حمض دسم مشبع

أنواع الدهون في الغذاء
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الأحماض الدسمة المشبعة Saturated Fatty Acid هي حموض دسمة تكون فيها جميع ذرات الكربون مشبعة بالهايدروجين فهي تحتوي على أكبر عدد من الهايدروجين تستطيع أن تحمله: مرتبطة مع ذرتين هايدروجين لذرات الكربون باستثناء ذرتي الكربون على طرفي السلسلة. بمعنى آخر تكون جميع الروابط بين ذرات الكربون في السلسلة روابط أحادية (تذكر أن ذرة الكربون لها دائماً أربع روابط)


كأي حمض دهني, تكون الاحماض الدهنية بالصيغة التالية:

ذرة كربون مشبعة بثلاث ذرات هايدروجين

ثم سلسلة مكررة من ذرات الكربون الوسطية.

الجزء الحمضي وهي COOH في نهاية السلسلة

عدد ذرات الكربون بالدهن الحمضي المشبع هو الذي يحدد نوع واسم الدهن المشبع


ملف:Fatty-Acide stearic-acid.JPG

بالأعلى مثال على الأحماض الدهنية المشبعة وفي هذا المثال الرسم الجزيئي لحمض الستريك المشبع ذي الـ 18 ذرة كربون حيث ترى أن كل ذرات الكربون مرتبطة بذرتي هايدروجين باستثناء الأطراف. في هذا المثال تستطيع أن ترى الجزء الحمضي باللون الأحمر والجزء الدهني باللون البرتقالي


وتكون جميع الاحماض الدهنية المشبعة بنفس الطريقة لكن تختلف بطول السلسلة

فحمض الستريك المشبع لديه 18 ذرة كربون كما في المثال أعلاه

Arachidic acid, a saturated fatty acid
Examples of saturated fatty acids
Common name Chemical structure C :D [أ]
Propionic acid CH3CH2COOH 3:0
Butyric acid CH3(CH2)2COOH 4:0
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

Production

Industrial

Fatty acids are usually produced industrially by the hydrolysis of triglycerides, with the removal of glycerol (see oleochemicals). Phospholipids represent another source. Some fatty acids are produced synthetically by hydrocarboxylation of alkenes.[1]

By animals

المقالة الرئيسية: Fatty acid synthesis

In animals, fatty acids are formed from carbohydrates predominantly in the liver, adipose tissue, and the mammary glands during lactation.[2]

Carbohydrates are converted into pyruvate by glycolysis as the first important step in the conversion of carbohydrates into fatty acids.[2] Pyruvate is then decarboxylated to form acetyl-CoA in the mitochondrion. However, this acetyl CoA needs to be transported into cytosol where the synthesis of fatty acids occurs. This cannot occur directly. To obtain cytosolic acetyl-CoA, citrate (produced by the condensation of acetyl-CoA with oxaloacetate) is removed from the citric acid cycle and carried across the inner mitochondrial membrane into the cytosol.[2] There it is cleaved by ATP citrate lyase into acetyl-CoA and oxaloacetate. The oxaloacetate is returned to the mitochondrion as malate.[3] The cytosolic acetyl-CoA is carboxylated by acetyl-CoA carboxylase into malonyl-CoA, the first committed step in the synthesis of fatty acids.[3][4]

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 form triglycerides, the main storage form of fatty acids, and thus of energy in animals. However, fatty acids are also important components of the phospholipids that form the phospholipid bilayers out of which all the membranes of the cell are constructed (the cell wall, and the membranes that enclose all the organelles within the cells, such as the nucleus, the mitochondria, endoplasmic reticulum, and the Golgi apparatus).[2]

The "uncombined fatty acids" or "free fatty acids" found in the circulation of animals come from the breakdown (or lipolysis) of stored triglycerides.[2][5] Because they are insoluble in water, these fatty acids are transported bound to plasma albumin. 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 the central nervous system). Fatty acids can only be broken down in mitochondria, by means of beta-oxidation followed by further combustion in the citric acid cycle to CO2 and water. Cells in the central nervous system, although they possess mitochondria, cannot take free fatty acids up from the blood, as the blood–brain barrier is impervious to most free fatty acids,[citation needed] excluding short-chain fatty acids and medium-chain fatty acids.[6][7] 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.[2]

Variation between animal species

Studies on the cell membranes of mammals and reptiles discovered that mammalian cell membranes are composed of a higher proportion of polyunsaturated fatty acids (DHA, omega−3 fatty acid) than reptiles.[8] Studies on bird fatty acid composition have noted similar proportions to mammals but with 1/3rd less omega−3 fatty acids as compared to omega−6 for a given body size.[9] 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 concomitant warm-bloodedness of mammals and birds.[8] However polyunsaturation of cell membranes may also occur in response to chronic cold temperatures as well. In fish increasingly 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 lower temperatures.[10][11]

Fatty acids in dietary fats

The following table gives the fatty acid, vitamin E and cholesterol composition of some common dietary fats.[12][13]

Saturated Monounsaturated Polyunsaturated Cholesterol Vitamin E
g/100g g/100g g/100g mg/100g mg/100g
Animal fats
Duck fat[14] 33.2 49.3 12.9 100 2.70
Lard[14] 40.8 43.8 9.6 93 0.60
Tallow[14] 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

Fatty acids exhibit reactions like other carboxylic acids, i.e. they undergo esterification and acid-base reactions.

Transesterification

All fatty acids transesterify. Typically, transesterification is practiced in the conversion of fats to fatty acid methyl esters. These esters are used for biodiesel. They are also hydrogenated to give fatty alcohols. Even vinyl esters can be made by transesterification using vinyl acetate.[15]

Acid-base reactions

Fatty acids do not show a great variation in their acidities, as indicated by their respective pKa. Nonanoic acid, for example, has a pKa of 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 the pH of an aqueous solution. Near neutral pH, fatty acids exist at their conjugate bases, i.e. oleate, etc.

Solutions of fatty acids in ethanol can be titrated with sodium hydroxide solution using phenolphthalein as an indicator. This analysis is used to determine the free fatty acid content of fats; i.e., the proportion of the triglycerides that have been hydrolyzed.

Neutralization of fatty acids, like saponification, is a widely practiced route to metallic soaps.[16]

Hydrogenation and hardening

Hydrogenation of unsaturated fatty acids is widely practiced. Typical conditions involve 2.0–3.0 MPa of H2 pressure, 150 °C, and nickel supported on silica as a catalyst. This treatment affords saturated fatty acids. The extent of hydrogenation is indicated by the iodine number. Hydrogenated fatty acids are less prone toward rancidification. Since the saturated fatty acids are higher melting than the unsaturated precursors, the process is called hardening. Related technology is used to convert vegetable oils into margarine. 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 from cis to trans configuration.[1]

More forcing hydrogenation, i.e. using higher pressures of H2 and higher temperatures, converts fatty acids into fatty alcohols. Fatty alcohols are, however, more easily produced from simpler fatty acid esters, like the fatty acid methyl esters ("FAME"s).

Decarboxylation

Ketonic decarboxylation is a method useful for producing symmetrical ketones from carboxylic acids. The process involves reactions of the carboxylic acid with an inorganic base. Stearone is prepared by heating magnesium stearate.[17]

Chemistry of saturated vs unsaturated acids

The reactivity of saturated fatty acids is usually associated with the carboxylic acid or the adjacent methylene group By conversion to their acid chlorides, they can be converted to the symmetrical fatty ketone laurone (O=C(C
nH
(2n+1))
2).[18] Treatment with sulfur trioxide gives the α-sulfonic acids.[19]

The reactivity of unsaturated fatty acids is often dominated by the site of unsaturation. These reactions are the basis of ozonolysis, hydrogenation, and the iodine number. Ozonolysis (degradation by ozone) is practiced in the production of azelaic acid ((CH2)7(CO2H)2) from oleic acid.[1]

Circulation

Digestion and intake

المقالة الرئيسية: Digestion § Fat digestion

Short- and medium-chain fatty acids are absorbed directly into the blood via intestine capillaries and travel through the portal vein just as other absorbed nutrients do. However, long-chain fatty acids are not directly released into the intestinal capillaries. Instead they are absorbed into the fatty walls of the intestine villi and reassemble again into triglycerides. The triglycerides are coated with cholesterol and protein (protein coat) into a compound called a chylomicron.

From within the cell, the chylomicron is released into a lymphatic capillary called a lacteal, which merges into larger lymphatic vessels. It is transported via the lymphatic system and the thoracic duct up to a location near the heart (where the arteries and veins are larger). The thoracic duct empties the chylomicrons into the bloodstream via the left subclavian vein. At this point the chylomicrons can transport the triglycerides to tissues where they are stored or metabolized for energy.

Metabolism

المقالة الرئيسية: Fatty acid metabolism

Fatty acids are broken down to CO2 and water by the intra-cellular mitochondria through beta oxidation and the citric acid cycle. In the final step (oxidative phosphorylation), reactions with oxygen release a lot of energy, captured in the form of large quantities of ATP. Many cell types can use either glucose or 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

المقالة الرئيسية: Essential fatty acid

Fatty 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 bond three carbon atoms away from the methyl end; the other has a double bond six carbon atoms away 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.[20] Two essential fatty acids are linoleic acid (LA) and alpha-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-chain omega-3 fatty acids — eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which can also be obtained from fish. Omega−3 and omega−6 fatty acids are biosynthetic precursors to endocannabinoids with antinociceptive, anxiolytic, and neurogenic properties.[21]

Distribution

المقالة الرئيسية: Blood fatty acids

Blood fatty acids adopt distinct forms in different stages in the blood circulation. They are taken in through the intestine in chylomicrons, but also exist in very low density lipoproteins (VLDL) and low density lipoproteins (LDL) after processing in the liver. In addition, when released from adipocytes, fatty acids exist in the blood as free fatty acids.

It is proposed that the blend of fatty acids exuded by mammalian skin, together with lactic acid and pyruvic acid, is distinctive and enables animals with a keen sense of smell to differentiate individuals.[22]

Skin

The stratum corneum  – the outermost layer of the epidermis  – is composed of terminally differentiated and enucleated corneocytes within a lipid matrix.[23] Together with cholesterol and ceramides, free fatty acids form a water-impermeable barrier that prevents evaporative water loss.[23] Generally, the epidermal lipid matrix is composed of an equimolar mixture of ceramides (about 50% by weight), cholesterol (25%), and free fatty acids (15%).[23] Saturated fatty acids 16 and 18 carbons in length are the dominant types in the epidermis,[23][24] while unsaturated fatty acids and saturated fatty acids of various other lengths are also present.[23][24] The relative abundance of the different fatty acids in the epidermis is dependent on the body site the skin is covering.[24] There are also characteristic epidermal fatty acid alterations that occur in psoriasis, atopic dermatitis, and other inflammatory conditions.[23][24]

Analysis

The chemical analysis of fatty acids in lipids typically begins with an interesterification step that breaks down their original esters (triglycerides, waxes, phospholipids etc.) and converts them to methyl esters, which are then separated by gas chromatography[25] or analyzed by gas chromatography and mid-infrared spectroscopy.

Separation of unsaturated isomers is possible by silver ion complemented thin-layer chromatography.[26] Other separation techniques include high-performance liquid chromatography (with short columns packed with silica gel with 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

Fatty acids are mainly used in the production of soap, both for cosmetic purposes and, in the case of metallic soaps, as lubricants. Fatty acids are also converted, via their methyl esters, to fatty alcohols and fatty amines, which are precursors to surfactants, detergents, and lubricants.[1] Other applications include their use as emulsifiers, texturizing agents, wetting agents, anti-foam agents, or stabilizing agents.[27]

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 as sorbitol, ethylene glycol, diethylene glycol, and polyethylene glycol are consumed in food, or used for personal care and water treatment, or used as synthetic lubricants or fluids for metal working.

Fatty acids[28] and their derivatives like dimer acids[29] have also been used by scientists to prepare polyurethane coatings of bio-based or bio-derived coatings.

See also

مشاع المعرفة فيه ميديا متعلقة بموضوع Fatty acids.

References

  1. ^ خطأ استشهاد: وسم <ref> غير صحيح؛ لا نص تم توفيره للمراجع المسماة c:d
  1. ^ أ ب ت ث Anneken, David J.; Both, Sabine; Christoph, Ralf; Fieg, Georg; Steinberner, Udo; Westfechtel, Alfred. "Fatty Acids". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a10_245.pub2. {{cite encyclopedia}}: Cite has empty unknown parameter: |authors= (help)
  2. ^ أ ب ت ث ج ح Stryer, Lubert (1995). "Fatty acid metabolism.". Biochemistry (4th ed.). New York: W. H. Freeman and Company. pp. 603–628. ISBN 978-0-7167-2009-6.
  3. ^ أ ب Ferre, P.; Foufelle, F. (2007). "SREBP-1c Transcription Factor and Lipid Homeostasis: Clinical Perspective". Hormone Research. 68 (2): 72–82. doi:10.1159/000100426. PMID 17344645. this process is outlined graphically in page 73
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  6. ^ Tsuji A (2005). "Small molecular drug transfer across the blood–brain barrier via carrier-mediated transport systems". NeuroRx. 2 (1): 54–62. doi:10.1602/neurorx.2.1.54. PMC 539320. PMID 15717057. Uptake of valproic acid was reduced in the presence of medium-chain fatty acids such as hexanoate, octanoate, and decanoate, but not propionate or butyrate, indicating that valproic acid is taken up into the brain via a transport system for medium-chain fatty acids, not short-chain fatty acids. ... Based on these reports, valproic acid is thought to be transported bidirectionally between blood and brain across the BBB via two distinct mechanisms, monocarboxylic acid-sensitive and medium-chain fatty acid-sensitive transporters, for efflux and uptake, respectively.
  7. ^ Vijay N, Morris ME (2014). "Role of monocarboxylate transporters in drug delivery to the brain". Curr. Pharm. Des. 20 (10): 1487–98. doi:10.2174/13816128113199990462. PMC 4084603. PMID 23789956. Monocarboxylate transporters (MCTs) are known to mediate the transport of short chain monocarboxylates such as lactate, pyruvate and butyrate. ... MCT1 and MCT4 have also been associated with the transport of short chain fatty acids such as acetate and formate which are then metabolized in the astrocytes [78].
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  17. ^ Dobson, A. G.; Hatt, H. H. (1953). "Stearone". Organic Syntheses. 33: 84. doi:10.15227/orgsyn.033.0084.
  18. ^ Sauer, J. C. (1951). "Laurone". Organic Syntheses. 31: 68. doi:10.15227/orgsyn.031.0068.
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  25. ^ Aizpurua-Olaizola O, Ormazabal M, Vallejo A, Olivares M, Navarro P, Etxebarria N, et al. (January 2015). "Optimization of supercritical fluid consecutive extractions of fatty acids and polyphenols from Vitis vinifera grape wastes". Journal of Food Science. 80 (1): E101-7. doi:10.1111/1750-3841.12715. PMID 25471637.
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  29. ^ SD Rajput, PP Mahulikar, VV Gite, Biobased dimer fatty acid containing two pack polyurethane for wood finished coatings, Progress in Organic Coatings 77 (1), 38–46

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