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Ionic bonds are important for life because they can also play a role in holding different parts of molecules together.

In Figure 3.5, you can see a typical protein chain. A protein is made up of individual amino acids strung together in a long chain. Some of these amino acids are charged (we will come back to amino acids and discuss their features in more detail in Chapter 4).

Figure 3.5 A typical protein. The colored ribbons and lines depict the chains made of amino acids strung together, a chain that is itself folded together into a complex three-dimensional molecule. This one (called NOD2) is involved in the human immune system.

Source: Reproduced with permission of LPKozlowski, https://commons.wikimedia.org/wiki/File:Nod2_protein.png.

Some amino acids have a negatively charged side group, and some a positively charged side group. These two different types of amino acids can therefore come together to form an ionic bond. For example, the negative charge in an aspartic acid, which is one type of amino acid, is attracted to the positive charge in a lysine, another type of amino acid, to form an ionic bond, which keeps the chains together. Figure 3.6 illustrates this bonding.

Figure 3.6 An ionic bond in a protein formed between two amino acids, the positively charged amino acid lysine, and the negatively charged amino acid aspartic acid. Hydrogen bonding is also involved giving rise to both ionic and hydrogen bonding across the link – usually called a “salt bridge.”

This bonding is important in proteins that have functions to perform in cells, for example proteins that are enzymes or biological catalysts, that play a role in accelerating chemical reactions in life. These ionic bonds help the proteins keep their three-dimensional shape, which is necessary if they are to perform their functions correctly. From a very colloquial perspective, you can think of ionic bonds as “bolts” that can help hold together the three-dimensional structure of biological molecules, such as proteins. It is important to note that in proteins, as well as this ionic interaction, hydrogen bonding (which is discussed later in this chapter) also plays a role in linking the two amino acids together. It is this combination of ionic bonds and hydrogen bonds that forms the complete link. This combination of ionic and hydrogen bond interaction is referred to by biochemists as a “salt bridge.”