Techniques for the characterization of physical gels - Physical Gels from Biological and Synthetic Polymers - page 31

Upper volume 0.2 cm 3

Volume 1 cm 3

Lower volume 0.55 cm 3

(a) (b)

Schematic views of the two types of cells in μDSC III from Setaram (Caluire, France): (a) closed
batch cell; (b) mixing batch cell before and after piston displacement. Reproduced courtesy of
Setaram.

Figure 2.8

2.3.4 Isothermal titration calorimetry (ITC)

Up until now, few results are known in which gelation has been studied using
isothermal titration calorimetry, even though ITC provides a complete thermodynamic
pro

le of molecular interactions, and will increase in importance. Thermodynamic
data reveals the energy contributions which drive molecular interactions and molecular
complex formation: conformational changes, hydrogen bonding, hydrophobic inter-
actions and electrostatic interactions, and how such interactions can be understood at a
molecular level.
Speci c binding and molecular recognition between biological molecules are essential

in biological processes (Blandamer et al., 1998 ;O ' Brien et al., 2001 ). Many proteins

have speci c binding sites for small molecules, including sugars, drugs, metals and

surfactants, referred to as ligands. Knowledge of these interactions is important in
understanding the role of proteins in biological activity. The method is well known to
biochemists, but nowadays it is used for other types of systems which are relevant to the
physical-chemical domain, such as association of amphiphilic polyelectrolytes (Raju
et al., 2001 ), dissociation of copolymer micelles upon dilution below the critical micellar
concentration (Paula et al., 1995 ; Garidel and Hildebrand, 2005 ) and binding of multi-
valent ions to a gelling polysaccharide (Fang et al., 2007).
An ITC instrument is composed, like DSC and μDSC, of two identical cells made of a
good conducting material and surrounded by an adiabatic jacket ( Figure 2.9 ). Very sensitive
thermocouples are used to detect temperature differences between the reference cell, lled

with the solvent (for example, water or buffer), and the titration cell, filled with the

macromolecule in solution. Solutions with well-known concentrations of small molecules