Triglycerides

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General chemical structure of fats (R 1 , R 2 and R 3 are alkyl or alkenyl radicals with mostly an odd number of carbon atoms). If the radicals R 1 and R 3 are different, the triglyceride is chiral .

Triglycerides , triglycerides , also glycerol triesters , more rarely out of date neutral fats , are triple esters of the trivalent alcohol glycerine with three acid molecules and, according to the IUPAC recommendation, should only be referred to as triacylglycerols , TAG s for short (or more precisely tri-O-acylglycerols ) . The prefix tri refers to three acyl acid residues that are esterified with glycerine.

Triacylglycerols with three fatty acids are the compounds in fats and fatty oils . Natural fats mainly consist of triglycerols with three long-chain fatty acids, which mostly consist of unbranched chains with 4 to 26, typically 12 to 22 carbon atoms. If they are liquid at room temperature, they are also known as oils or, to distinguish them from mineral oils or essential oils , fatty oils . Pure triacylglycerols of fatty acids are also known as neutral fats .

Substance groups and names

Catalytic fat hardening of a triglyceride (mixed triacylglycerol) with hydrogen: Above a triglyceride in an oil with a blue marked saturated fatty acid residue ( palmitic acid ), a green marked monounsaturated fatty acid residue ( oleic acid ) and a red marked triple unsaturated fatty acid residue ( linolenic acid ). The triple acylated glycerine ( marked in black ) can be seen in the center of the structural formula above . Below the hydrogenation product (a fat) - a triglyceride with saturated fatty acid residues - which has a higher melting point than the starting material (oil, above).

One can differentiate between medium- and long-chain triglycerides. While having medium-chain triglycerides (engl .: medium-chain triglycerides , MCT) fatty acids of medium length (6 to 12 C atoms) and long chain triglycerides (engl .: long-chain triglycerides , LCT) fatty acids of great length (14 to 24 C-atoms ) bound. Short-chain triglycerides (SCT) are, for example, triacetin and tributyrin (fatty acids with 2 or 4 carbon atoms).

There are also two types of triacylglycerols: simple and mixed triacylglycerols. With simple triacylglycerols the side chains (i.e. the fatty acid residues) are identical, with mixed ones they are different.

The reason why fat is solid and oil is liquid is the significantly higher proportion of unsaturated fatty acid residues in the triglycerides of oils. The unsaturated fatty acids usually have cis double bonds, which makes it difficult for the triglycerides to form crystals and thus lowers the melting point.

If the side chains R 1 and R 3 in a triacylglycerol are different, then a chiral molecule is present and an optical activity can be observed, i. that is, the mirror image of the molecule is not congruent with the original and a solution of the molecule is able to rotate incident polarized light .

Phosphoglycerides such as B. Lecithins are triesters of glycerol with two fatty acids and an organic phosphoric acid. Both types of compounds belong to the class of lipids with mostly plant and animal occurrences in nature, but they can also be represented by chemical synthesis.

The sn nomenclature also applies to chiral derivatives of glycerol .

Analysis of triglycerides

The explanations under substance groups and designations show that native triglyceride mixtures can be very complex. If n different types of fatty acid are present, n 3 different isomers (including the positional isomers and the optical isomers) are possible. Since more than 100 relatively common naturally occurring fatty acids are now known, it can be seen that only very powerful analytical methods are suitable for clearly characterizing and quantifying the different triglycerides.

For the qualitative and quantitative analysis of the triglycerides, chromatographic methods are therefore preferably used. Depending on the matrices to be examined, adequate sample preparation methods are required. For example, the triglycerides must be isolated from physiological matrices (e.g. serum or liquor ) or from foods using suitable extraction methods prior to analysis . Native oils and fats can usually be examined without laborious sample preparation using argentation chromatography or HPLC with propionitrile as the eluent . The high-temperature gas chromatography is used in the determination of the individual triglycerides.

Acid number

At elevated temperatures and long storage times, the quality and purity of the triglycerides decrease, the triglycerides increasingly decompose and "free fatty acids" are formed. This undesirable aging process can be followed analytically by determining the acid number .

Medical importance

In the medical field, the triglyceride levels in the blood are measured when a blood analysis is carried out . Elevated values ​​(over 150 mg per dl or 1.7 mmol per l) indicate a lipid metabolism disorder ( hypertriglyceridaemia ) or obesity . These values ​​are also increased in other diseases such as hypothyroidism or kidney disease . Elevated triglyceride levels are a cardiovascular risk factor , especially if they are associated with elevated cholesterol levels .

Examples

Examples of some triacylglycerols with three identical acid residues:

However, the three acid residues contained in the triacylglycerols are often different.

Web links

Commons : Triglycerides  - Collection of pictures, videos and audio files
Wikibooks: Biochemistry and Pathobiochemistry: Triacylglycerol Biosynthesis  - Learning and Teaching Materials
Wikibooks: Biochemistry and Pathobiochemistry: Triacylglycerol Breakdown  - Learning and Teaching Materials

Individual evidence

  1. ↑ Ewa Rogalska, Claire Cudrey, Francine Ferrato, Robert Verger: Stereoselective hydrolysis of triglycerides by animal and microbial lipases. In: Chirality . 5, 1993, 24-30, doi : 10.1002 / chir.530050106 .
  2. ↑ Entry on triacylglycerols. In: Römpp Online . Georg Thieme Verlag, accessed on January 6, 2013.
  3. ^ GP Moss, PAS Smith, D. Tavernier: Lipids or Nomenclature of Lipids. Recommendations Lip-1 and Lip-2 In: Glossary of class names of organic compounds and reactive intermediate based on structure. IUPAC Recommendations, 1994.
  4. ↑ Otto Wolfgang Thiele: Lipids, isoprenoids with steroids. Thieme, Stuttgart 1979, ISBN 3-13-576301-3 .
  5. ↑ H.-U. Melchert, K. Kemper: Triglyceride pattern of human sera as marker of vegetarian nutrition. , in: Biomarkers of dietary exposure , Proceedings of the 3rd. Meeting on Nutritional Epidemiology, Rotterdam 1991, Ed .: Frans J. Kok, Smith-Gordon Verlag, London 1992, ISBN 1-85463-071-7
  6. ^ V. Ruiz-Gutiérrez, LJ Barron: Methods for the analysis of triacylglycerols. In: Journal of Chromatography B: Biomedical Sciences and Applications . 671 (1-2), 1995, 133-168, PMID 8520690 .
  7. ↑ Katharina Kemper: Effects of a vegetarian diet on the triglyceride distribution pattern of the serum. Dissertation. Technical University, Berlin 1987, DNB 880167696 .
  8. ↑ K. Kemper, H. - U. Melchert, K. Rubach, H. Hoffmeister: Characterization of the triglyceride pattern of vegetable and animal fats as well as human and animal sera by means of HPLC after pre-separation on AgNO 3 -impregnated silica gel mini columns . In: Fresenius' Journal of Analytical Chemistry. 331 (6), 1988, 634-641, doi : 10.1007 / BF01032542 .
  9. ↑ Good fats, bad fats In: Pharmazeutische Zeitung . on-line.
  10. ↑ Triglycerides on internisten-im-netz.de.