In this module:

There are three amino acids that have basic side chains at neutral pH. These are arginine (Arg), lysine (Lys), and histidine (His). Their side chains contain nitrogen and resemble ammonia, which is a base. Their pKa's are high enough that they tend to bind protons, gaining a positive charge in the process.

Click on the structures below to switch between their protonated and deprotonated forms. For these amino acids, the protonated forms predominate at physiological pH (about 7).

Two amino acids have acidic side chains at neutral pH. These are aspartic acid or aspartate (Asp) and glutamic acid or glutamate (Glu). Their side chains have carboxylic acid groups whose pKa's are low enough to lose protons, becoming negatively charged in the process.

Click on the structures below to switch between their protonated and deprotonated forms. For these amino acids, the deprotonated forms predominate at physiological pH (about 7).

Click on the Protein 1 icon at left to see a chart of all the amino acids, classified according to the chemistry of their side chains. See if you can tell why each amino acid has been sorted in that way.