Structure
Home Up Structure Nomenclature Common Names of Amino Acids Optical Isomerism Reactions

 

Structure

The most important of the amino acids are the alpha amino acids. These are amino acids such as shown here (and in Example 11-a in your workbook).

Amino acid structure with R group down. [struct1.jpg]

Amino acid structure with R group to left. [struct2.jpg]

 

The alpha is a carry-over from a previous system of nomenclature that indicates that the amino group is attached to the carbon atom next to the carboxyl group. This is the carbon we would refer to as the number two carbon in the carbon chain of the carboxylic acid.

Structur of carboxylic acid chain with number and letter (alpha) designations for the carbon atoms. [ 65str02.JPG]

The alpha amino acids are so much more important than other amino acids that we will concentrate on the alpha amino acids, and we will ignore the others.

The way that you recognize an amino acid from its structural formula is to see both the carboxylic acid group and the amino group in the same molecule.

Structure of an amino acid. [65str03.JPG]

 

You will see the structural formulas for amino acids written in a variety of orientations. The orientation shown here (and on the right in Example 11-a) has the advantage of pointing out that the amino group is on the alpha or number-2 carbon atom of the carboxylic acid chain.

Amino acid structure with R group to left. [struct2.jpg]
The orientation shown here (and on the left in Example 11-a) emphasizes the importance of both the amino and the acid group, deemphasizing the carbon chain by deflecting it down.

Amino acid structure with R group down. [struct1.jpg]
This orientation also gets that other group of atoms out of the way, and will make it easier to show how the amino group and the carboxylic acid group can each form amide bonds to create the long-chain protein molecules that we will study later in this course.

Equation using amino acid structures to show formation of amode bonds. [struct3.jpg]
You will see other orientations as well. Sometimes a particular orientation is used for a particular purpose. Pay attention to that when you see the structural formula for any amino acid.

Specific Examples

Here you can see two examples of amino acids. The part of each of these compounds that makes them amino acids is the right-hand part, which is the same in each formula. Notice that quite different groups can be attached to that alpha carbon atom.

str5ala.jpg (3359 bytes) str5leu.jpg (4987 bytes)

 

Next you can see that the groups attached to the second carbon do not have to be simple alkyl groups. Here is a hydroxyl group and a benzene ring.

str5ser.jpg (3582 bytes) str5phe.jpg (5847 bytes)

 

In these amino acids, you can see that sulfur atoms can also be included in the groups attached to the second carbon atom.

str5cys.jpg (3582 bytes) str5met.jpg (4803 bytes)

 

Here you can see that carboxylic acid groups and amide groups can be attached to the second carbon atom.

str5asp.jpg (4053 bytes) str5asn.jpg (4212 bytes)

 

So, you can see that there is quite a variety of alpha-amino acids, even though they all have the carboxylic group and an adjacent amino group.

 

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E-mail instructor: Sue Egglilng

Clackamas Community College
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