Keywords: amino, acid, acids, zwitterion, zwitterions, base
Description: The acid-base behaviour of amino acids

This page looks at what happens to amino acids as you change the pH by adding either acids or alkalis to their solutions.

For simplicity, the page only looks at amino acids which contain a single -NH2 group and a single -COOH group.

There is an internal transfer of a hydrogen ion from the -COOH group to the -NH2 group to leave an ion with both a negative charge and a positive charge.

This is the form that amino acids exist in even in the solid state. If you dissolve the amino acid in water, a simple solution also contains this ion.

A zwitterion is a compound with no overall electrical charge, but which contains separate parts which are positively and negatively charged.

If you increase the pH of a solution of an amino acid by adding hydroxide ions, the hydrogen ion is removed from the -NH3 + group.

In its simplest form, electrophoresis can just consist of a piece of moistened filter paper on a microscope slide with a crocodile clip at each end attached to a battery. A drop of amino acid solution is placed in the centre of the paper.

Although the amino acid solution is colourless, its position after a time can be found by spraying it with a solution of ninhydrin . If the paper is allowed to dry and then heated gently, the amino acid shows up as a coloured spot.

If you decrease the pH by adding an acid to a solution of an amino acid, the -COO - part of the zwitterion picks up a hydrogen ion.

This time, during electrophoresis, the amino acid would move towards the cathode (the negative electrode).

Suppose you start with the ion we've just produced under acidic conditions and slowly add alkali to it.

That ion contains two acidic hydrogens - the one in the -COOH group and the one in the -NH3 + group.

The more acidic of these is the one in the -COOH group, and so that is removed first - and you get back to the zwitterion.

So when you have added just the right amount of alkali, the amino acid no longer has a net positive or negative charge. That means that it wouldn't move towards either the cathode or anode during electrophoresis.

The pH at which this lack of movement during electrophoresis happens is known as the isoelectric point of the amino acid. This pH varies from amino acid to amino acid.

If you go on adding hydroxide ions, you will get the reaction we've already seen, in which a hydrogen ion is removed from the -NH3 + group.

Note: You might have expected the isoelectric point to be at pH 7 - when the solution is neither acidic nor alkaline. In fact, the isoelectric point for many amino acids is about pH 6. Explaining why it isn't at pH 7 is quite long-winded and almost certainly beyond what you will need for UK A level (or its equivalent) purposes. If you are interested, the problem is discussed at the bottom of this page.

You can, of course, reverse the whole process by adding an acid to the ion we've just finished up with.

That ion contains two basic groups - the -NH2 group and the -COO - group. The -NH2 group is the stronger base, and so picks up hydrogen ions first. That leads you back to the zwitterion again.

Photogallery Zwitterion:

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