Zwitterions are molecules that have a positive charge on one atom or group and a negative charge on another. Since these two charges cancel out, the molecule as a whole is neutral. The most common example is an amino acid. As the pH of water changes, so too will the fraction of the total present in zwitterionic form. At the isoelectric point, the majority of the amino acid will be present in zwitterionic form. You can estimate the pH of water at the isoelectric point using simple formulas.
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Determine which amino acid is present in the water. If you're working this problem as a quiz or homework question, the identity of the amino acid will typically be given to you.
Look up the pKa values for the amino acid, using the link in the Resources section. Use the pKa values for the carboxyl group (CO2H) and the amine group (NH3+). Also determine whether the amino acid has an acidic or basic side chain; if it does, a pKa value will be listed for the side chain (the R group).
If the side chain is neither acidic nor basic, take the average of the carboxyl group pKa and the amine group pKa to find the pH of the water at isoelectric point. At this pH, over 99 per cent of the amino acid molecules will be present in zwitterionic form.
If the amino acid does have a titratable side chain, determine whether it is acidic or basic. It helps to draw the structure of the amino acid in fully protonated form. Acidic amino acids act as proton donors in water whereas basic amino acids act as proton acceptors. Histidine, lysine and arginine are all basic while aspartic acid, asparagine, glutamic acid and glutamine are all acidic.
Write down the three pKa values for the amino acid--the pKa of the carboxyl group, of the amine group and of the side chain. If the amino acid is basic, take the average of the two highest pKas to find the isoelectric point pH. If the amino acid is acidic, take the average of the two lowest pKas.
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