Question

Why aren't all amino acid R groups ionizable?

Answer 1

There are only seven amino acids that can be ionizable. Ionizable indicates that the amino acid can accept ions and can be protonated or deprotonated and forms ionic bonds. Ionization is due to the presence of R side chains of amino acids.

If the amino acid has an acidic side chain, then COOH gets deprotonated first. Then the R group is protonated followed by the amine. This ionization depends on the pKa. For amino acids with basic R groups, COOH gets deprotonated first. Then the amino group is deprotonated, followed by the R group as the R group has a higher pka.

There are only seven amino acids that can be Ionizable. These are tyrosine, cysteine, arginine, lysine, histidine, aspartic acid, and glutamic acid. The other amino acids have an uncharged R group, which cannot be ionized due to absence of positive or negative charge. Aspartic acid, glutamic acid, Tyrosine, cysteine have charged Ionizable groups below their Pka while arginine, lysine, histidine have charged Ionizable groups above their pKa