Editing Side chain

{{short description|Chemical group attached to backbone}}
{{About|the chemistry term|other uses|Side-chain (disambiguation)}}
{{About|the side chain of any type of long molecule|replacement of a hydrogen atom on a hydrocarbon chain|Substituent}}

{{Quote box
|title=[[International Union of Pure and Applied Chemistry|IUPAC]] definition
|quote='''Branch'''<br>'''Side-chain'''<br>'''Pendant chain'''<br>An [[oligomer]]ic or [[polymer]]ic offshoot from a [[macromolecular]] chain.

''Notes''

# An oligomeric branch may be termed a ''short-chain branch''.
# A polymeric branch may be termed a ''long-chain branch''.<ref>{{cite journal |title=Glossary of basic terms in polymer science (IUPAC Recommendations 1996) |journal=[[Pure and Applied Chemistry]] |year=1996 |volume=68 |issue=12 |pages=2287–2311 |doi=10.1351/pac199668122287 |url=http://pac.iupac.org/publications/pac/pdf/1996/pdf/6812x2287.pdf |last1=Jenkins |first1=A. D. |last2=Kratochvíl |first2=P. |last3=Stepto |first3=R. F. T. |last4=Suter |first4=U. W. |s2cid=98774337}}</ref> 
}}

In [[organic chemistry]] and [[biochemistry]], a '''side chain''' is a [[substituent|chemical group]] that is attached to a core part of the molecule called the "main chain" or [[backbone chain|backbone]]. The side chain is a [[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.<ref>{{cite book |last=Wade |first=L.G. |url=https://archive.org/details/organicchemistry00wade_1 |title=Organic Chemistry, 7th Edition |publisher=Pearson Prentice Hall |year=2010 |isbn=978-0-321-59231-6 |location=Upper Saddle River, NJ |pages=70–78 |url-access=registration}}</ref> A side chain is also known as a '''pendant chain''', but a [[pendant group]] (side group) has a different definition.

==Conventions==
The placeholder '''R''' is often used as a generic placeholder for [[alkyl]] (saturated hydrocarbon) group side chains in [[structural formula]]e. 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]], who advocated its adoption on the grounds that it would be widely recognizable and intelligible given its correspondence in multiple [[European languages]] to the initial letter of "root" or "residue": French ''{{Lang|fr|racine}}'' ("root") and ''{{Lang|fr|résidu}}'' ("residue"), these terms' respective English translations along with ''[[radical (chemistry)|radical]]'' (itself derived from Latin ''{{Lang|la|radix}}'' below), Latin ''{{Lang|la|radix}}'' ("root") and ''{{Lang|la|residuum}}'' ("residue"), and German ''{{Lang|de|Rest}}'' ("remnant" and, in the context of chemistry, both "residue" and "radical").<ref>[[William B. Jensen|Jensen W.B.]], ''Journal of Chemical Education'' '''87''', 360 (2010)</ref>

==Usage==

===Organic chemistry===
In [[polymer science]], the side chain of an [[oligomer]]ic or [[polymer]]ic offshoot extends from the [[backbone chain]] of a polymer. Side chains have noteworthy influence on a polymer's properties, mainly its [[polymer#Crystallinity|crystallinity]] and [[density]]. An oligomeric branch may be termed a short-chain branch, and a polymeric branch may be termed a long-chain branch. [[Side group]]s are different from side chains; they are neither oligomeric nor polymeric.<ref>{{GoldBookRef|title=branch (side chain, pendant chain)|file=B00720}}</ref>

===Biochemistry===
In [[protein]]s, which are composed of [[amino acid]] residues, the side chains are attached to the [[alpha-carbon]] atoms of the [[amide]] backbone. The side chain connected to the alpha-carbon is specific for each amino acid and is responsible for determining [[ion|charge]] and [[Chemical polarity|polarity]] of the amino acid. The amino acid side chains are also responsible for many of the interactions that lead to proper [[protein folding]] and function.<ref name="Textbook">{{cite book |last1=Voet |first1=Donald |url=https://archive.org/details/fundamentalsofbi00voet_0 |title=Fundamentals of Biochemistry: Life at the Molecular Level |last2=Voet |first2=Judith |last3=Pratt |first3=Charlotte |date=2013 |publisher=John Wiley & Sons, Inc. |isbn=9781118129180 |edition=Fourth |location=Hoboken, NJ |url-access=registration}}</ref> 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.<ref>{{cite journal |last1=Andrew |first1=C. D. |last2=Penel |first2=S. |last3=Jones |first3=G. R. |last4=Doig |first4=A. J. |date=2001-12-01 |title=Stabilizing nonpolar/polar side-chain interactions in the alpha-helix |journal=Proteins |volume=45 |issue=4 |pages=449–455 |issn=0887-3585 |pmid=11746692 |doi=10.1002/prot.1161 |s2cid=25739520}}</ref> Spatial positions of side-chain atoms can be predicted based on protein backbone geometry using computational tools for side-chain reconstruction.<ref>{{cite journal |last1=Badaczewska-Dawid |first1=Aleksandra E. |last2=Kolinski |first2=Andrzej |last3=Kmiecik |first3=Sebastian |date=2019-12-26 |title=Computational reconstruction of atomistic protein structures from coarse-grained models |journal=Computational and Structural Biotechnology Journal |volume=18 |pages=162–176 |doi=10.1016/j.csbj.2019.12.007 |pmid=31969975 |pmc=6961067 |issn=2001-0370}}</ref>
[[File:AAs table.png|thumb|Table of amino acids]]

==See also==
*[[Alkyl]]
*[[Backbone-dependent rotamer library]]
*[[Backbone chain]]
*[[Branching (polymer chemistry)]]
*[[Functional group]]
*[[Pendant group]]
*[[Residue (chemistry)]]
*[[Substituent]]

==References==
{{Reflist}}

[[Category:Organic chemistry]]