Inchemistry,particularly inbiochemistry,afatty acidis acarboxylic acidwith analiphaticchain, which is eithersaturated or unsaturated.Most naturally occurring fatty acids have anunbranched chainof an even number of carbon atoms, from 4 to 28.[1]Fatty acids are a major component of the lipids (up to 70% by weight) in some species such as microalgae[2]but in some other organisms are not found in their standalone form, but instead exist as three main classes ofesters:triglycerides,phospholipids,andcholesteryl esters.In any of these forms, fatty acids are both importantdietarysources of fuel for animals and important structural components forcells.
History
editThe concept of fatty acid (acide gras) was introduced in 1813 byMichel Eugène Chevreul,[3][4][5]though he initially used some variant terms:graisse acideandacide huileux( "acid fat" and "oily acid" ).[6]
Types of fatty acids
edit
Fatty acids are classified in many ways: by length, by saturation vs unsaturation, by even vs odd carbon content, and by linear vs branched.
Length of fatty acids
edit- Short-chain fatty acids(SCFAs) are fatty acids withaliphatictails of five or fewercarbons(e.g.butyric acid).[7]
- Medium-chain fatty acids (MCFAs) are fatty acids with aliphatic tails of 6 to 12[8]carbons, which can formmedium-chain triglycerides.
- Long-chain fatty acids (LCFAs) are fatty acids with aliphatic tails of 13 to 21 carbons.[9]
- Very long chain fatty acids(VLCFAs) are fatty acids with aliphatic tails of 22 or more carbons.
Saturated fatty acids
editSaturated fatty acids have no C=C double bonds. They have the formula CH3(CH2)nCOOH, for differentn.An important saturated fatty acid isstearic acid(n= 16), which when neutralized withsodium hydroxideis the most common form ofsoap.
| Common name | Chemical structure | C:D[a] |
|---|---|---|
| Caprylic acid | CH3(CH2)6COOH | 8:0 |
| Capric acid | CH3(CH2)8COOH | 10:0 |
| Lauric acid | CH3(CH2)10COOH | 12:0 |
| Myristic acid | CH3(CH2)12COOH | 14:0 |
| Palmitic acid | CH3(CH2)14COOH | 16:0 |
| Stearic acid | CH3(CH2)16COOH | 18:0 |
| Arachidic acid | CH3(CH2)18COOH | 20:0 |
| Behenic acid | CH3(CH2)20COOH | 22:0 |
| Lignoceric acid | CH3(CH2)22COOH | 24:0 |
| Cerotic acid | CH3(CH2)24COOH | 26:0 |
Unsaturated fatty acids
editUnsaturated fatty acids have one or more C=Cdouble bonds.The C=C double bonds can give eithercisortransisomers.
- cis
- Acisconfiguration means that the two hydrogen atoms adjacent to the double bond stick out on the same side of the chain. The rigidity of the double bond freezes its conformation and, in the case of thecisisomer, causes the chain to bend and restricts the conformational freedom of the fatty acid. The more double bonds the chain has in thecisconfiguration, the less flexibility it has. When a chain has manycisbonds, it becomes quite curved in its most accessible conformations. For example,oleic acid,with one double bond, has a "kink" in it, whereaslinoleic acid,with two double bonds, has a more pronounced bend.α-Linolenic acid,with three double bonds, favors a hooked shape. The effect of this is that, in restricted environments, such as when fatty acids are part of a phospholipid in a lipid bilayer or triglycerides in lipid droplets, cis bonds limit the ability of fatty acids to be closely packed, and therefore can affect the melting temperature of the membrane or of the fat. Cis unsaturated fatty acids, however, increase cellular membrane fluidity, whereas trans unsaturated fatty acids do not.
- trans
- Atransconfiguration, by contrast, means that the adjacent two hydrogen atoms lie onoppositesides of the chain. As a result, they do not cause the chain to bend much, and their shape is similar to straight saturated fatty acids.
In most naturally occurring unsaturated fatty acids, each double bond has three (n-3), six (n-6), or nine (n-9) carbon atoms after it, and all double bonds have a cis configuration. Most fatty acids in thetransconfiguration (trans fats) are not found in nature and are the result of human processing (e.g.,hydrogenation). Some trans fatty acids also occur naturally in the milk and meat ofruminants(such as cattle and sheep). They are produced, by fermentation, in the rumen of these animals. They are also found indairy productsfrom milk of ruminants, and may be also found inbreast milkof women who obtained them from their diet.
The geometric differences between the various types of unsaturated fatty acids, as well as between saturated and unsaturated fatty acids, play an important role in biological processes, and in the construction of biological structures (such as cell membranes).
| Common name | Chemical structure | Δx[b] | C:D[a] | IUPAC[10] | n−x[c] |
|---|---|---|---|---|---|
| Myristoleic acid | CH3(CH2)3CH=CH(CH2)7COOH | cis-Δ9 | 14:1 | 14:1(9) | n−5 |
| Palmitoleic acid | CH3(CH2)5CH=CH(CH2)7COOH | cis-Δ9 | 16:1 | 16:1(9) | n−7 |
| Sapienic acid | CH3(CH2)8CH=CH(CH2)4COOH | cis-Δ6 | 16:1 | 16:1(6) | n−10 |
| Oleic acid | CH3(CH2)7CH=CH(CH2)7COOH | cis-Δ9 | 18:1 | 18:1(9) | n−9 |
| Elaidic acid | CH3(CH2)7CH=CH(CH2)7COOH | trans-Δ9 | 18:1 | 18:1(9t) | n−9 |
| Vaccenic acid | CH3(CH2)5CH=CH(CH2)9COOH | trans-Δ11 | 18:1 | 18:1(11t) | n−7 |
| Linoleic acid | CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH | cis,cis-Δ9,Δ12 | 18:2 | 18:2(9,12) | n−6 |
| Linoelaidic acid | CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH | trans,trans-Δ9,Δ12 | 18:2 | 18:2(9t,12t) | n−6 |
| α-Linolenic acid | CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH | cis,cis,cis-Δ9,Δ12,Δ15 | 18:3 | 18:3(9,12,15) | n−3 |
| Arachidonic acid | CH3(CH2)4CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOHNIST | cis,cis,cis,cis-Δ5Δ8,Δ11,Δ14 | 20:4 | 20:4(5,8,11,14) | n−6 |
| Eicosapentaenoic acid | CH3CH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOH | cis,cis,cis,cis,cis-Δ5,Δ8,Δ11,Δ14,Δ17 | 20:5 | 20:5(5,8,11,14,17) | n−3 |
| Erucic acid | CH3(CH2)7CH=CH(CH2)11COOH | cis-Δ13 | 22:1 | 22:1(13) | n−9 |
| Docosahexaenoic acid | CH3CH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)2COOH | cis,cis,cis,cis,cis,cis-Δ4,Δ7,Δ10,Δ13,Δ16,Δ19 | 22:6 | 22:6(4,7,10,13,16,19) | n−3 |
Even- vs odd-chained fatty acids
editMost fatty acids are even-chained, e.g. stearic (C18) and oleic (C18), meaning they are composed of an even number of carbon atoms. Some fatty acids have odd numbers of carbon atoms; they are referred to as odd-chained fatty acids (OCFA). The most common OCFA are the saturated C15 and C17 derivatives,pentadecanoic acidandheptadecanoic acidrespectively, which are found in dairy products.[11][12]On a molecular level, OCFAs are biosynthesized and metabolized slightly differently from the even-chained relatives.
Branching
editMost common fatty acids arestraight-chain compounds,with no additional carbon atoms bonded asside groupsto the main hydrocarbon chain.Branched-chain fatty acidscontain one or moremethyl groupsbonded to the hydrocarbon chain.
Nomenclature
editCarbon atom numbering
edit
Most naturally occurring fatty acids have anunbranched chainof carbon atoms, with acarboxyl group(–COOH) at one end, and amethyl 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 numberxis often abbreviated C-x(or sometimes Cx), withx= 1, 2, 3, etc. This is the numbering scheme recommended by theIUPAC.
Another convention uses letters of theGreek alphabetin sequence, starting with the first carbonafterthe 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.[d]
In either numbering scheme, the position of adouble bondin a fatty acid chain is always specified by giving the label of the carbon closest to thecarboxylend.[d]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 Δx,y,...is traditionally used to specify a fatty acid with double bonds at positionsx,y,.... (The capital Greek letter "Δ" (delta) corresponds toRoman"D", forDouble bond). Thus, for example, the 20-carbonarachidonic acidis Δ5,8,11,14,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 ofmultiple double bondssuch as theessential fatty acids.Thuslinoleic acid(18 carbons, Δ9,12),γ-linolenic acid(18-carbon, Δ6,9,12), and arachidonic acid (20-carbon, Δ5,8,11,14) are all classified as "ω−6" fatty acids; meaning that theirformulaends with –CH=CH–CH
2–CH
2–CH
2–CH
2–CH
3.
Fatty acids with anodd numberof carbon atoms are calledodd-chain fatty acids,whereas the rest are even-chain fatty acids. The difference isrelevant to gluconeogenesis.
Naming of fatty acids
editThe following table describes the most common systems of naming fatty acids.
| Nomenclature | Examples | Explanation |
|---|---|---|
| Trivial | Palmitoleic acid | Trivial names(orcommon 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 theirsystematic names(see below). These names frequently do not follow any pattern, but they are concise and often unambiguous. |
| Systematic | cis-9-octadec-9-enoic acid (9Z)-octadec-9-enoic acid |
Systematic names(orIUPACnames) derive from the standardIUPAC Rules for the Nomenclature of Organic Chemistry,published in 1979,[13]along with a recommendation published specifically for lipids in 1977.[14]Carbon atom numberingbegins from thecarboxylicend of the molecule backbone.Double bondsare labelled withcis-/trans-notation orE-/Z- notation, where appropriate. This notation is generally more verbose than common nomenclature, but has the advantage of being more technically clear and descriptive. |
| Δx | cis-Δ9,cis-Δ12octadecadienoic acid | InΔx(ordelta-x)nomenclature,each double bond is indicated by Δx,where the double bond begins at thexth carbon–carbon bond,countingfromcarboxylicend of the molecule backbone. Each double bond is preceded by acis- ortrans- prefix, indicating the configuration of the molecule around the bond. For example,linoleic acidis designated "cis-Δ9,cis-Δ12octadecadienoic acid ". This nomenclature has the advantage of being less verbose than systematic nomenclature, but is no more technically clear or descriptive.[citation needed] |
| n−x (or ω−x) |
n−3 (orω−3) |
n−x(nminusx;alsoω−xoromega-x)nomenclatureboth provides names for individual compounds and classifies them by their likely biosynthetic properties in animals. A double bond is located on thexthcarbon–carbon bond,countingfrom themethylend of the molecule backbone. For example,α-linolenic acidis classified as an−3oromega-3fatty 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, butIUPAChas deprecated it in favor ofn−xnotation in technical documents.[13]The most commonly researched fatty acid biosynthetic pathways aren−3andn−6. |
| Lipid numbers | 18:3 18:3n3 18:3,cis,cis,cis-Δ9,Δ12,Δ15 18:3(9,12,15) |
Lipid numberstake the formC:D,[a]whereCis the number of carbon atoms in the fatty acid andDis the number of double bonds in the fatty acid. If D is more than one, the double bonds are assumed to be interrupted byCH 2units,i.e.,at intervals of 3 carbon atoms along the chain. For instance,α-linolenic acidis an 18:3 fatty acid and its three double bonds are located at positions Δ9,Δ12,and Δ15.This notation can be ambiguous, as some different fatty acids can have the sameC:Dnumbers. Consequently, when ambiguity exists this notation is usually paired with either a Δxorn−xterm.[13]For instance, althoughα-linolenic acidandγ-linolenic acidare 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.[10]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
editWhencirculatingin theplasma(plasma fatty acids), not in theirester,fatty acids are known as non-esterified fatty acids (NEFAs) or free fatty acids (FFAs). FFAs are always bound to atransport protein,such asalbumin.[15]
FFAs also form fromtriglyceridefood oils and fats by hydrolysis, contributing to the characteristicrancidodor.[16]An analogous process happens inbiodieselwith risk of part corrosion.
Production
editIndustrial
editFatty acids are usually produced industrially by thehydrolysisoftriglycerides,with the removal ofglycerol(seeoleochemicals).Phospholipidsrepresent another source. Some fatty acids are produced synthetically byhydrocarboxylationof alkenes.[17]
By animals
editIn animals, fatty acids are formed from carbohydrates predominantly in theliver,adipose tissue,and themammary glandsduring lactation.[18]
Carbohydrates are converted intopyruvatebyglycolysisas the first important step in the conversion of carbohydrates into fatty acids.[18]Pyruvate is then decarboxylated to formacetyl-CoAin themitochondrion.However, this acetyl CoA needs to be transported intocytosolwhere the synthesis of fatty acids occurs. This cannot occur directly. To obtain cytosolic acetyl-CoA,citrate(produced by the condensation of acetyl-CoA withoxaloacetate) is removed from thecitric acid cycleand carried across the inner mitochondrial membrane into the cytosol.[18]There it is cleaved byATP citrate lyaseinto acetyl-CoA and oxaloacetate. The oxaloacetate is returned to the mitochondrion asmalate.[19]The cytosolic acetyl-CoA is carboxylated byacetyl-CoA carboxylaseintomalonyl-CoA,the first committed step in the synthesis of fatty acids.[19][20]
Malonyl-CoA is then involved in a repeating series of reactions that lengthens the growing fatty acid chain by two carbons at a time. Almost all natural fatty acids, therefore, have even numbers of carbon atoms. When synthesis is complete the free fatty acids are nearly always combined with glycerol (three fatty acids to one glycerol molecule) to formtriglycerides,the main storage form of fatty acids, and thus of energy in animals. However, fatty acids are also important components of thephospholipidsthat form thephospholipid bilayersout of which all the membranes of the cell are constructed (thecell wall,and the membranes that enclose all theorganelleswithin the cells, such as thenucleus,themitochondria,endoplasmic reticulum,and theGolgi apparatus).[18]
The "uncombined fatty acids" or "free fatty acids" found in the circulation of animals come from the breakdown (orlipolysis) of stored triglycerides.[18][21]Because they are insoluble in water, these fatty acids are transported bound to plasmaalbumin.The levels of "free fatty acids" in the blood are limited by the availability of albumin binding sites. They can be taken up from the blood by all cells that have mitochondria (with the exception of the cells of thecentral nervous system). Fatty acids can only be broken down in mitochondria, by means ofbeta-oxidationfollowed by further combustion in thecitric acid cycleto CO2and water. Cells in the central nervous system, although they possess mitochondria, cannot take free fatty acids up from the blood, as theblood–brain barrieris impervious to most free fatty acids,[citation needed]excludingshort-chain fatty acidsandmedium-chain fatty acids.[22][23]These cells have to manufacture their own fatty acids from carbohydrates, as described above, in order to produce and maintain the phospholipids of their cell membranes, and those of their organelles.[18]
Variation between animal species
editStudies on thecell membranesofmammalsandreptilesdiscovered that mammalian cell membranes are composed of a higher proportion of polyunsaturated fatty acids (DHA,omega-3 fatty acid) thanreptiles.[24]Studies on bird fatty acid composition have noted similar proportions to mammals but with 1/3rd less omega-3 fatty acids as compared toomega-6for a given body size.[25]This fatty acid composition results in a more fluid cell membrane but also one that is permeable to various ions (H+&Na+), resulting in cell membranes that are more costly to maintain. This maintenance cost has been argued to be one of the key causes for the high metabolic rates and concomitantwarm-bloodednessof mammals and birds.[24]However polyunsaturation of cell membranes may also occur in response to chronic cold temperatures as well. Infishincreasingly cold environments lead to increasingly high cell membrane content of both monounsaturated and polyunsaturated fatty acids, to maintain greater membrane fluidity (and functionality) at the lowertemperatures.[26][27]
Fatty acids in dietary fats
editThe following table gives the fatty acid,vitamin Eandcholesterolcomposition of some common dietary fats.[28][29]
| Saturated | Monounsaturated | Polyunsaturated | Cholesterol | Vitamin E | |
|---|---|---|---|---|---|
| g/100g | g/100g | g/100g | mg/100g | mg/100g | |
| Animal fats | |||||
| Duck fat[30] | 33.2 | 49.3 | 12.9 | 100 | 2.70 |
| Lard[30] | 40.8 | 43.8 | 9.6 | 93 | 0.60 |
| Tallow[30] | 49.8 | 41.8 | 4.0 | 109 | 2.70 |
| Butter | 54.0 | 19.8 | 2.6 | 230 | 2.00 |
| Vegetable fats | |||||
| Coconut oil | 85.2 | 6.6 | 1.7 | 0 | .66 |
| Cocoa butter | 60.0 | 32.9 | 3.0 | 0 | 1.8 |
| Palm kernel oil | 81.5 | 11.4 | 1.6 | 0 | 3.80 |
| Palm oil | 45.3 | 41.6 | 8.3 | 0 | 33.12 |
| Cottonseed oil | 25.5 | 21.3 | 48.1 | 0 | 42.77 |
| Wheat germ oil | 18.8 | 15.9 | 60.7 | 0 | 136.65 |
| Soybean oil | 14.5 | 23.2 | 56.5 | 0 | 16.29 |
| Olive oil | 14.0 | 69.7 | 11.2 | 0 | 5.10 |
| Corn oil | 12.7 | 24.7 | 57.8 | 0 | 17.24 |
| Sunflower oil | 11.9 | 20.2 | 63.0 | 0 | 49.00 |
| Safflower oil | 10.2 | 12.6 | 72.1 | 0 | 40.68 |
| Hemp oil | 10 | 15 | 75 | 0 | 12.34 |
| Canola/Rapeseed oil | 5.3 | 64.3 | 24.8 | 0 | 22.21 |
Reactions of fatty acids
editFatty acids exhibit reactions like other carboxylic acids, i.e. they undergoesterificationand acid-base reactions.
Acidity
editFatty acids do not show a great variation in their acidities, as indicated by their respectivepKa.Nonanoic acid,for example, has a pKaof 4.96, being only slightly weaker than acetic acid (4.76). As the chain length increases, the solubility of the fatty acids in water decreases, so that the longer-chain fatty acids have minimal effect on thepHof an aqueous solution. Near neutral pH, fatty acids exist at their conjugate bases, i.e. oleate, etc.
Solutions of fatty acids inethanolcan betitratedwithsodium hydroxidesolution usingphenolphthaleinas an indicator. This analysis is used to determine the free fatty acid content of fats; i.e., the proportion of the triglycerides that have beenhydrolyzed.
Neutralization of fatty acids, one form ofsaponification(soap-making), is a widely practiced route tometallic soaps.[31]
Hydrogenation and hardening
editHydrogenationof unsaturated fatty acids is widely practiced. Typical conditions involve 2.0–3.0 MPa of H2pressure, 150 °C, and nickel supported on silica as a catalyst. This treatment affords saturated fatty acids. The extent of hydrogenation is indicated by theiodine number.Hydrogenated fatty acids are less prone towardrancidification.Since the saturated fatty acids arehigher meltingthan the unsaturated precursors, the process is called hardening. Related technology is used to convert vegetable oils intomargarine.The hydrogenation of triglycerides (vs fatty acids) is advantageous because the carboxylic acids degrade the nickel catalysts, affording nickel soaps. During partial hydrogenation, unsaturated fatty acids can be isomerized fromcistotransconfiguration.[17]
More forcing hydrogenation, i.e. using higher pressures of H2and higher temperatures, converts fatty acids intofatty alcohols.Fatty alcohols are, however, more easily produced from fatty acidesters.
In theVarrentrapp reactioncertain unsaturated fatty acids are cleaved in molten alkali, a reaction which was, at one point of time, relevant to structure elucidation.
Auto-oxidation and rancidity
editUnsaturated fatty acids and their esters undergoauto-oxidation,which involves replacement of a C-H bond with C-O bond. The process requires oxygen (air) and is accelerated by the presence of traces of metals, which serve as catalysts. Doubly unsaturated fatty acids are particularly prone to this reaction. Vegetable oils resist this process to a small degree because they contain antioxidants, such astocopherol.Fats and oils often are treated withchelating agentssuch ascitric acidto remove the metal catalysts.
Ozonolysis
editUnsaturated fatty acids are susceptible to degradation by ozone. This reaction is practiced in the production ofazelaic acid((CH2)7(CO2H)2) fromoleic acid.[17]
Circulation
editDigestion and intake
editShort-andmedium-chain fatty acidsare absorbed directly into the blood via intestine capillaries and travel through theportal veinjust as other absorbed nutrients do. However,long-chain fatty acidsare not directly released into the intestinal capillaries. Instead they are absorbed into the fatty walls of the intestinevilliand reassemble again intotriglycerides.The triglycerides are coated withcholesteroland protein (protein coat) into a compound called achylomicron.
From within the cell, the chylomicron is released into alymphaticcapillary called alacteal,which merges into larger lymphatic vessels. It is transported via the lymphatic system and thethoracic ductup to a location near the heart (where the arteries and veins are larger). The thoracic duct empties the chylomicrons into the bloodstream via the leftsubclavian vein.At this point the chylomicrons can transport the triglycerides to tissues where they are stored or metabolized for energy.
Metabolism
editFatty acids are broken down to CO2and water by the intra-cellularmitochondriathroughbeta oxidationand thecitric acid cycle.In the final step (oxidative phosphorylation), reactions with oxygen release a lot of energy, captured in the form of large quantities ofATP.Many cell types can use eitherglucoseor fatty acids for this purpose, but fatty acids release more energy per gram. Fatty acids (provided either by ingestion or by drawing on triglycerides stored in fatty tissues) are distributed to cells to serve as a fuel for muscular contraction and general metabolism.
Essential fatty acids
editFatty acids that are required for good health but cannot be made in sufficient quantity from other substrates, and therefore must be obtained from food, are called essential fatty acids. There are two series of essential fatty acids: one has a double bondthree carbon atomsaway from the methyl end; the other has a double bondsix carbon atomsaway from the methyl end. Humans lack the ability to introduce double bonds in fatty acids beyond carbons 9 and 10, as counted from the carboxylic acid side.[32]Two essential fatty acids arelinoleic acid(LA) andalpha-linolenic acid(ALA). These fatty acids are widely distributed in plant oils. The human body has a limited ability to convert ALA into the longer-chainomega-3 fatty acids—eicosapentaenoic acid(EPA) anddocosahexaenoic acid(DHA), which can also be obtained from fish. Omega-3 andomega-6fatty acids arebiosyntheticprecursors toendocannabinoidswithantinociceptive,anxiolytic,andneurogenicproperties.[33]
Distribution
editBlood fatty acids adopt distinct forms in different stages in the blood circulation. They are taken in through the intestine inchylomicrons,but also exist invery low density lipoproteins(VLDL) andlow density lipoproteins(LDL) after processing in the liver. In addition, when released fromadipocytes,fatty acids exist in the blood asfree fatty acids.
It is proposed that the blend of fatty acids exuded by mammalian skin, together withlactic acidandpyruvic acid,is distinctive and enables animals with a keen sense of smell to differentiate individuals.[34]
Skin
editThestratum corneum– the outermost layer of theepidermis– is composed of terminallydifferentiatedandenucleatedcorneocyteswithin a lipid matrix.[35]Together withcholesterolandceramides,free fatty acids form a water-impermeable barrier that preventsevaporative water loss.[35]Generally, the epidermal lipid matrix is composed of an equimolar mixture of ceramides (about 50% by weight), cholesterol (25%), and free fatty acids (15%).[35]Saturated fatty acids 16 and 18 carbons in length are the dominant types in the epidermis,[35][36]while unsaturated fatty acids and saturated fatty acids of various other lengths are also present.[35][36]The relative abundance of the different fatty acids in the epidermis is dependent on the body site the skin is covering.[36]There are also characteristic epidermal fatty acid alterations that occur inpsoriasis,atopic dermatitis,and otherinflammatory conditions.[35][36]
Analysis
editThe chemical analysis of fatty acids in lipids typically begins with aninteresterificationstep that breaks down their original esters (triglycerides, waxes, phospholipids etc.) and converts them tomethylesters, which are then separated by gas chromatography[37]or analyzed bygas chromatographyand mid-infrared spectroscopy.
Separation of unsaturated isomers is possible bysilver ion complemented thin-layer chromatography.[38]Other separation techniques includehigh-performance liquid chromatography(with short columns packed withsilica gelwith bonded phenylsulfonic acid groups whose hydrogen atoms have been exchanged for silver ions). The role of silver lies in its ability to form complexes with unsaturated compounds.
Industrial uses
editFatty acids are mainly used in the production ofsoap,both for cosmetic purposes and, in the case ofmetallic soaps,as lubricants. Fatty acids are also converted, via their methyl esters, tofatty alcoholsandfatty amines,which are precursors to surfactants, detergents, and lubricants.[17]Other applications include their use asemulsifiers,texturizing agents, wetting agents,anti-foam agents,or stabilizing agents.[39]
Esters of fatty acids with simpler alcohols (such as methyl-, ethyl-, n-propyl-, isopropyl- and butyl esters) are used as emollients in cosmetics and other personal care products and as synthetic lubricants. Esters of fatty acids with more complex alcohols, such assorbitol,ethylene glycol,diethylene glycol,andpolyethylene glycolare consumed in food, or used for personal care and water treatment, or used as synthetic lubricants or fluids for metal working.
See also
edit
References
edit- ^abc"C:D" is the numerical symbol: total amount of (C)arbon atoms of the fatty acid, and the number of (D)ouble (unsaturated) bonds in it; if D > 1 it is assumed that the double bonds are separated by one or moremethylene bridge(s).
- ^Each double bond in the fatty acid is indicated by Δx, where the double bond is located on the xth carbon–carbon bond, counting from the carboxylic acid end.
- ^Inn minus x(also ω−x or omega-x) nomenclature a double bond of the fatty acid is located on the xth carbon–carbon bond, counting from the terminal methyl carbon (designated as n or ω) toward the carbonyl carbon.
- ^abcA common mistake is to say that the last carbon is "ω−1".
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.
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.
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
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),Omega-three fatty acids and diabetes.
- ^Moss, G. P.; Smith, P. A. S.; Tavernier, D. (1997)."IUPAC Compendium of Chemical Terminology".Pure and Applied Chemistry.67(8–9). International Union of Pure and Applied Chemistry: 1307–1375.doi:10.1351/pac199567081307.S2CID95004254.Retrieved2007-10-31.
- ^Chen, Lin (2012). "Biodiesel production from algae oil high in free fatty acids by two-step catalytic conversion".Bioresource Technology.111:208–214.doi:10.1016/j.biortech.2012.02.033.PMID22401712.
- ^Chevreul, M. E. (1813)."Sur plusieurs corps gras, et particulièrement sur leurs combinaisons avec les alcalis".Annales de Chimie.88.Paris: H. Perronneau: 225–261 – via Gallica.
- ^Chevreul, M. E. (1823).Recherches chimiques sur les corps gras d'origine animale.Paris: Levrault – via Internet Archive.
- ^Leray, Claude (11 November 2017)."Chronological history of lipid science".Cyberlipid Center.Archived fromthe originalon 2017-10-06.
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