# Side chain

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This article is about the chemistry term. For other uses, see [Side-chain (disambiguation)](/source/Side-chain_(disambiguation)).

This article is about the side chain of any type of long molecule. For replacement of a hydrogen atom on a hydrocarbon chain, see [Substituent](/source/Substituent).

**Branch** **Side-chain** **Pendant chain** An [oligomeric](/source/Oligomer) or [polymeric](/source/Polymer) offshoot from a [macromolecular](/source/Macromolecular) chain.

*Notes*

1. An oligomeric branch may be termed a *short-chain branch*.
1. A polymeric branch may be termed a *long-chain branch*.[1]

— *[IUPAC](/source/International_Union_of_Pure_and_Applied_Chemistry) definition*

In [organic chemistry](/source/Organic_chemistry) and [biochemistry](/source/Biochemistry), a **side chain** is a [chemical group](/source/Substituent) that is attached to a core part of the molecule called the "main chain" or [backbone](/source/Backbone_chain). The side chain is a [hydrocarbon](/source/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] A side chain is also known as a **pendant chain**, but a [pendant group](/source/Pendant_group) (side group) has a different definition.

## Conventions

The placeholder **R** is often used as a generic placeholder for [alkyl](/source/Alkyl) (saturated hydrocarbon) group side chains in [structural formulae](/source/Structural_formula). To indicate other non-carbon groups in structure diagrams, **X**, **Y**, or **Z** are often used.

## History

The *R* symbol was introduced by 19th-century French chemist [Charles Frédéric Gerhardt](/source/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](/source/European_languages) to the initial letter of "root" or "residue": French **racine** ("root") and **résidu** ("residue"), these terms' respective English translations along with *[radical](/source/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]

## Usage

### Organic chemistry

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

### Biochemistry

In [proteins](/source/Protein), which are composed of [amino acid](/source/Amino_acid) residues, the side chains are attached to the [alpha-carbon](/source/Alpha-carbon) atoms of the [amide](/source/Amide) backbone. The side chain connected to the alpha-carbon is specific for each amino acid and is responsible for determining [charge](/source/Ion) and [polarity](/source/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](/source/Protein_folding) and function.[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] Spatial positions of side-chain atoms can be predicted based on protein backbone geometry using computational tools for side-chain reconstruction.[7]

## See also

- [Alkyl](/source/Alkyl)
- [Backbone-dependent rotamer library](/source/Backbone-dependent_rotamer_library)
- [Backbone chain](/source/Backbone_chain)
- [Branching (polymer chemistry)](/source/Branching_(polymer_chemistry))
- [Functional group](/source/Functional_group)
- [Pendant group](/source/Pendant_group)
- [Residue (chemistry)](/source/Residue_(chemistry))
- [Substituent](/source/Substituent)

## References

1. Jenkins, A. D.; Kratochvíl, P.; Stepto, R. F. T.; Suter, U. W. (1996). ["Glossary of basic terms in polymer science (IUPAC Recommendations 1996)"](http://pac.iupac.org/publications/pac/pdf/1996/pdf/6812x2287.pdf). *[Pure and Applied Chemistry](/source/Pure_and_Applied_Chemistry)*. **68** (12): 2287–2311. [doi:10.1351/pac199668122287](https://doi.org/10.1351/pac199668122287). [S2CID 98774337](https://api.semanticscholar.org/CorpusID:98774337)

1. Wade, L.G. (2010). [*Organic Chemistry, 7th Edition*](https://archive.org/details/organicchemistry00wade_1). Upper Saddle River, NJ: Pearson Prentice Hall. pp. 70–78. ISBN 978-0-321-59231-6.

1. [Jensen W.B.](/source/William_B._Jensen), *Journal of Chemical Education* **87**, 360 (2010)

1. Voet, Donald; Voet, Judith; Pratt, Charlotte (2013). [*Fundamentals of Biochemistry: Life at the Molecular Level*](https://archive.org/details/fundamentalsofbi00voet_0). Fourth ed. Hoboken, NJ: John Wiley & Sons, Inc. ISBN 9781118129180.

1. Andrew, C. D.; Penel, S.; Jones, G. R.; Doig, A. J. (2001-12-01). "Stabilizing nonpolar/polar side-chain interactions in the alpha-helix". *Proteins*. **45** (4): 449–455. [doi:10.1002/prot.1161](https://doi.org/10.1002/prot.1161). [ISSN 0887-3585](https://www.worldcat.org/issn/0887-3585). [PMID 11746692](https://pubmed.ncbi.nlm.nih.gov/11746692). [S2CID 25739520](https://api.semanticscholar.org/CorpusID:25739520)

1. Badaczewska-Dawid, Aleksandra E.; Kolinski, Andrzej; Kmiecik, Sebastian (2019-12-26). "Computational reconstruction of atomistic protein structures from coarse-grained models". *Computational and Structural Biotechnology Journal*. **18**: 162–176. [doi:10.1016/j.csbj.2019.12.007](https://doi.org/10.1016/j.csbj.2019.12.007). [ISSN 2001-0370](https://www.worldcat.org/issn/2001-0370). [PMC 6961067](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6961067). [PMID 31969975](https://pubmed.ncbi.nlm.nih.gov/31969975)

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Adapted from the Wikipedia article [Side chain](https://en.wikipedia.org/wiki/Side_chain) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Side_chain?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.