Amino Acids

Edexcel

Topic 18B/16. 

To be able to understand the properties of 2-amino acids, including: 

i  acidity and basicity in solution, as a result of the formation of zwitterions 


ii  effect of aqueous solutions on plane-polarised monochromatic light 


OCR

6.2.2 Reactions of amino acids

(a) the general formula for an α-amino acid as RCH(NH2)COOH and the following reactions of amino acids: 

(i)  reaction of the carboxylic acid group with alkalis and in the formation of esters


(ii)  reaction of the amine group with acids 


AQA

Amino acids 

Amino acids have both acidic and basic properties, including the formation of zwitterions. 

Students should be able to draw the structures of amino acids as zwitterions and the ions formed from amino acids: 

• in acid solution 

• in alkaline solution.

Amino Acids

Amino acids sometimes referred to as α-amino acids or 2-amino acids depending on the A level specification you are studying from.

Either way their structure looks like this:

The 2– refers to the second carbon atom from the carboxylic acid group to which both the R group and the NH₂groups are attached.

The R– groups varies from amino acid to amino acid and leans each its distinctive properties.

Here are the skeletal formulae of the 21 essential amino acids with their generic names and abbreviations.  

 

They are classified above according to their type of side chain or R– group.

They have certain properties common to them all.

1.  They are acids so react as acids in aqueous solution with the common reagents such as a reactive metal like magnesium or a base such as a carbonate e.g. sodium carbonate Na₂CO₃.

2.  Every amino acid also contains at least one amino group which is basic so amino acids accept protons from strong acids such as Hydrochloric acid (HCl)

3.  Coupling the above two properties together we find that the acid group of the amino acid protonates the amine group and creates what is known as a zwitter ion (on the right in the illustration below)

So in high pH alkaline conditions the amino acid carries an amino group and the carboxyl group is ionised

But in low acidic pH the amino group is protonated and the carboxyl group is free as in the illustration below:

And therefore the amino acid carries two Ka values.

4. If a solution of an amino acid is allowed to evaporate and crystallise then a white crystalline solid forms with a fairly high melting point.

The solid is the crystalline zwitter ion and as that is an ionic structure, the substance has a fairly high melting point.

So amino acids have both acidic and basic properties they both accept protons and donate protons under the appropriate conditions. 

5. Furthermore as we noted in a previous blog (here) amino acids (except glycine are optically active having a chiral centre that is a carbon atom attached to four different functional groups.  

This structural feature a chiral centre means that aqueous solutions of amino acids will rotate the plane of plane-polarised monochromatic light.  

The significance of this behaviour is that all amino acids that occur naturally have the same type of rotation of the plane of plane polarised monochromatic light.  They are all laevo rotatory.  Some have colloquially called this the left-handedness of the natural world!

You can read more about this feature of the natural world hereand also here.