Biology Reference
In-Depth Information
CHCL
0mol%
1mol%
5mol%
10mol%
20
25
30
35
40
T (˚C)
45
50
55
60
FIGURE 9.24
The effect of increasing levels of cholesterol on the main transition of DPPC as followed by DSC.
The main transition shifts to lower temperature and exhibits a decrease in enthalpy (decrease in cooperativity) with
increasing cholesterol. Courtesy of Alfred C. Dumaual.
and by 10 mol% cholesterol, most of the transition has disappeared. By 30 mol% cholesterol,
the DPPC main transition is completely obliterated. From the shape of the curve, one can esti-
mate the number of lipids melting as a unit. This empirical number, referred to as the 'coop-
erativity unit,' can be simply calculated from the DSC curve, as first described by Susan
Mabrey and Julian Sturtevant in the mid 1970s
[37]
and nicely summarized in 1986 by Donald
Small
[39]
. The cooperativity unit is defined as:
H
vH
=D
H
cal
Cooperativity Unit
¼ D
H
cal
is the effective enthalpy which is the
calculated area beneath the curve of the thermal transition expressed in cal/mol/gram. The
van't Hoff enthalpy is defined by Julian Sturtevant as:
H
vH
is the van't Hoff enthalpy and
Where
D
D
2
where T
m
is the midpoint of the transition (
K) and the expression (T
2
T
m
2
=
T
H
vH
¼
6
:
9
D
2
T
1
T
1
)/2 is the peak
width at one half the peak height. T
1
is the onset temperature and T
2
is the final (offset)
temperature of the transition, both expressed in
K.
