Epimer, defined

Compounds that have the same chemical formula but have different structures are called isomers. For example, fructose, glucose, mannose, and galactose are all isomers of each other, having the same chemical formula, C6H1206. If two monosaccharides differ in configuration around only one specific carbon atom (with the exception of the carbonyl carbon, see anomers below), they are defined as epimers of each other. (Of course, they are also isomers ) For example, glucose... 

Define chiral, enantomer, diastereomer, epimer, anomer (see Chapter 4), prochiral (see Chapter 9). What is meant by the statement that biochemical reactions are stereochemically specific Why is such stereospecificity to be expected in organisms (which are constructed from asymmetric units) See Chapter 9 for further discussion. 

Subsequent modifications of the polymers involve extensive formation of O-sulfate esters,1903 193 197 N-deacetylation and N-sulfation,198/199 and epimerization at C5.10 In some tissues almost all GluA is epimer-ized.200 The modifications are especially extensive in dermatan, heparan sulfates, and heparin (see also p. 177).196 201 203b The modifications are not random and follow a defined order. N-Deacetylation must precede N-sulfation, and O-sulfation is initiated only after N-sulfation of the entire chain is complete. The modifications occur within the Golgi (see Fig. 20-7) but not all... 

The crystal structures of the alkaloids containing a hydroxyl group at C-2 have not been determined. The stereochemistry of erythratine (42) was established as 2R,3R,5S by Barton et al. (79) and that of erythratidine (39) as 2S,3R,5S by the same group (27) on the basis of optical rotation and NMR data for both pairs of C-2 epimers (see Section II,C,4b). The configuration at C-2 for erysosalvine (45), erysotine (48), and erysopitine (50) has not been defined. 

Terms in bold are defined glycoconjugate 238 monosaccharide 238 oligosaccharide 238 disaccharide 238 polysaccharide 238 aldose 239 ketose 239 Fischer projection formulas 240 epimers 240... 

Recently, paradisiol has been identified with intermedeol, which re-defines its structure as the C-4 epimer of (243) see J. W. Hoffmann and L. H. Zalkow, Tetrahedron Letters, 1973. 751. 

Diastereoisomers have different chemical properties and thus can often be resolved by conventional separation techniques. There are two other terms that need to be defined regarding diastereoisomers and they zx anomers and epimers. 

Now let us see the behaviour of the trans epimer ( ) which reacted quite differently with sodium cyanide. The reaction was considerably slower. Heating in DMSO at 100 °C for about four days was needed for disappearance of the starting material, and even then a well-defined derivative (1 ) could be isolated only in poor yield (25 % ). Thus the behaviour of the trans epimer compared with the ois isomer is strikingly different. 

When cooling a melt of an 1 1 mixture of the epimers, at To (Figure 3.11) first S-rich a-mixed crystals are formed with a composition defined by the corresponding tie line. On further cooling, the composition of the a-mixed crystals and the melt alter along the solidus and liquidus lines as indicated by arrows. At the eutectic temperature Tm, a crystalline phase of composition C and amount A-B will be in equilibrium with a melt having eutectic composition A of amount B-C. 

As a measure of their biological activity, all E vitamers can be assigned a potency, determined in a bioassay. One international unit (lU) is defined as the activity of 1 mg of flZZ-rac-a-tocopheryl acetate. Accordingly, the biopotencies of RRR-a-tocopheryl acetate, RRR-a-tocopherol and a/Z-rac-a-tocopherol have been calculated to be 1.36, 1.49, and 1.10 lU, respectively. The chirality of the C-2 atom is crucial to the activity, the 2-epimers having only one-third of the potency of the 2R forms. Biopotencies of tocopherols and tocotrienols can be expressed relative to that of a-tocopherol. Reported values (in %) are 100 (a-tocopherol), 40 (p-tocopherol), 8 (y-tocopherol), 1 (8-tocopherol), 21 (a-tocotrie-nol), and 4 (p-tocotrienol). This order parallels that of the relative antioxidant activity of the various vitamers (see B). 

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