So far, we have only considered the way that molecular interactions influence the spatial distribution of molecules in a system. Molecular interactions can also determine the three-dimensional conformation and flexibility of individual molecules (Lehninger et al. 1993, Atkins 1994, Gelin 1994). Small molecules, such as H2O and CH4, normally exist in a single conformation which is determined by the relatively strong covalent bonds that hold the atoms together (Karplus and Porter 1970, Atkins 1994). On the other hand, many larger molecules can exist in a number of different conformations because of the possibility of rotation around saturated covalent bonds (e.g., proteins and polysaccharides) (Baianu 1992, Bergethon and Simons 1990, Lehninger et al. 1993, Fennema 1996a). A macromolecule will tend to adopt the conformation that has the lowest free energy under the prevailing environmental conditions (Alber 1989). The conformational free energy of a molecule is determined by the interaction energies and entropy of the system that contains it (Dill 1990). The molecular interactions may be between different parts of the same molecule (intramolecular) or between the molecule and its neighbors (intermolecular). Similarly, the entropy is determined by the number of conformations that the molecule can adopt, as well as by any changes in the entropy caused by interactions with its neighbors (e.g., restriction of their translational or rotational motion) (Alber 1989, Dill 1990).
To highlight the importance of molecular interactions and entropy in determining the conformation of molecules in solution, it is useful to examine a specific example. Consider a hydrophilic biopolymer molecule in an aqueous solution that can exist in either a helical or a random-coil conformation depending on the environmental conditions (Figure 2.8). Many types of food biopolymers are capable of undergoing this type of transformation, including the protein gelatin (Walstra 1996b) and the polysaccharide xanthan (BeMiller and Whistler 1996). The free energy associated with the transition (helix ^ coil) between these two different conformations is given by:
FIGURE 2.8 The conformation of a molecule in solution is governed by a balance of interaction energies and entropic effects. A helical molecule unfolds when it is heated above a certain temperature because the random-coil conformation is entropically more favorable than the helical conformation.
where AGh^c, AEh^c, and ASh^c are the free energy, interaction energy, and entropy changes associated with the helix-to-coil transformation. If AGh^c is negative, the random-coil conformation is favored; if AGh^c is positive, the helix conformation is favored; and if AGh^c ~ 0, the molecule spends part of its time in each of the conformations. A helical conformation often allows a molecule to maximize the number of energetically favorable intermolecular and intramolecular interactions while minimizing the number of energetically unfavorable ones (Bergethon and Simons 1990, Dickinson and McClements 1995). Nevertheless, it has a much lower entropy than the random-coil state because the molecule can only exist in a single conformation, whereas in the random-coil state the molecule can exist in a large number of different conformations that have similar low energies. At low temperatures, the interaction energy term dominates the entropy term and so the molecule tends to exist as a helix, but as the temperature is raised, the entropy term (-TA5h^c) becomes increasingly important until eventually it dominates and the molecule unfolds. The temperature at which the helix-to-coil transformation takes place is referred to as the transition temperature (Th^c), which occurs when AGh^c = 0. Similar arguments can be used to account for the unfolding of globular proteins when they are heated above a particular temperature, although the relative contribution of the various types of interaction energy is different (Dickinson and McClements 1995). It must be stressed that many food molecules are unable to adopt their thermodynamically most stable conformation because of the presence of various kinetic energy barriers (Section 1.2.1). When an energy barrier is much greater than the thermal energy of the system, a molecule may be "trapped" in a metastable state indefinitely.
The flexibility of molecules in solution is also governed by both thermodynamic and kinetic factors. Thermodynamically, a flexible molecule must be able to exist in a number of conformations that have fairly similar (±kT) low free energies. Kinetically, the energy barriers that separate these energy states must be small compared to the thermal energy of the system. When both of these criteria are met, a molecule will rapidly move between a number of different configurations and therefore be highly flexible. If the free energy difference between the conformations is large compared to the thermal energy, the molecule will tend to exist predominantly in the minimum free energy state (unless it is locked into a metastable state by the presence of a large kinetic energy barrier).
Knowledge of the conformation and flexibility of a macromolecule under a particular set of environmental conditions is particularly important in understanding and predicting the behavior of many ingredients in food emulsions. The conformation and flexibility of a molecule determine its chemical reactivity, catalytic activity, intermolecular interactions, and functional properties (e.g., solubility, dispersability, water-holding capacity, gelation, foaming, and emulsification) (Damodaran 1994, 1996, 1997).
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