Amino acids Amino acids The twenty amino acids commonly found in proteins. Functional groups not illustrated elsewhere are labelled in red. Full names and one-letter codes are given. Amino acids are categorized as non-polar, uncharged polar, acidic and basic according to the composition of their side chains. Methionine and tryptophan are classified as non-polar, despite including the polar atoms sulphur and nitrogen respectively, as both are typically found in the interior of a protein. The sulphur atom in methionine does not participate in hydrogen bonds or act as a nucleophile in enzymatic reactions. In tryptophan, the nitrogen lone pair is delocalized through the indole ring, ensuring that it is likewise unavailable as a nucleophile. Side chains which frequently form hydrogen bonds are classified as polar, those that lose a proton at physiological pH are acidic, and those that gain a proton are basic. Histidine could be considered both polar uncharged and basic as it exists in both neutral and positively charged state at physiological pH. Amanda L. Jonsson et al. Essays Biochem. 2017;61:401-427 ©2017 by Portland Press Ltd