Epimer, definition

Epimerization of sugar, mechanisms 778 Epimers, definition of 163 Epinephrine (adrenaline) 542,553, 553s Episomes. See plasmid Epithelial cells 29 Epitheliocytes 25 Epoxides, alkyation by 254 Epoxide hydrolases 591 EPR (electron paramagnetic resonance) spectroscopy 398, 399 of glutamate mutase 873 in study of phosphotransferases 639 EPSP (enolpyruvoylshikimate-3-phosphate) 687s... 

Colorless deoxyvomicine is reduced by allylic fission, together with reduction of the 12,13-double bond, to give F (CCLXXIV). Without definite experimental evidence, Huisgen claims F and C to be C-13 epimers. Further reduction gives bases G and H (CCLXXV), epimeric at C-21, and stereoisomeric with bases D and E (CCLXXIII). 

THC (27). [a]D +114° with absolute optical purity (Srebnik et al. 1984). These enantiomers have now been tested in human volunteers (Hollister et al., in press). Subjects received progressively increasing intravenous doses of (lS)-delta-3-THC (26) and (lR)-delta-3-THC (27), beginning with 1 mg (in 1 ml ethanol) followed by progressive doubling of doses until, definite effects were observed. Cannabimimetic effects with the (IS) epimer were noted at doses of 8 mg or higher. No effects were noted with the (IR) epimer. (lS)-delta-3-THC (26) is thus estimated to have a potency from one-third to one-sixth that of delta-1-THC. 

Epimers Ratio Index. Chiral centers occur in the materia prima in one definite configuration. The degree of epimerization of these centers in the materia actuale have been interpreted as a measure of the degree of maturation of the sediments. 

To answer this problem, one must know the structures of the molecules in question and a couple of definitions. By definition, epimers are a pair of molecules that differ from each other only in their configuration at a single asymmetric center. Anomers are special epimers that differ only in their configuration at a carbonyl carbon hence, they are usually acetals or hemiacetals. An aldose-ketose pair is obvious. Inspection of Fischer representations of the molecular pairs leads to the conclusion that (a), (c), and (e) are aldose-ketose pairs (b) and (f) are epimers and (e) are anomers. 

The conversion of 3a,7a,12a-trihydroxy-5)5-cholestanoic acid into cholic acid (cholyl coenzyme A) occurs by means of a /9-oxidation with release of propionic acid (propionyl coenzyme A) (Fig. 2). Although no definite experimental evidence is available, it may be assumed that the reactions involve the coenzyme-A esters of the steroid acids. Only one of the intermediates in the /9-oxidation of 3a,7a,12a-trihydroxy-5/9-cholestanoic acid has been isolated. Masui and Staple (88,89) have shown that the mitochondrial fraction of rat liver homogenate supplemented with the 100,000g supernatant fluid catalyzes the conversion of 3a,7a,12a-trihydroxy-5/9-cholestanoic acid into one of the two C-24 epimers of 3a,7a,12a,24-tetrahydroxy-5/9-choles-tanoic acid. It is probable that the C-24 isomer formed is the 24a-hydroxy compound (89,90). The transformation of 3a,7a,12a,24a-tetrahydroxy-5/9-cholestanoic acid into cholic acid is catalyzed by the 100,000g supernatant fluid of rat liver homogenate (89). No studies have yet been carried out to compare the enzymes catalyzing /9-oxidation of 3a,7a,12a-trihydroxy-5/9-... 

Definition Calcium sait of the aipha epimer of giucoheptonic acid... 

In an extensive series of studies, Yoshikawa,Yano and co-workers have confirmed that various kinds of aldose can be complexed with nickel and amino compounds such as diamines and amino acid in a methanolic solution [43]. They studied the nickel(ll) complexes coordinating N-glycosides derived from diamine and monosaccharide. The structure of the complex was determined from three-dimensional X-ray data [43 d]. Their further studies elucidated that aldoses can be epimerized by reaction systems containing a nickel/diamine complex. This novel epimerization proceeds through an alternative mechanism that is definitely distinct from the epimerization mechanism by reversible enolization that was introduced in Sect. 2.1. It results in the formation of an equilibrium mixture of the two epimers. According to their study [ 39], three significant pecu-harities of this epimerization are ... 

Isomers that originally differed from each other at C-2, for example, D-glucose and d-mannose. Now the definition has been expanded to include any pair of diasteromers that differ from each other at only one chiral center. For example, D-galactose and D-allose are epimers of D-glucose and D-mannose. 

According to the lUPAC definition, any diastereo-mer that has the opposite configuration at only one or two tetrahedral stereogenic centers is called an epimer, and the process of epimer formation by changing one of the asymmetric centers in the molecule is known as epimerization. The positions in two diastereomers that have different configurations are called epimeric. For example, for 2-(methylamino)-l-phenylpropan-l-ol (ephedrine), the IR,2R- and 1/ ,25-stereoisomers have the same configuration at Cl, but they are epimeric at the C2 atom (Figure 1.20). 

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