Fructose isomers

In contrast man may have a B-D-fructopyranose site since this compound is believed to be the sweetest fructose isomer (2, 26). Further evidence for a D-fructose site in otEer animals completely distinct from a sucrose or a D-glucose site is evident in single fiber responses (27,28) or in biochemical studies (29). 

The enzymatic conversion of 6-azido-6-deoxy-D-glucose into its D-fructose isomer was utilised as a key step in a five-step synthesis from sucrose [80] of the natural product and powerful D-mannosidase inhibitor 1-deoxymannojirimycin (l,5-dideoxy-l,5-imino-D-mannitol, 66) [81]. Subsequently, it was shown that the immobilised enzyme from Streptomyces murinus sp. (Sweetzyme T from Novo A/S) was able to isomerise D-glucose derivatives with modifications at C-3 and C-6 such as 6-azido-6-deoxy-3-0-methyl-D-glucose (67) as well as the corresponding 3-deoxy derivative 69 (Scheme 24) [64,82]. 

Displacement-purge forms the basis for most simulated continuous countercurrent systems (see hereafter) such as the UOP Sorbex processes. UOP has licensed close to one hundred Sorbex units for its family of processes Parex to separate p-xylene from C3 aromatics, Molex tor /i-paraffin from branched and cyclic hydrocarbons, Olex for olefins from paraffin, Sarex for fruc tose from dextrose plus polysaccharides, Cymex forp- or m-cymene from cymene isomers, and Cresex for p- or m-cresol from cresol isomers. Toray Industries Aromax process is another for the production of p-xylene [Otani, Chem. Eng., 80(9), 106-107, (1973)]. Illinois Water Treatment [Making Wave.s in Liquid Processing, Illinois Water Treatment Company, IWT Adsep System, Rockford, IL, 6(1), (1984)] and Mitsubishi [Ishikawa, Tanabe, and Usui, U.S. Patent 4,182,633 (1980)] have also commercialized displacement-purge processes for the separation of fructose from dextrose. 

Another naturally occurring sugar is fructose, also CsHijOe. It is an isomer of glucose but the... 

Draw a structural formula for the fructose molecule (remember that fructose is an isomer of glucose). Explain why fructose cannot be oxidized to a six-carbon acid. 

The new compounds were assigned structures by examination of their 13C NMR spectra and of the H NMR spectra of the peracetates. A similar mechanism to that previously postulated for fructose and inulin,31 and involving a sor-bofuranosyl fluoride was suggested for the formation of these isomers. In both Refs. 31 and 80, formation of the 2,3-linkage was associated with more-rigorous conditions. 

The presence of asymmetric carbon atoms also confers optical activity on the compound. When a beam of plane-polarized hght is passed through a solution of an optical isomer, it will be rotated either to the right, dextrorotatory (+) or to the left, levorotatory (—). The direction of rotation is independent of the stereochemistry of the sugar, so it may be designated d(—), d(+), l(—), or l(+). For example, the naturally occurring form of fructose is the d(—) isomer. 

By application of first-order, kinetic equations, B. Anderson and Degn claimed that an equilibrated (25°) aqueous solution of D-fructose contains 31.56% of jS-D-fructofuranose and 68.44% of -D-fructopyranose. N.m.r. studies, however, showed that, at equilibrium, a solution of D-fructose contains /3-D-fructopyranose, -D-fructofuranose, a-D-fructofuranose, and a trace of a-D-fructopyranose the distribution of these isomers was shown by gas-liquid chromatography to be 76,19.5, and 4%, respectively. Based on Anderson and Degn s result, Shallenberger reasoned that, as 0.68 X 1.8 = 1.22 (which approximates the reported sweetness of mutarotated D-fructose ), the furanose form(s) must possess very little sweetness. 

D-fructose, and of their optical isomers, a truly remarkable achievement (see Fig. 1). Thus, encouragement was given to the formaldehyde theory. Paper chromatography shows formose to be a complex mixture containing glycolaldehyde, trioses, tetroses, pentoses, and hexoses.62- 63 Schmitz64 re-... 

Even more interesting is the observed regioselectivity of 37 its reaction with 2, 3 -cCMP and 2, 3 -cUMP resulted in formation of more than 90% of 2 -phosphate (3 -OH) isomer. The postulated mechanisms for 37 consists of a double Lewis-acid activation, while the metal-bound hydroxide and water act as nucleophilic catalyst and general acid, respectively (see 39). The substrate-ligand interaction probably favors only one of the depicted substrate orientations, which may be responsible for the observed regioselectivity. Complex 38 may operate in a similar way but with single Lewis-acid activation, which would explain the lower bimetallic cooperativity and the lack of regioselectivity. Both proposed mechanisms show similarities to that of the native phospho-monoesterases (37 protein phosphatase 1 and fructose 1,6-diphosphatase, 38 purple acid phosphatase). 

Both steric and electronic factors are used for chiral recognition of saccharides by the R and S forms of S-3. A difference in PET efficiency is created by the asymmetric immobilization of the amine groups relative to the binaphthyl moiety upon 1 1 complexation of saccharides by d- or L-isomers. For instance, D-fructose is recognized by the R form of S-2 with a large fluorescence enhancement. 

The first phosphatase step is very important FBPase converts fructose,1-6-bisphos-phate into fructose-6-phosphate under allosteric control of several factors but during fasting, glucagon-induced regulation is crucial. One effect of glucagon stimulation of liver cells is to reduce the concentration of fructose-2,6-bisphosphate, an isomer that activates PFK-1 and is itself synthesized by PFK-2 when fructose-6-phosphate concentration rises... 

The carbonyl group in glucose and ribose is an aldehyde such compounds are termed aldoses. Fructose, by contrast, has a ketone group and is therefore classified as a ketose. Glucose could also be termed an aldohexose and fructose a ketohexose, whereas ribose would be an aldopentose, names which indicate both the number of carbons and the nature of the carbonyl group. Another aspeet of nomenclature is the use of the suffix -ulose to indicate a ketose. Fructose could thus be referred to as a hexulose, though we are more likely to see this suffix in the names of specific sugars, e.g. ribulose is a ketose isomer of the aldose ribose. 

When the original methyl D-fructofuranoside sirup was fermented with yeast, the unstable beta isomer was selectively eliminated and the residue yielded a crystalline methyl D-fructoside melting at 81° and with [a] D +93° in water. The ring structure of this new isomer was proved to be furan by methylation to the liquid tetramethyl derivative, of [a] °D +129.4°, and subsequent hydrolysis to 1,3,4,6-tetramethyl-D-fructofuranose (structure IX) with the correct specific rotation of +29.8° in water. Both the methyl D-fructoside and its fully methylated derivative were therefore of the alpha configuration, since the latter was more dextrorotatory than the tetramethyl-D-fructose and also since the former was more dextrorotatory than the isomer, of [a] D —51°, unstable to invertase. Similar work with the benzyl D-fructofuranoside sirup produced the crystalline alpha isomer, melting point 89°, [a] D +45.7° in water, the liquid tetramethyl derivative, [a] D +83.3° in chloroform and, after acid hydrolysis of the latter, 1,3,4,6-tetramethyl-D-fructofuranose. 

It will be observed that the constants correspond rather well and we may conclude that in common fructose the hydroxyl groups occupy a less favorable position than in the isomer (or isomers) formed during the mutarotation reaction. 

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