Reactions

Another very important feature about the structure of amino acids that relates to the way that amino acids react. So let's take another look at that structure.

Acid-Base Reactions

The carboxylic group is an acid group. When it is in solution, it can lose a hydrogen ion (donate a proton) and become negatively charged. The amino group can act as a base. This means it can accept a proton (hydrogen ion) and become positively charged.

Structure of neutral amino acid. [65str04.JPG]

In solution, amino acids actually do both of these things to a considerable extent. In solution, amino acids can be found as dipolar ions, as shown here (and in Example 11-b in your workbook). These are often called zwitter ions. These ions are unlike anything that you have dealt with so far in this course because they have both a positive and a negative charge. The existence of amino acids as dipolar ions makes them very polar and allows for fairly high solubility in water, which is a very important biological property since water is the biological solvent. It would be most difficult to imagine life existing as we know it, if one of the most important building blocks of life were not soluble in water.

Structure of amino acid zwitter ion. [65str05.JPG]

These formulas (also shown in Examples 11-c and d) show what happens to amino acids when they are dissolved in acidic or basic solution. You can see that they are ionized either way. An acidic solution will ionize the amino group.

Structure of acidic ion of an amino acid. [65str06.JPG]

A basic solution will ionize the carboxylic acid group. Because amino acids can react with both acids and bases they are called amphoteric compounds and in solutions they could serve as pH buffers.

Structure of basic ion of an amino acid. [65str07.JPG]

Intermolecular Dehydration Reactions

If you continue to look at the structures of the amino acids and think about what you have already studied in this lesson, I think you can see that an amino acid can form two amide bonds by using both of its functional groups.

It has an amino group that can form an amide bond with an acid group from another molecule. And there is a carboxylic group which can form an amide bond with an amino group from another molecule. When amino acids form amide bonds from both ends of the amino acid molecule, the resulting larger molecules are called peptides or polypeptides.

Structure of an amino acid emphsizing the amino and acid groups. [65str08.JPG]

The formation of an amide bond between two amino acid molecules is shown here. When an amide bond joins two amino acids it is generally called a peptide bond or peptide linkage. As the two react, hydrogen from the amino group combines with -OH from the carboxylic group to form water. This results in the C in the carboxylic group and the N in the amino group bonding to one another to form the amide bond (or peptide bond or peptide linkage as it is generally called, when amino acids are joined).

Equation showing the intermolecular dehydration of two amino acids to form a dipeptide. [65rxn05.JPG]

The compound formed here is called a dipeptide. It consists of two amino acids joined to one another by an intermolecular dehydration reaction. Notice that this molecule still has an amino group on the left and a carboxylic acid group on the right. If those groups were to bond to other amino acids in the same way, we would get an even larger molecule which would still have an amino group on the left end and a carboxylic acid group on the right end. This process can go on and on, resulting in polymers called polypeptides. Very long polypeptides are known as proteins. (Actually a few proteins are as short as nine amino acids long. Others are hundreds or thousands of amino acids long.) Proteins are very important compounds which will be studied in a later lesson.

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