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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

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