View source for Chemistry:Side chain

{{short description|Chemical group attached to backbone}}

{{Quote box
 |title = [[Organization:International Union of Pure and Applied Chemistry|IUPAC]] definition
 |quote = '''Branch'''<br/>'''Side-chain'''<br/>'''Pendant chain'''<br/>An [[Chemistry:Oligomer|oligomer]]ic or [[Chemistry:Polymer|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=[[Chemistry:Pure and Applied Chemistry|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 [[Chemistry:Organic chemistry|organic chemistry]] and [[Biology:Biochemistry|biochemistry]], a '''side chain''' is a [[Chemistry:Substituent|chemical group]] that is attached to a core part of the molecule called the "main chain" or [[Chemistry:Backbone chain|backbone]]. The side chain is a [[Chemistry:Hydrocarbon|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 [[Chemistry:Pendant group|pendant group]] (side group) has a different definition.

==Conventions==
The placeholder '''R''' is often used as a generic placeholder for [[Chemistry:Alkyl|alkyl]] (saturated hydrocarbon) group side chains in chemical structure diagrams. 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 ''[[Biology:Radical|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>Jensen W.B., ''Journal of Chemical Education'' '''87''', 360 (2010)</ref>

==Usage==
===Organic chemistry===
In [[Physics:Polymer science|polymer science]], the side chain of an [[Chemistry:Oligomer|oligomer]]ic or [[Chemistry:Polymer|polymer]]ic offshoot extends from the [[Chemistry:Backbone chain|backbone chain]] of a polymer. Side chains have noteworthy influence on a polymer's properties, mainly its [[Chemistry:Polymer#Crystallinity|crystallinity]] and [[Physics:Density|density]]. An oligomeric branch may be termed a short-chain branch, and a polymeric branch may be termed a long-chain branch. Side groups are different from side chains; they are neither oligomeric nor polymeric.<ref>{{cite book|url=http://goldbook.iupac.org/B00720.html|title=IUPAC Compendium of Chemical Terminology|first=International Union of Pure and Applied|last=Chemistry|publisher=IUPAC|website=iupac.org|doi=10.1351/goldbook.B00720}}</ref>

===Biochemistry===
In [[Biology:Protein|protein]]s, which are composed of [[Chemistry:Amino acid|amino acid]] residues, the side chains are attached to the alpha-carbon atoms of the [[Chemistry:Amide|amide]] backbone. The side chain connected to the alpha-carbon is specific for each amino acid and is responsible for determining [[Physics:Ion|charge]] and [[Chemistry:Chemical polarity|polarity]] of the amino acid. The amino acid side chains are also responsible for many of the interactions that lead to proper [[Biology:Protein folding|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==
*[[Chemistry:Alkyl|Alkyl]]
*[[Chemistry:Backbone chain|Backbone chain]]
*[[Chemistry:Branching (polymer chemistry)|Branching (polymer chemistry)]]
*[[Chemistry:Functional group|Functional group]]
*[[Chemistry:Pendant group|Pendant group]]
*[[Chemistry:Residue|Residue]]
*[[Chemistry:Substituent|Substituent]]
*[[Physics:Backbone-dependent rotamer library|Backbone-dependent rotamer library]]

==References==
{{reflist}}

[[Category:Organic chemistry]]

{{Sourceattribution|Side chain}}