# Side chain

By [Senia](https://paragraph.com/@senia) · 2023-05-30

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In [organic chemistry](https://en.wikipedia.org/wiki/Organic_chemistry) and [biochemistry](https://en.wikipedia.org/wiki/Biochemistry), a **side chain** is a [chemical group](https://en.wikipedia.org/wiki/Substituent) that is attached to a core part of the molecule called the "main chain" or [backbone](https://en.wikipedia.org/wiki/Backbone_chain). The side chain is a [hydrocarbon](https://en.wikipedia.org/wiki/Hydrocarbon) branching element of a molecule that is attached to a larger hydrocarbon backbone. It is one factor in determining a molecule's properties and reactivity.[\[2\]](https://en.wikipedia.org/wiki/Side_chain#cite_note-2) A side chain is also known as a **pendant chain**, but a [pendant group](https://en.wikipedia.org/wiki/Pendant_group) (side group) has a different definition.

Conventions
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The placeholder **R** is often used as a generic placeholder for [alkyl](https://en.wikipedia.org/wiki/Alkyl) (saturated hydrocarbon) group side chains in [chemical structure diagrams](https://en.wikipedia.org/wiki/Chemical_structure_diagram). To indicate other non-carbon groups in structure diagrams, **X**, **Y**, or **Z** are often used.

History
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The _R_ symbol was introduced by 19th-century French chemist [Charles Frédéric Gerhardt](https://en.wikipedia.org/wiki/Charles_Fr%C3%A9d%C3%A9ric_Gerhardt), who advocated its adoption on the grounds that it would be widely recognizable and intelligible given its correspondence in multiple [European languages](https://en.wikipedia.org/wiki/European_languages) to the initial letter of "root" or "residue": French _racine_ ("root") and _résidu_ ("residue"), these terms' respective English translations along with [_radical_](https://en.wikipedia.org/wiki/Radical_\(chemistry\)) (itself derived from Latin _radix_ below), Latin _radix_ ("root") and _residuum_ ("residue"), and German _Rest_ ("remnant" and, in the context of chemistry, both "residue" and "radical").[\[3\]](https://en.wikipedia.org/wiki/Side_chain#cite_note-3)

Usage
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### Organic chemistry

In [polymer science](https://en.wikipedia.org/wiki/Polymer_science), the side chain of an [oligomeric](https://en.wikipedia.org/wiki/Oligomer) or [polymeric](https://en.wikipedia.org/wiki/Polymer) offshoot extends from the [backbone chain](https://en.wikipedia.org/wiki/Backbone_chain) of a polymer. Side chains have noteworthy influence on a polymer's properties, mainly its [crystallinity](https://en.wikipedia.org/wiki/Polymer#Crystallinity) and [density](https://en.wikipedia.org/wiki/Density). An oligomeric branch may be termed a short-chain branch, and a polymeric branch may be termed a long-chain branch. [Side groups](https://en.wikipedia.org/wiki/Side_group) are different from side chains; they are neither oligomeric nor polymeric.[\[4\]](https://en.wikipedia.org/wiki/Side_chain#cite_note-4)

### Biochemistry

In [proteins](https://en.wikipedia.org/wiki/Protein), which are composed of [amino acid](https://en.wikipedia.org/wiki/Amino_acid) residues, the side chains are attached to the [alpha-carbon](https://en.wikipedia.org/wiki/Alpha-carbon) atoms of the [amide](https://en.wikipedia.org/wiki/Amide) backbone. The side chain connected to the alpha-carbon is specific for each amino acid and is responsible for determining [charge](https://en.wikipedia.org/wiki/Ion) and [polarity](https://en.wikipedia.org/wiki/Chemical_polarity) of the amino acid. The amino acid side chains are also responsible for many of the interactions that lead to proper [protein folding](https://en.wikipedia.org/wiki/Protein_folding) and function.[\[5\]](https://en.wikipedia.org/wiki/Side_chain#cite_note-Textbook-5) Amino acids with similar polarity are usually attracted to each other, while nonpolar and polar side chains usually repel each other. Nonpolar/polar interactions can still play an important part in stabilizing the secondary structure due to the relatively large amount of them occurring throughout the protein.[\[6\]](https://en.wikipedia.org/wiki/Side_chain#cite_note-6) Spatial positions of side-chain atoms can be predicted based on protein backbone geometry using computational tools for side-chain reconstruction.[\[7\]](https://en.wikipedia.org/wiki/Side_chain#cite_note-7)

Table of amino acids

nft://137/0xbe352d4E2519f3FacEa7dBEBd71f3Fb717C54628/?showBuying=true&showMeta=true

See also
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*   [Alkyl](https://en.wikipedia.org/wiki/Alkyl)
    
*   [Backbone chain](https://en.wikipedia.org/wiki/Backbone_chain)
    
*   [Branching (polymer chemistry)](https://en.wikipedia.org/wiki/Branching_\(polymer_chemistry\))
    
*   [Functional group](https://en.wikipedia.org/wiki/Functional_group)
    
*   [Pendant group](https://en.wikipedia.org/wiki/Pendant_group)
    
*   [residue](https://en.wikipedia.org/wiki/Residue_\(chemistry\))
    
*   [Substituent](https://en.wikipedia.org/wiki/Substituent)
    
*   [Backbone-dependent rotamer library](https://en.wikipedia.org/wiki/Backbone-dependent_rotamer_library)

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*Originally published on [Senia](https://paragraph.com/@senia/side-chain)*