Amino acids contain 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 positive (COO-) and one negative (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.