sidechain

The part of an amino acid that gives it its identity.

		          R1  H   O
		          |   |   "
		          C   N   C
		       \ / \ / \ / \
			N   C   C
		        |   "   |
		        H   O   R2

Here, R is the sidechain, sometimes called the R-group. The remaining atoms belong to the backbone, a central polymer which connects the protein together. For the twenty natural amino acids, there are twenty different sidechains. The simplest sidechain is a hydrogen, resulting in the amino acid glycine. The biggest sidechain is 3-methylindole, which when attached to a backbone unit, gives tryptophan. The different physical and chemical characteristics of sidechains determine how a protein assembles, and gives it a basic toolset to build environments which can carry out complex chemistry, or make very specific interactions.

Sidechains of the twenty natural amino acids

Nonpolar

glycine - H alanine - CH₃ valine - CH - CH₃ | CH₃ leucine - CH₂ - CH - CH₃ | CH₃ methionine - CH₂ - CH₂ - S - CH₃ tryptophan - CH₂ - indole phenylalanine - CH₂ - benzene isoleucine - CH - CH₂ - CH₃ | CH₃

Polar, Uncharged

serine - CH₂ - OH asparagine - CH₂ - C = O | NH₂ glutamine - CH₂ - CH₂ - C = O | NH₂ threonine - CH - OH | CH₃ tyrosine - CH₂ - phenol cysteine - CH₂ - SH

Acidic

aspartic acid - CH₂ - COOH glutamic acid - CH₂ - CH₂ - COOH

Basic

histidine - CH₂ - imidazole lysine - CH₂ - CH₂ - CH₂ - CH₂ - NH₃⁺ arginine - CH₂ - CH₂ - CH₂ - NH - C = NH₂⁺ | NH₂

Cyclic

proline sidechain in bold O \ " HN C / \ / \ H₂C C | | H₂C - CH₂