Editing Fatty acid (section)

==Nomenclature==

===Carbon atom numbering===
{{See also|Essential fatty acid#Nomenclature and terminology}}
[[File:Fatty acid carbon numbering.svg|thumb|upright=2|Numbering of carbon atoms. The systematic (IUPAC) C-''x'' numbers are in blue. The omega-minus "ω−''x''" labels are in red. The Greek letter labels are in green.{{efn|name=note.omega}} Note that [[#Unsaturated fatty acids|unsaturated fatty acids]] with a ''cis'' configuration are actually "kinked" rather than straight as shown here.]]

Most naturally occurring fatty acids have an [[branched chain fatty acids|unbranched chain]] of carbon atoms, with a [[carboxyl group]] (–COOH) at one end, and a [[methyl group]] (–CH3) at the other end.

The position of each carbon atom in the backbone of a fatty acid is usually indicated by counting from 1 at the −COOH end. Carbon number ''x'' is often abbreviated C-''x'' (or sometimes C''x''), with ''x'' = 1, 2, 3, etc. This is the numbering scheme recommended by the [[IUPAC]].

Another convention uses letters of the [[Greek alphabet]] in sequence, starting with the first carbon ''after'' the carboxyl group. Thus carbon α ([[alpha]]) is C-2, carbon β ([[beta]]) is C-3, and so forth.

Although fatty acids can be of diverse lengths, in this second convention the last carbon in the chain is always labelled as ω ([[omega]]), which is the last letter in the Greek alphabet. A third numbering convention counts the carbons from that end, using the labels "ω", "ω−1", "ω−2". Alternatively, the label "ω−''x''" is written "n−''x''", where the "n" is meant to represent the number of carbons in the chain.{{efn|name=note.omega|A common mistake is to say that the last carbon is "ω−1".<br />Another common mistake is to say that the position of a bond in omega-notation is the number of the carbon closest to the END.<br />For double bonds, these two mistakes happen to compensate each other; so that a "ω−3" fatty acid indeed has the double bond between the 3rd and 4th carbons from the end, counting the methyl as 1.<br />However, for substitutions and other purposes, they don't: a hydroxyl "at ω−3" is on carbon 15 (4th from the end), not 16. See for example this article. {{doi|10.1016/0005-2760(75)90089-2}}<br />Note also that the "−" in the omega-notation is a minus sign, and "ω−3" should in principle be read "omega minus three". However, it is very common (especially in non-scientific literature) to write it "ω-3" (with a hyphen/dash) and read it as "omega-three". See for example Karen Dooley (2008), [https://podcasts.ufhealth.org/omega-three-fatty-acids-and-diabetes/ Omega-three fatty acids and diabetes].}}

In either numbering scheme, the position of a [[double bond]] in a fatty acid chain is always specified by giving the label of the carbon closest to the '''carboxyl''' end.{{efn|name=note.omega}} Thus, in an 18 carbon fatty acid, a double bond between C-12 (or ω−6) and C-13 (or ω−5) is said to be "at" position C-12 or ω−6. The IUPAC naming of the acid, such as "octadec-12-enoic acid" (or the more pronounceable variant "12-octadecanoic acid") is always based on the "C" numbering.

The notation Δ<sup>''x'',''y'',...</sup> is traditionally used to specify a fatty acid with double bonds at positions ''x'',''y'',.... (The capital Greek letter "Δ" ([[Delta (letter)|delta]]) corresponds to [[Latin alphabet|Roman]] "D", for '''D'''ouble bond). Thus, for example, the 20-carbon [[arachidonic acid]] is Δ<sup>5,8,11,14</sup>, meaning that it has double bonds between carbons 5 and 6, 8 and 9, 11 and 12, and 14 and 15.

In the context of human diet and fat metabolism, unsaturated fatty acids are often classified by the position of the double bond closest between to the ω carbon (only), even in the case of [[polyunsaturated fatty acid|multiple double bonds]] such as the [[essential fatty acid]]s. Thus [[linoleic acid]] (18 carbons, Δ<sup>9,12</sup>), [[gamma-Linolenic acid|γ-linole'''n'''ic acid]] (18-carbon, Δ<sup>6,9,12</sup>), and arachidonic acid (20-carbon, Δ<sup>5,8,11,14</sup>) are all classified as "ω−6" fatty acids; meaning that their [[condensed structural formula|formula]] ends with –CH=CH–{{chem|CH|2}}–{{chem|CH|2}}–{{chem|CH|2}}–{{chem|CH|2}}–{{chem|CH|3}}.

Fatty acids with an [[odd number]] of carbon atoms are called [[odd-chain fatty acid]]s, whereas the rest are even-chain fatty acids. The difference is [[gluconeogenesis#Precursors|relevant to gluconeogenesis]].

===Naming of fatty acids===
The following table describes the most common systems of naming fatty acids.

{{Clear}}
{| class="wikitable"
|-
!Nomenclature
!Examples
!Explanation
|-
!Trivial
|[[Palmitoleic acid]]
|'''[[Trivial name]]s''' (or '''common names''') are non-systematic historical names, which are the most frequent naming system used in literature. Most common fatty acids have trivial names in addition to their ''systematic names'' (see below). These names frequently do not follow any pattern, but they are concise and often unambiguous.
|-
!Systematic
|[[Oleic acid|cis-9-octadec-9-enoic acid]]<br />[[Oleic acid|(9''Z'')-octadec-9-enoic acid]]
|'''[[Systematic name]]s''' (or '''[[International Union of Pure and Applied Chemistry|IUPAC]] names''') derive from the standard ''[[IUPAC nomenclature of organic chemistry|IUPAC Rules for the Nomenclature of Organic Chemistry]]'', published in 1979,<ref name="nomenclature-1979">{{cite book |title=Nomenclature of Organic Chemistry |last1=Rigaudy|first1= J. |last2=Klesney|first2=S. P. |publisher=[[Pergamon]] |year=1979 |isbn=978-0-08-022369-8 |oclc=5008199}}</ref> along with a recommendation published specifically for lipids in 1977.<ref name="nomenclature-1977">{{cite journal |year=1977 |title=The Nomenclature of Lipids. Recommendations, 1976 |volume=79 |issue=1 |pages=11–21 |doi=10.1111/j.1432-1033.1977.tb11778.x  |journal=[[European Journal of Biochemistry]]|doi-access=free }}</ref> [[#Numbering of the carbon atoms in a fatty acid|Carbon atom numbering]] begins from the [[carboxylic group|carboxylic]] end of the molecule backbone. [[Double bond]]s are labelled with [[cis-trans isomerism|''cis''-/''trans''-]] notation or ''[[E-Z notation|E]]''-/''[[E-Z notation|Z]]''- notation, where appropriate. This notation is generally more verbose than common nomenclature, but has the advantage of being more technically clear and descriptive.
|-
!Δ<sup>''x''</sup>
|[[Linoleic acid|''cis''-Δ<sup>9</sup>, ''cis''-Δ<sup>12</sup> octadecadienoic acid]]
|In '''Δ<sup>''x''</sup>''' (or '''delta-''x''''') '''nomenclature''', each double bond is indicated by Δ<sup>''x''</sup>, where the double bond begins at the ''x''th carbon–carbon bond, [[#Numbering of the carbon atoms in a fatty acid|counting]] from [[carboxylic group|carboxylic]] end of the molecule backbone. Each double bond is preceded by a ''[[cis-trans isomerism|cis]]''- or ''[[cis-trans isomerism|trans]]''- prefix, indicating the configuration of the molecule around the bond. For example, [[linoleic acid]] is designated "''cis''-Δ<sup>9</sup>, ''cis''-Δ<sup>12</sup> octadecadienoic acid". This nomenclature has the advantage of being less verbose than systematic nomenclature, but is no more technically clear or descriptive.{{citation needed|reason=citation needed for this verbose delta notation, sometimes in chemical it's seen simpler notation with single delta, locants in superscript comma-separated, for various types of unsaturated natural organic compounds |date=September 2018}}
|-
!''n''−''x'' <br />(or ω−''x'')
|[[Omega-3 fatty acid|''n''−3]]<br />(or [[Omega-3 fatty acid|ω−3]])
|'''''n''−''x''''' ('''''n'' minus ''x'''''; also '''ω−''x''''' or '''omega−''x''''') '''nomenclature''' both provides names for individual compounds and classifies them by their likely biosynthetic properties in animals. A double bond is located on the ''x''<sup>th</sup> carbon–carbon bond, [[#Numbering of the carbon atoms in a fatty acid|counting]] from the [[Methyl group|methyl]] end of the molecule backbone. For example, [[α-Linolenic acid|α-linolenic acid]] is classified as a [[omega-3 fatty acid|''n''−3]] or [[omega-3|omega−3]] fatty acid, and so it is likely to share a biosynthetic pathway with other compounds of this type. The ω−''x'', omega−''x'', or "omega" notation is common in popular nutritional literature, but [[IUPAC nomenclature|IUPAC]] has deprecated it in favor of ''n''−''x'' notation in technical documents.<ref name="nomenclature-1979" /> The most commonly researched fatty acid biosynthetic pathways are [[omega-3 fatty acid|''n''−3]] and [[omega-6 fatty acid|''n''−6]].
|-
!Lipid numbers
|18:3<br />[[α-Linolenic acid|18:3n3]]<br />[[α-Linolenic acid|18:3,&nbsp;''cis'',''cis'',''cis''-Δ<sup>9</sup>,Δ<sup>12</sup>,Δ<sup>15</sup>]]<br />[[α-Linolenic acid|18:3(9,12,15)]]
|'''Lipid numbers''' take the form ''C'':''D'',{{efn|name=c:d}} where ''C'' is the number of carbon atoms in the fatty acid and ''D'' is the number of double bonds in the fatty acid. If D is more than one, the double bonds are assumed to be interrupted by [[methylene bridge|{{chem|CH|2}} units]], ''i.e.'', at intervals of 3 carbon atoms along the chain. For instance, [[α-Linolenic acid|α-linolenic acid]] is an 18:3 fatty acid and its three double bonds are located at positions Δ<sup>9</sup>, Δ<sup>12</sup>, and Δ<sup>15</sup>. This notation can be ambiguous, as some different fatty acids can have the same ''C'':''D'' numbers. Consequently, when ambiguity exists this notation is usually paired with either a Δ<sup>''x''</sup> or ''n''−''x'' term.<ref name="nomenclature-1979" /> For instance, although [[α-Linolenic acid|α-linolenic acid]] and [[gamma-Linolenic acid|γ-linolenic acid]] are both 18:3, they may be unambiguously described as 18:3n3 and 18:3n6 fatty acids, respectively. For the same purpose, IUPAC recommends using a list of double bond positions in parentheses, appended to the C:D notation.<ref name="IUPAClipid">{{cite web |title=IUPAC Lipid nomenclature: Appendix A: names of and symbols for higher fatty acids |url=http://www.sbcs.qmul.ac.uk/iupac/lipid/appABC.html#appA |website=www.sbcs.qmul.ac.uk}}</ref> For instance, IUPAC recommended notations for α- and γ-linolenic acid are 18:3(9,12,15) and 18:3(6,9,12), respectively.
|}

=== Free fatty acids ===
{{See also|Deep frying#Oil deterioration and chemical changes}}
When [[circulatory system|circulating]] in the [[blood plasma|plasma]] (plasma fatty acids), not in their [[ester]], fatty acids are known as non-esterified fatty acids (NEFAs) or free fatty acids (FFAs). FFAs are always bound to a [[transport protein]], such as [[albumin]].<ref name="Dorlands">{{cite book|title=Dorland's Illustrated Medical Dictionary |publisher=[[Elsevier]] |url=http://dorlands.com/}}</ref>

FFAs also form from [[triglyceride]] food oils and fats by hydrolysis, contributing to the characteristic [[Rancidification|rancid]] odor.<ref>{{Cite journal|last1=Mariod|first1=Abdalbasit|last2=Omer|first2=Nuha|last3=Al|first3=El Mugdad|first4=Mohammed|last4=Mokhtar |date=2014-09-09|title=Chemical Reactions Taken Place During deep-fat Frying and Their Products: A review|url=https://www.researchgate.net/publication/270727167|journal=Sudan University of Science & Technology SUST Journal of Natural and Medical Sciences|volume=Supplementary issue|pages=1–17}}</ref> An analogous process happens in [[biodiesel]] with risk of part corrosion.