Furanose ring structures

A pyranose or furanose ring structure and an a-glycol configuration leading to the anti glycol conformation... 

They then prepared the completely methylated derivatives of adeno-gine . and guanosine by simultaneous deacetylation and methylation of the acetylated nucleosides. In this way, trimethyl-iV-methyl adenosine and trimethyl-A -methyl guanosine were formed, and isolated as the hydrochlorides. On hydrolysis of the adenosine derivative by means of dilute hydrochloric acid, 6-iV-methyladenine and trimethyl-D-ribo-furanose were isolated. The same trimethyl sugar was isolated from the methylated guanosine and was identified in each case by oxidation, first to trimethyl-7-D-ribonolactone and then to meso-dimethoxy succinic acid. It follows that the sugar component has the furanose ring-structure. 

These formulations were confirmed by treatment of the diacetyl-adenosine with p-toluenesulfonyl chloride, giving a ditosyl-diacetyl-adenosine, only one tosyloxy group of which was substituted by iodine on treatment with sodium iodide. Hence the ribose residue of adenosine has the furanose ring structure. 

The ring structure of the ribose residue was ascertained" in the same general manner as for adenosine and guanosine. Triacetyl-dihydrouridine was prepared by the hydrogenation of triacetyl-uridine. On simultaneous deacetylation and methylation this was transformed to the fully methylated dihydro-uridine. By simultaneous hydrolysis and oxidation of this product, with hydrobromic acid and bromine, trimethyl -D-ribonolactone was formed, its identity being confirmed by oxidation to meso-dimethoxy-succinic acid. It follows that the ribose component has the furanose ring structure, and that uridine is 3 -D-ribofuranosyl-uracil. 

Furthermore Levene and Tipson discovered that uridine will condense with acetone in the presence of sulfuric acid and anhydrous copper sulfate to give a mono-isopropylidene-uridine. On treatment with tosyl chloride it gave a tosyl-isopropylidene-uridine which reacted readily with sodium iodide in acetone to give a crystalline monoiodo-isopropyli-dene-uridine, showing that the tosyl group is at position (5) of the sugar chain. Hence the isopropylidene derivative is 2,3-isopropylidene-uridine with a furanose ring structure, and uridine is D-ribofuranosyl-uracil. 

During the phosphoglucose isomerase reaction, the pyranose structure of glucose 6-phos-phate is converted into the furanose ring structure of fructose 6-phosphate. Does this conversion require an additional enzyme Explain. 

One head group - one chain may seem a simple structural architecture for an amphiphile, but for glycolipids the situation is complicated by the variation of the structures of the sugar units available for head group design, i.e. the head group may have an open chain structure, or a pyranose or a furanose ring structure. 

Fructose differs from glucose only at C-1 and C-2. When free, fructose occurs in the more stable pyranose ring form, but when combined, as in sucrose or its phosphate esters, it assumes the five-membered furanose ring structure. 

The same trend was observed for o-fructose and its disaccharide derivatives lactulose and leucrose. Strong fluorescence responses were observed with d-fructose and lactulose (saccharides which can interconvert between their pyranose and furanose forms). Conversely, only a weak fluorescence response was observed with leucrose, which is covalently bound at the 5-position (the fourth carbon as numbered from the anomeric centre) and as such cannot isomerise to adopt a furanose ring structure (but still allows approaching boronic acids unfettered access to the C2,C3-diol). 

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