Food Chemistry and Fish in Nutrition ( 2+1)

7.3.2.2.Types of bound water associated within proteins

Types of bound water associated within proteins

 Types of water associated with proteins are of 5 types- structural, monolayer, hydrophobic, hydration, unfreezable and capillary.

1. Structural: In some proteins, a molecule of water may be intimately associated with the final structure of the molecule. In these cases, the water serves as a bridge to hydrogen bond charged groups within the molecule. These molecules of structural water do not behave like bulk phase water and are unavailable for chemical reactions.

2. Monolayer: The water molecules that are bound through ion-dipole or dipole-dipole interactions are described as monolayer water, in which the molecules form a layer of tightly bound water around the protein molecule.

3. Hydrophobic Hydration: Hydrophobic groups that are exposed to the aqueous phase tend to cause an increase in the order of the water molecules near them. The nature of the water bound near these hydrophobic groups is not well defined, but it does                                                                                                                       not behave as normal bulk water.

4. Unfreezable: All of the above types of water interact strongly with the protein molecule and exhibit properties that are different from those of free water. One of the most notable characteristics of bound water is that it does not freeze at normal temperatures and is thus termed unfreezable water.  This is also the type of water that can be predicted from a knowledge of the amino acid composition of proteins as previously mentioned.

5. Capillary water: Capillary water refers to water that is associated with proteins, but that freezes at normal temperatures and is free to act as a solvent for small molecules. This water behaves very much like bulk phase water, but is very difficult to remove from the protein mass. The amount of this type of water associated with a protein is dependent upon the nature of the measurement being utilized. Some of this water is entrained in a three-dimensional network of protein molecules. The incorporation of large amounts of water into a protein structure can lead to the formation of a gel.