Structure and Bonding:  The Duality of Amino Acids

Examine the following molecules and identify the functional groups in each.

ammonia                ethylamine                acetic acid                glycine

How would you characterize the acid-base behavior of the four molecules above?

What is unusual about the glycine molecule?

Glycine, the simplest of amino acids, is amphoteric. Amino acids are organic compounds that contain an amine group (-NH2) bonded to a carbon atom that is bonded to a carboxylic acid group (-COOH) as seen below.  

For glycine the R group is a hydrogen, H.  Other side chains or R groups make up other amino acids.  There are about 20 amino acids that naturally occur in nature. We can classify amino acids into four major groups, each characterized by its type of side chain: non-polar, polar but neutral, acidic, and basic.  Click here to go to a good site displaying the amino acids.

For glycine shown below, what has to happen for the neutral amino acid (on the left) to form the zwitterion on the right?

What makes the zwitterion unusual?

What is the overall charge or net charge on a zwitterion?

Here is another amino acid in the zwitterion form, redraw the neutral structure of leucine.

 

A zwitterion ion has both a positive and negative charge on the same structure due to the transfer of a hydrogen ion from the carboxylic acid group to the amine group.  The net charge on a zwitterion is zero.  This makes the carboxylate ion group act as a base and the protonated amine group act as a acid.  Complete the acid-base reactions for this behavior.

        -COO-   +    _______       _______

        -NH3+   +   _______      _______    +    _______

Since amino acids have the capability of reacting with both acid and base, they naturally act as buffers. Amino acids are always in ionic form in aqueous solutions.

The zwitterion form of most amino acids is typical near pH = 7 (physiological pH).  Using glycine as an example, draw the structures of glycine that would be found at a very low pH (1-2) and at a very high pH (12-13).

 low pH high pH
 

 

 

 

   

What is the overall charge of the form at very low pH? _____    at very high pH? _____

As the pH increases, how does the charge on an amino acid change?

Amino acids react with each other to form peptides (short chains) and proteins (long chains).  Proteins, especially in blood, are important buffers.  See Exploring the Molecules of Life:  Proteins for more information on protein formation, structure, and behavior.

The image below illustrates two glycine molecules that have reacted to form a dipeptide.  The two amino acids are bound together by an amide linkage (or peptide linkage as the biochemists call it).  The amide linkage is seen at the center of the molecule.  Is the molecule as shown neutral or in the zwitterion form?

Would this dipeptide still be able to be a buffer?  Explain why or why not.

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