Amino Acids

Amino acids are a type of organic acid that contains both a carboxyl group (COOH) and an amino group (NH₂). The general formula for an amino acid is given below. Although the neutrally-charged structure is commonly written, it is inaccurate because the acidic COOH and basic NH₂ groups react with one another to form an internal salt called a zwitterion. The zwitterion has no net charge; there is one negative (COO^-) and one positive (NH₃⁺) charge.

There are 20 amino acids derived from proteins. While there are several methods of categorizing them, one of the most common is to group them according to the nature of their side chains.

Nonpolar Side Chains

There are eight amino acids with nonpolar side chains. Glycine, alanine, and proline have small, nonpolar side chains and are all weakly hydrophobic. Phenylalanine, valine, leucine, isoleucine, and methionine have larger side chains and are more strongly hydrophobic.

Polar, Uncharged Side Chains

There are also eight amino acids with polar, uncharged side chains. Serine and threonine have hydroxyl groups. Asparagine and glutamine have amide groups. Histidine and tryptophan have heterocyclic aromatic amine side chains. Cysteine has a sulfhydryl group. Tyrosine has a phenolic side chain. The sulfhydryl group of cysteine, phenolic hydroxyl group of tyrosine, and imidazole group of histidine all show some degree of pH-dependent ionization.

Charged Side Chains

There are four amino acids with charged side chains. Aspartic acid and glutamic acid have carboxyl groups on their side chains. Each acid is fully ionized at pH 7.4. Arginine and lysine have side chains with amino groups. Their side chains are fully protonated at pH 7.4.