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IUPAC definition

Main chain or Backbone
That linear chain to which all other chains, long or short or both,
may be regarded as being pendant.
Note: Where two or more chains
could equally be considered to be the main chain, that one is
selected which leads to the simplest representation of the
molecule.^[1]

In polymer science, the polymer chain or simply backbone of a polymer is the main chain of a polymer. Polymers are often classified according to the elements in the main chains. The character of the backbone, i.e. its flexibility, determines the properties of the polymer (such as the glass transition temperature). For example, in polysiloxanes (silicone), the backbone chain is very flexible, which results in a very low glass transition temperature of −123 °C (−189 °F; 150 K).^[2] The polymers with rigid backbones are prone to crystallization (e.g. polythiophenes) in thin films and in solution. Crystallization in its turn affects the optical properties of the polymers, its optical band gap and electronic levels.^[3]

Organic polymers

Common synthetic polymers have main chains composed of carbon, i.e. C-C-C-C.... Examples include polyolefins such as polyethylene ((CH₂CH₂)_n) and many substituted derivative ((CH₂CH(R))_n) such as polystyrene (R = C₆H₅), polypropylene (R = CH₃), and acrylates (R = CO₂R').

Other major classes of organic polymers are polyesters and polyamides. They have respectively -C(O)-O- and -C(O)-NH- groups in their backbones in addition to chains of carbon. Major commercial products are polyethyleneterephthalate ("PET"), ((C₆H₄CO₂C₂H₄OC(O))_n) and nylon-6 ((NH(CH₂)₅C(O))_n).

Inorganic polymers

Polydimethylsiloxane is classified as an "inorganic polymer", because the backbone lacks carbon.

Siloxanes are a premier example of an inorganic polymer, even though they have extensive organic substituents. Their backbond is composed of alternating silicon and oxygen atoms, i.e. Si-O-Si-O... The silicon atoms bear two substituents, usually methyl as in the case of polydimethylsiloxane. Some uncommon but illustrative inorganic polymers include polythiazyl ((SN)x) with alternating S and N atoms, and polyphosphates ((PO₃⁻)_n).

Biopolymers

Major families of biopolymers are polysaccharides (carbohydrates), peptides, and polynucleotides. Many variants of each are known.^[4]

Proteins and peptides

Proteins are characterized by amide linkages (-N(H)-C(O)-) formed by the condensation of amino acids. The sequence of the amino acids in the polypeptide backbone is known as the primary structure of the protein. Like almost all polymers, protein fold and twist, forming into the secondary structure, which is rigidified by hydrogen bonding between the carbonyl oxygens and amide hydrogens in the backbone, i.e. C=O---HN. Further interactions between residues of the individual amino acids form the protein's tertiary structure. For this reason, the primary structure of the amino acids in the polypeptide backbone is the map of the final structure of a protein, and it therefore indicates its biological function.^[5]^[4] Spatial positions of backbone atoms can be reconstructed from the positions of alpha carbons using computational tools for the backbone reconstruction.^[6]

Carbohydrates

Carbohydrates arise by condensation of monosaccharides such as glucose. The polymers can be classified into oligosaccharides (up to 10 residues) and polysaccharides (up to about 50,000 residues). The backbone chain is characterized by an ether bond between individual monosaccharides. This bond is called the glycosidic linkage.^[7] These backbone chains can be unbranched (containing one linear chain) or branched (containing multiple chains). The glycosidic linkages are designated as alpha or beta depending on the relative stereochemistry of the anomeric (or most oxidized) carbon. In a Fischer Projection, if the glycosidic linkage is on the same side or face as carbon 6 of a common biological saccharide, the carbohydrate is designated as beta and if the linkage is on the opposite side it is designated as alpha. In a traditional "chair structure" projection, if the linkage is on the same plane (equatorial or axial) as carbon 6 it is designated as beta and on the opposite plane it is designated as alpha. This is exemplified in sucrose (table sugar) which contains a linkage that is alpha to glucose and beta to fructose. Generally, carbohydrates which our bodies break down are alpha-linked (example: glycogen) and those which have structural function are beta-linked (example: cellulose).^[4]^[8]

Nucleic acids

Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the main examples of polynucleotides. They arise by condensation of nucleotides. Their backbones form by the condensation of a hydroxy group on a ribose with the phosphate group on another ribose. This linkage is called a phosphodiester bond. The condensation is catalyzed by enzymes called polymerases. DNA and RNA can be millions of nucleotides long thus allowing for the genetic diversity of life. The bases project from the pentose-phosphate polymer backbone and are hydrogen bonded in pairs to their complementary partners (A with T and G with C). This creates a double helix with pentose phosphate backbones on either side, thus forming a secondary structure.^[9]^[4]^[10]

References

^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "main chain (backbone) of a polymer". doi:10.1351/goldbook.M03694
^ "Polymers". Archived from the original on 2015-10-02. Retrieved 2015-09-17.
^ Brabec, C.J.; Winder, C.; Scharber, M.C; Sarıçiftçi, S.N.; Hummelen, J.C.; Svensson, M.; Andersson, M.R. (2001). "Influence of disorder on the photoinduced excitations in phenyl substituted polythiophenes" (PDF). Journal of Chemical Physics. 115 (15): 7235. Bibcode:2001JChPh.115.7235B. doi:10.1063/1.1404984.
^ ^a ^b ^c ^d Voet, Donald; Voet, Judith G.; Pratt, Charlotte W. (2016). Fundamentals of Biochemistry: Life at the Molecular Level (5th ed.). Wiley. ISBN 978-1-118-91840-1.V
^ Berg JM, Tymoczko JL, Stryer L (2002). "3.2 Primary Structure: Amino Acids Are Linked by Peptide Bonds to Form Polypeptide Chains". Biochemistry (5th ed.). W.H. Freeman. ISBN 0-7167-3051-0. NBK22364.
^ Badaczewska-Dawid, Aleksandra E.; Kolinski, Andrzej; Kmiecik, Sebastian (2020). "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. ISSN 2001-0370. PMC 6961067. PMID 31969975.
^ Buschiazzo, Alejandro (2004). "Crystal structure of glycogen synthase: homologous enzymes catalyze glycogen synthesis and degradation". The EMBO Journal. 23 (16): 3196–3205. doi:10.1038/sj.emboj.7600324. PMC 514502. PMID 15272305.
^ Bertozzi CR, Rabuka D (2009). "Structural Basis of Glycan Diversity". In Varki A, Cummings RD, Esko JD, et al. (eds.). Essentials of Glycobiology (2nd ed.). Cold Spring Harbor Laboratory Press. ISBN 9780879697709. PMID 20301274.
^ Alberts B, Johnson A, Lewis J, et al. (2002). "DNA Replication Mechanisms". Molecular Biology of the Cell (4th ed.). Garland Science. ISBN 0-8153-3218-1. NBK26850.
^ Lodish H, Berk A, Zipursky SL, et al. (2000). "4.1, Structure of Nucleic Acids". Molecular Cell Biology (4th ed.). W.H. Freeman. ISBN 0-7167-3136-3. NBK21514.

@@ Line 1: / Line 1: @@
+{{short description|Longest chain of covalently-bonded atoms in a polymer}}
-{{Refimprove|date=February 2014}}
 {{Quote box
  |title = [[International Union of Pure and Applied Chemistry|IUPAC]] definition
- |quote = '''Main chain'''<br/>'''Backbone'''<br/>That linear chain to which all other chains, long or short or both,<br/>may be regarded as being pendant.
+ |quote = '''Main chain''' or '''Backbone'''<br />That linear chain to which all other chains, long or short or both,<br />may be regarded as being pendant.
-''Note'': Where two or more chains could equally be considered to be the<br/>main chain, that one is selected which leads to the simplest representation<br/>of the molecule.<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}}</ref>
+''Note'': Where two or more chains <br /> could equally be considered to be the main chain, that one is <br />selected which leads to the simplest representation of the <br />molecule.<ref>{{GoldBookRef |title=main chain (backbone) ''of a polymer'' |file=M03694 }}</ref>
 }}
+In [[polymer science]], the '''polymer chain''' or simply '''backbone''' of a [[polymer]] is the main chain of a polymer. Polymers are often classified according to the elements in the main chains. The character of the backbone, i.e. its flexibility, determines the properties of the polymer (such as the [[glass transition]] temperature). For example, in [[Silicone|polysiloxanes]] (silicone), the backbone chain is very flexible, which results in a very low [[glass transition]] temperature of {{Cvt|-123|C|F K}}.<ref>{{Cite web |url=http://courses.chem.psu.edu/chem112/materials/polymers.html |title=Polymers |access-date=2015-09-17 |archive-url=https://web.archive.org/web/20151002172625/http://courses.chem.psu.edu/chem112/materials/polymers.html |archive-date=2015-10-02 |url-status=dead }}</ref> The polymers with rigid backbones are prone to [[crystallization]] (e.g. [[polythiophenes]]) in [[thin film]]s and in [[Solution (chemistry)|solution]]. Crystallization in its turn affects the optical properties of the polymers, its optical [[band gap]] and electronic levels.<ref>{{cite journal|last1=Brabec|first1=C.J.|last2=Winder|first2=C.|last3=Scharber|first3=M.C|last4=Sarıçiftçi|first4=S.N.|last5=Hummelen|first5=J.C.|last6=Svensson|first6=M.|last7=Andersson|first7=M.R.|title=Influence of disorder on the photoinduced excitations in phenyl substituted polythiophenes|journal=Journal of Chemical Physics |date=2001|volume=115|issue=15|page=7235|doi=10.1063/1.1404984|bibcode=2001JChPh.115.7235B|url=https://pure.rug.nl/ws/files/6636890/2001BrabecJChemPhys.pdf|author4-link=Niyazi Serdar Sarıçiftçi}}</ref>
-In [[polymer science]], the backbone chain of a [[polymer]] is the longest series of [[Covalent bond|covalently bonded]] atoms that together create the continuous chain of the [[molecule]]. This science is subdivided into the study of organic polymers, which consist of a carbon backbone, and [[inorganic polymer]]s which have backbones containing only [[Main-group element|main group]] elements.
-[[File:Prtn backbone 3.svg|thumb|An example of a biological backbone (polypeptide)]]
-In [[biochemistry]], organic backbone chains make up the primary structure of [[macromolecule]]s. The backbones of these biological macromolecules consist of central chains of covalently bonded atoms. The characteristics and order of the monomer residues in the backbone make a map for the complex structure biological polymers. The backbone is, therefore, directly related to biological molecules’ function. The macromolecules within the body can be divided into four main subcategories, each of which are involved in very different and important biological processes: [[Protein]]s, [[Carbohydrate]]s, [[Lipid]]s, and [[Nucleic acid]]s.<ref name=":0">Voet, Donald, Judith G. Voet, and Charlotte W. Pratt. ''Fundamentals of Biochemistry: Life at the Molecular Level''. 5th ed. Hoboken, NJ: Wiley, 2008. Print</ref> Each of these molecules has a different backbone and consists of different monomers each with distinctive residues and functionalities. This is the driving factor of their different structures and functions in the body. Although lipids have a "backbone," they are not true biological polymers as their backbone is a three carbon molecule, [[glycerol]], with longer substituent "side chains." For this reason, only proteins, carbohydrates, and nucleic acids should be considered as biological macromolecules with polymeric backbones.<ref>Cox RA, García-Palmieri MR. Cholesterol, Triglycerides, and Associated Lipoproteins. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Boston: Butterworths; 1990. Chapter 31. Available from: <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK351/</nowiki></ref>
+==Organic polymers==
-==Character of the backbone==
+:[[File:Polystyrene formation.PNG|left|thumb|390 px|Formation of polystyrene, a polymer with an organic backbone.]]
-<u>Polymer Chemistry:</u>
+Common synthetic polymers have main chains composed of carbon, i.e. C-C-C-C....  Examples include [[polyolefin]]s such as [[polyethylene]] ((CH<sub>2</sub>CH<sub>2</sub>)<sub>n</sub>) and many substituted derivative ((CH<sub>2</sub>CH(R))<sub>n</sub>) such as [[polystyrene]] (R = C<sub>6</sub>H<sub>5</sub>), [[polypropylene]] (R = CH<sub>3</sub>), and [[acrylate]]s (R = CO<sub>2</sub>R').
+Other major classes of organic polymers are [[polyester]]s and [[polyamide]]s.  They have respectively -C(O)-O- and -C(O)-NH- groups in their backbones in addition to chains of carbon.  Major commercial products are [[polyethyleneterephthalate]] ("PET"), ((C<sub>6</sub>H<sub>4</sub>CO<sub>2</sub>C<sub>2</sub>H<sub>4</sub>OC(O))<sub>n</sub>) and [[nylon-6]] ((NH(CH<sub>2</sub>)<sub>5</sub>C(O))<sub>n</sub>).
-The character of the backbone chain depends on the type of polymerization: in [[step-growth polymerization]], the [[monomer]] moiety becomes the backbone, and thus the backbone is typically functional. These include [[polythiophenes]] or low band gap polymers in [[organic semiconductor]]s.<ref>{{cite journal|last1=Budgaard|first1=Eva|last2=Krebs|first2=Frederik|title=Low band gap polymers for organic photovoltaics|journal=Solar Energy Materials and Solar Cells|date=2006|volume=91|issue=11|pages=954–985}}</ref> In [[chain-growth polymerization]], typically applied for [[alkenes]], the backbone is not functional, but bears the functional [[side chains]] or pendant groups.
+==Inorganic polymers==
-The character of the backbone, i.e. its flexibility, determines the thermal properties of the polymer (such as the [[glass transition]] temperature). For example, in polisiloxanes, the backbone chain is very flexible, which results in a very low glass transition temperature of -123&nbsp;°C.<ref>[http://courses.chem.psu.edu/chem112/materials/polymers.html Polymers]</ref> The polymers with rigid backbones are prone to [[crystallization]] (e.g. [[polythiophenes]]) in [[thin film]]s and in [[solution]]. Crystallization in its turn affects the optical properties of the polymers, its optical [[band gap]] and electronic levels.<ref>{{cite journal|last1=Brabec|first1=C.J.|last2=Winder|first2=C.|last3=Scharber|first3=M.C|last4=[[Niyazi Serdar Sarıçiftçi|Sarıçiftçi]]|first4=S.N.|last5=Hummelen|first5=J.C.|last6=Svensson|first6=M.|last7=Andersson|first7=M.R.|title=Influence of disorder on the photoinduced excitations in phenyl substituted polythiophenes|journal=Journal of Chemical Physics |date=2001|volume=115|page=7235|doi=10.1063/1.1404984}}</ref>
+[[File:PmdsStructure.png|230px|right|Polydimethylsiloxane is classified as an "[[inorganic polymer]]", because the backbone lacks carbon.|thumb]]
+[[Siloxane]]s are a premier example of an inorganic polymer, even though they have extensive organic substituents.  Their backbond is composed of alternating silicon and oxygen atoms, i.e. Si-O-Si-O...  The silicon atoms bear two substituents, usually [[methyl]] as in the case of [[polydimethylsiloxane]].  Some uncommon but illustrative inorganic polymers include [[polythiazyl]] ((SN)x) with alternating S and N atoms, and polyphosphates ((PO<sub>3</sub><sup>−</sup>)<sub>n</sub>).
+==Biopolymers==
-<u>Biochemistry:</u>
+Major families of biopolymers are [[polysaccharide]]s (carbohydrates), [[peptide]]s, and [[polynucleotide]]s.  Many variants of each are known.<ref name=Voet16>{{cite book |first1=Donald |last1=Voet |first2=Judith G. |last2=Voet |first3=Charlotte W. |last3=Pratt |title=Fundamentals of Biochemistry: Life at the Molecular Level |url=https://books.google.com/books?id=9T7hCgAAQBAJ |date=2016 |publisher=Wiley |edition=5th |isbn=978-1-118-91840-1}}V</ref>
+===Proteins and peptides===
-There are some similarities and many differences inherent in the character of biopolymer backbones. The backbone of each of the three biological polymers; [[Protein|proteins]], [[Carbohydrate|carbohydrates]], and [[Nucleic acid|nucleic acids]], is formed through a net [[condensation reaction]]. In a condensation reaction, monomers are covalently connected along with the loss of some small molecule, most commonly water.<ref>IUPAC Gold Book: http://goldbook.iupac.org/C01238.html</ref> Because they are polymerized through complex enzymatic mechanisms, none of the biopolymers' backbones are formed through the elimination of water but through the elimination of other small biological molecules. Each of these biopolymers can be characterized as either a [[Copolymer|heteropolymer,]] meaning it consists of more than one monomer ordered in the backbone chain, or a homopolymer, which consists of just one repeating monomer. [[Peptide|Polypeptides]] and [[Nucleic acid|nucleic acids]] are very commonly heteropolymers whereas common carbohydrate macromolecules such as [[glycogen]] can be homopolymers. This is because the chemical differences of peptide and nucleotide monomers determines the biological function of their polymers whereas common carbohydrate monomers have one general function such as for energy storage and delivery.
+Proteins are characterized by [[Peptide bond|amide linkages]] (-N(H)-C(O)-) formed by the condensation of [[amino acid]]s. The sequence of the amino acids in the polypeptide backbone is known as the [[Protein primary structure|primary structure]] of the protein. Like almost all polymers, protein fold and twist, forming into the [[Protein secondary structure|secondary structure]], which is rigidified by [[hydrogen bonding]] between the [[Carbonyl group|carbonyl]] oxygens and amide hydrogens in the backbone, i.e. C=O---HN. Further interactions between residues of the individual amino acids form the protein's [[Protein tertiary structure|tertiary structure]]. For this reason, the primary structure of the amino acids in the polypeptide backbone is the map of the final structure of a protein, and it therefore indicates its biological function.<ref>{{cite book |vauthors=Berg JM, Tymoczko JL, Stryer L |chapter=3.2 Primary Structure: Amino Acids Are Linked by Peptide Bonds to Form Polypeptide Chains |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK22364/ |id=NBK22364 |title=Biochemistry |publisher=W.H. Freeman |edition=5th |year=2002 |isbn=0-7167-3051-0 |url=https://www.ncbi.nlm.nih.gov/books/NBK21154/}}</ref><ref name=Voet16 /> Spatial positions of backbone atoms can be reconstructed from the positions of alpha carbons using computational tools for the backbone reconstruction.<ref>{{Cite journal|last=Badaczewska-Dawid|first=Aleksandra E.|last2=Kolinski|first2=Andrzej|last3=Kmiecik|first3=Sebastian|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|year=2020}}</ref>
+[[File:Sucrose condensation.svg|thumb|A simplified example of condensation showing the ''alpha'' and ''beta'' classification. [[Glucose]] and [[fructose]] form [[sucrose]]. The synthesis of glycogen in the body is driven by the enzyme [[glycogen synthase]] which uses a [[uridine diphosphate]] (UDP) leaving group.]]
+=== Carbohydrates ===
-==Overview of common backbones==
+Carbohydrates arise by condensation of [[monosaccharide]]s such as [[glucose]]. The polymers can be classified into [[oligosaccharide]]s (up to 10 residues) and [[polysaccharide]]s (up to about 50,000 residues). The backbone chain is characterized by an ether bond between individual monosaccharides.  This bond is called the [[Glycosidic bond|glycosidic linkage]].<ref>{{Cite journal|last=Buschiazzo|first=Alejandro|year=2004|title=Crystal structure of glycogen synthase: homologous enzymes catalyze glycogen synthesis and degradation|journal=The EMBO Journal |volume=23|issue=16|pages=3196–3205|doi=10.1038/sj.emboj.7600324|pmc=514502|pmid=15272305}}</ref> These backbone chains can be unbranched (containing one linear chain) or branched (containing multiple chains). The glycosidic linkages are designated as [[Anomer|''alpha'' or ''beta'']] depending on the relative [[stereochemistry]] of the [[anomer]]ic (or most [[oxidized]]) carbon. In a [[Fischer projection|Fischer Projection]], if the glycosidic linkage is on the same side or face as carbon 6 of a common biological saccharide, the carbohydrate is designated as ''beta'' and if the linkage is on the opposite side it is designated as ''alpha''. In a traditional "[[Cyclohexane conformation|chair structure]]" projection, if the linkage is on the same plane (equatorial or axial) as carbon 6 it is designated as ''beta'' and on the opposite plane it is designated as ''alpha''. This is exemplified in [[sucrose]] (table sugar) which contains a linkage that is ''alpha'' to glucose and ''beta'' to [[fructose]]. Generally, carbohydrates which our bodies break down are ''alpha''-linked [[Glycogen|(example: glycogen)]] and those which have structural function are ''beta''-linked (example: [[cellulose]]).<ref name=Voet16 /><ref>{{cite book |vauthors=Bertozzi CR, Rabuka D |chapter=Structural Basis of Glycan Diversity |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK1955/ |veditors=Varki A, Cummings RD, Esko JD, et al |title=Essentials of Glycobiology |publisher=Cold Spring Harbor Laboratory Press |year=2009 |isbn=9780879697709 |edition=2nd |url=https://www.ncbi.nlm.nih.gov/books/NBK1908/ |pmid=20301274}}</ref>
-<u>In polymer chemistry:</u>
-*saturated alkane (typical for [[vinyl]] polymers)
-*step-growth polymers ([[polyaniline]], [[polythiophene]], [[PEDOT]]) backbone. These often have derivatized [[heterocyclic compound|heterocycles]] as monomers, such as [[thiophene]]s, [[diazole]]s or [[pyrrole]]s.
-*[[fullerene]] backbone<ref>{{cite journal|last1=Hirsch|first1=Andreas|title=Fullerene polymers|journal=Advanced Materials|date=1993|volume=5|issue=11|pages=859–861|doi=10.1002/adma.19930051116}}</ref>
-[[File:Polypeptide condensation.svg|thumb|A simplified condensation reaction between two amino acids forming a polypeptide backbone. This occurs  in the ribosome through a complex catalytic mechanism involving the liberation of tRNA.]]
-<u>In Biology:</u>
-* '''Proteins (polypeptides)'''
-Proteins are important biological molecules and play an integral role in the structure and function of [[virus]]es, [[bacteria]], and [[Eukaryote|eukaryotic cells.]] Their backbones are characterized by [[Peptide bond|amide linkages]] formed by the polymerization between [[Amine|amino]] and [[carboxylic acid]] groups attached to the alpha carbon of each of the twenty [[amino acid]]s. These amino acid sequences are translated from cellular [[Messenger RNA|mRNAs]] by [[ribosome]]s in the [[cytoplasm]] of the cell.<ref>{{cite journal | last1 = Noller | first1 = HF | year = 2017 | title = The parable of the caveman and the Ferrari: protein synthesis and the RNA world | doi = 10.1098/rstb.2016.0187 | journal = Phil. Trans. R. Soc. B | volume = 372 | issue = | page = 20160187 }}</ref> The ribosomes have enzymatic activity which directs the condensation reaction forming the amide linkage between each successive amino acid. This happens during a biological process known as [[Translation (biology)|translation]]. In this enzymatic mechanism a covalently bonded [[Transfer RNA|tRNA]] shuttle acts as the leaving group for the condensation reaction. The newly liberated tRNA can "pick up" another peptide and continuously participate in this reaction.<ref>{{Cite journal|last=Weinger|first=Joshua|year=2006|title=Participation of the tRNA A76 hydroxyl groups throughout translation|journal=Biochemistry |volume=45 |pages=5939–5948 |doi=10.1021/bi060183n |pmc=2522371 |pmid=16681365}}</ref> The sequence of the amino acids in the polypeptide backbone is known as the [[Protein primary structure|primary structure]] of the protein. This primary structure leads to folding of the protein into the [[Protein secondary structure|secondary structure]], formed by hydrogen bonding between the [[Carbonyl group|carbonyl]] oxygens and amine hydrogens in the backbone. Further interactions between residues of the individual amino acids form the protein's [[Protein tertiary structure|tertiary structure]]. For this reason, the primary structure of the amino acids in the polypeptide backbone is the map of the final structure of a protein, and it therefore indicates its biological function.<ref name=":1">{{cite web|url=http://www.ncbi.nlm.nih.gov/books/NBK22364/|title=Biochemistry. 5th edition.Section 3.2 Primary Structure: Amino Acids Are Linked by Peptide Bonds to Form Polypeptide Chains|website=NCBI Bookshelf|accessdate=10 September 2015}}</ref><ref name=":0" />
-[[File:Sucrose condensation.svg|thumb|A simplified example condensation showing Alpha and Beta classification. Glucose and fructose form sucrose. The synthesis of glycogen in the body is driven by the enzyme glycogen synthase which uses a UDP leaving group.]]
-* '''Carbohydrates'''
-Carbohydrates have many roles in the body including functioning as structural units, [[Cofactor (biochemistry)|enzyme cofactors]] and cell surface [[Receptor (biochemistry)|recognition sites]]. Their most prevalent role is in energy storage and delivery in cellular [[metabolic pathway]]s. The most simple carbohydrates are single sugar residues called [[monosaccharide]]s like [[glucose]], our body’s energy delivery molecule. [[Oligosaccharide]]s (up to 10 residues) and [[polysaccharide]]s (up to about 50,000 residues) consist of saccharide residues bonded in a backbone chain, which is characterized by an ether bond known as a [[Glycosidic bond|glycosidic linkage]]. In the body's formation of [[glycogen]], the energy storage polymer, this glycosidic linkage is formed by the enzyme [[glycogen synthase]]. The mechanism of this enzymatically driven condensation reaction is not well studied but it is known that the molecule [[Uridine diphosphate glucose|UDP]] acts as an intermediary linker and is lost in the synthesis.<ref>{{Cite journal|last=Buschiazzo|first=Alejandro|year=2004|title=Crystal structure of glycogen synthase: homologous enzymes catalyze glycogen synthesis and degradation|journal=The EMBO Journal |volume=23|pages=3196|via=|doi=10.1038/sj.emboj.7600324|pmc=514502|pmid=15272305}}</ref> These backbone chains can be unbranched (containing one linear chain) or branched (containing multiple chains). The glycosidic linkages are designated as [[Anomer|Alpha or Beta]] depending on the relative [[stereochemistry]] of the [[anomer]]ic (or most oxidized) carbon. In a [[Fischer projection|Fischer Projection,]] if the glycosidic linkage is on the same side or face as carbon 6 of a common biological saccharide, the carbohydrate is designated as Beta and if the linkage is on the opposite side it is designated as Alpha. In a traditional "[[Cyclohexane conformation|chair structure]]" projection, if the linkage is on the same plane (equatorial or axial) as Carbon 6 it is designated as Beta and on the opposite plain it is designated as Alpha. This is exemplified in [[sucrose]] (table sugar) which contains a linkage that is alpha to glucose and beta to [[fructose]]. Generally, carbohydrates which our bodies break down are alpha-linked [[Glycogen|(example: glycogen)]] and those which have structural function are beta-linked [[Cellulose|(example: cellulose)]].<ref name=":0" /><ref name=":2">Bertozzi CR, Rabuka D. Structural Basis of Glycan Diversity. In: Varki A, Cummings RD, Esko JD, et al., editors. Essentials of Glycobiology. 2nd edition. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 2009. Chapter 2. Available from: https://www.ncbi.nlm.nih.gov/books/NBK1955/</ref>
-* '''Nucleic Acids'''
+=== Nucleic acids ===
-[[File:DNA condensation.svg|thumb|In this Condensation of Adenine and Guanine forming a phosphodiester bond, the triphosphorylated ribose of the incoming nucleotide is attacked by the 3' hydroxyl of the polymer, releasing pyrophosphate.]]
+[[File:DNA condensation.svg|thumb|Condensation of [[adenine]] and [[guanine]] forming a [[phosphodiester bond]], the [[Nucleoside triphosphate|triphosphorylated ribose]] of the incoming nucleotide is attacked by the 3' [[Hydroxy group|hydroxyl]] of the polymer, releasing [[pyrophosphate]].]]
-Nucleic acids [[DNA]] and [[RNA]] are of great importance because they code for the production of all cellular proteins. They are made up of monomers called [[nucleotide]]s which consist of an organic base: A, G, C and T or U, a pentose sugar, and a phosphate group. They have backbones in which the 3’ carbon of the [[ribose]] sugar is connected to the [[phosphate]] group via a [[phosphodiester bond]]. This bond is formed through the with the help of a class of cellular enzymes called [[polymerase]]s. In this enzymatically driven condensation reaction all incoming nucleotides have a triphosphorylated ribose which loses a [[pyrophosphate]] group to form the inherent phosphodiester bond. This reaction is driven by the large negative free energy change associated with the release of pyrophosphate. The sequence of bases in the nucleic acid backbone is also known as the [[Nucleic acid structure|primary structure.]] Nucleic acids can be millions of nucleotides long thus leading to the genetic diversity of life. The bases stick out from the pentose-phosphate polymer backbone in DNA and are hydrogen bonded in pairs to their complementary partners (A with T and G with C). This creates a [[Nucleic acid double helix|double helix]] with pentose phosphate backbones on either side, thus forming a secondary structure.<ref>Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. DNA Replication Mechanisms. Available from: <nowiki>https://www.ncbi.nlm.nih.gov/books/NBK26850</nowiki></ref><ref name=":0" /><ref name=":3">Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 4.1, Structure of Nucleic Acids. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21514/</ref>
+[[Deoxyribonucleic acid]] (DNA) and [[RiboNucleic Acid|ribonucleic acid]] (RNA) are the main examples of [[polynucleotide]]s. They arise by condensation of nucleotides. Their backbones form by the condensation of a hydroxy group on a [[ribose]] with the [[phosphate]] group on another ribose. This linkage is called a [[phosphodiester bond]]. The condensation is catalyzed by [[enzyme]]s called [[polymerase]]s. DNA and RNA can be millions of nucleotides long thus allowing for the [[genetic diversity]] of life. The bases project from the pentose-phosphate polymer backbone and are [[hydrogen bond]]ed in pairs to their [[Complementary nucleotide|complementary]] partners (A with T and G with C). This creates a [[Nucleic acid double helix|double helix]] with pentose phosphate backbones on either side, thus forming a [[Protein secondary structure|secondary structure]].<ref>{{cite book |vauthors=Alberts B, Johnson A, Lewis J, et al |chapter=DNA Replication Mechanisms |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK26850/ |id=NBK26850 |title=Molecular Biology of the Cell |publisher=Garland Science |edition=4th |year=2002 |isbn=0-8153-3218-1 |url=https://www.ncbi.nlm.nih.gov/books/NBK21054/}}</ref><ref name=Voet16 /><ref>{{cite book |vauthors=Lodish H, Berk A, Zipursky SL, et al |chapter=4.1, Structure of Nucleic Acids |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK21514/ |title=Molecular Cell Biology |publisher=W.H. Freeman |edition=4th |year=2000 |isbn=0-7167-3136-3 |url=https://www.ncbi.nlm.nih.gov/books/NBK21475/ |id=NBK21514}}</ref>
 ==References==