Amino acids and zwitterions

In a nutshell

Amino acids are amphoteric. They can exist as zwitterions at the isoelectric point. Amino acids can be identified using paper chromatography and TLC. This is because each amino acid has a different solubility, therefore have a different $$R_f$$​ value, in a given solvent. 

​

Nomenclature

Amino acids are amphoteric. This means they have both acidic and basic properties. This is because amino acids have an amino group (basic) and a carboxyl group (acidic).

Many amino acids have a common name as well as a systematic name.  

Example

The following amino acid is called valine. Give the systematic name for valine.

Firstly, find the longest carbon chain which includes the carbon chain and write down the parent name.

$$butanoic \ acid$$​​

Next, write down the position of the amino group. Amino groups are always on  $$C_2$$.

​$$2-aminobutanoic \ acid$$​​

Then, write down all the side chains in the amino acid. Valine has one methyl group on $$C_3$$.

Therefore the systematic name of valine is $$\underline{2-amino-3-methylbutanoic \ acid}$$.

Zwitterions

Amino acids exist as zwitterions. A zwitterion has both a positive and negative charge within the species, but the overall charge is neutral. Amino acids exist as zwitterions at their isoelectric point. The isoelectric point is the pH when the net charge of the amino acid is zero. The isoelectric point is dependant on the R group of the amino acids.

Acidic conditions will lead to the amino group to become protonated. At the isoelectric point, both the amino and the carboxyl group are ionised. Under basic conditions the carboxyl loses a proton and becomes a carboxylate.

Chirality

Most amino acids are chiral because they have four different atoms or groups of atoms attached to $$C_2$$. They are therefore able to rotate plane polarised light and exist as optical isomers.​

​

Paper chromatography

Amino acids can be identified using paper chromatography. This is because each amino acid has a different R group which gives rise to different solubilities in a given solvent.

PROCEDURE

Amino acids can also be separated and identified using thin-layer chromatography (TLC). The difference is that the stationary phase is a thin layer of silica $$(SiO_2)$$ or alumina ($$Al_2O_3)$$​.