o
C.
Table 1. Thermodynamic parameters obtained from DSC scans of BSA alone and in the
presence of gelatin at pH 5.4
H
vH
H
vH
System
T
m
(
o
C) - (
o
K)
T
(1/2)
H
H
( S
cal
)
J mol
-1
K
-1
( ),
J g
-1
kJ mol
-1
(
),
cal
vH
kJ mol
-1
BSA
68.6 - 341.6
9.1
16.8 - 1122
379.8
2.95
3285
+gelatin
q= 0.4)
68.1 - 341.1
9.1
15.5 - 1035
421.5
2.46
3035
+gelatin
(q=1.0)
68.1 - 341.1
9.4
16.8 - 1122
366.2
3.06
3290
+gelatin
(q=4.0)
68.8 - 341.8
9.2
15.9 - 1062
380
2.79
3107
4.
D
ISCUSSION
4.1. Behaviour of BSA Molecules in Water
Although the behaviour of BSA molecules in aqueous solutions was subject of many
studies, the results from literature differ due to different experimental conditions, composition
of solvent, purity of the protein sample. The calorimetric and spectroscopic information is
very sensitive to minor changes in the above conditions. Because the capacity of all proteins,
in particular BSA, to interact with other polymers depends strongly on the local structural
details of the components in water, we are in the following briefly dealing with the binary
subsystem BSA/water
.
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