Ketoses composition

V. Composition in Aqueous Solution Ketoses 1. Hexuloses and Pentuloses... 

The composition of solutions of the 2-heptuloses has been determined, and discussed, by Angyal and Tran.92 These ketoses are different from other reducing sugars inasmuch as there are two hydroxymethyl side chains attached to the pyranose ring. In the a-pyranose form, they are cis to each other and will therefore both be equatorial in the preponderant chair form. In the / -pyranose, however, one or other of the hydroxymethyl groups has to be axial and, in consequence, the / anomers are disfavored in only one solution (that of the altro isomer) was the yS-pyra-nose detected in the 13C-n.m.r. spectrum.92... 

Whereas the composition of solutions of the unsubstituted aldoses and ketoses has been systematically investigated, very few studies have been conducted on substituted and on derived sugars, such as amino sugars,... 

The simple sugars, or monosaccharides, are poly-hydroxyaldehydes (aldoses) or polyhydroxyketones (ketoses).1-5 All have the composition (CH20)n, hence the family name carbohydrate. A typical sugar, and the one with the widest distribution in nature, is glucose. 

The composition of many aldoses and two ketoses has been determined14 by, 3C-n.m.r. spectroscopy the results agreed well with those from previous determinations made from H-n.m.r. spectra. 

The Composition (%) of Aqueous Solutions of Ketoses and Amino Sugars... 

Campbell, J.M., Selected fructooligosaccharide (1-ketose, nystose, and lF-beta-fructofuranosylnystose) composition of foods and feeds, J. Agric. Food Chem., 45, 3076-3082, 1997. 

Integrated, 13C-n.m.r. spectra have been used extensively to study mutarotational equilibria of monosaccharides, especially of ketoses, which do not have well-resolved, H-n.m.r. spectra,26-33 and also have been used to determine the composition of crude reaction-mixtures.34... 

Following the production of 1,2-enol forms of aldose and ketose amines, a series of degradations and condensations results in the formation of melanoidins. The a-fi-dicarbonyl compounds enter into aldol type condensations, which lead to the formation of polymers, initially of small size, highly hydrated, and in colloidal form. These initial products of condensation are fluorescent, and continuation of the reaction results in the formation of the brown melanoidins. These polymers are of nondistinct composition and contain... 

As the name implies, carbohydrates are characterized by a nominal (CH20) composition. Next to proteins, carbohydrates combined into polysaccharides are generally the most abundant (Table 8.6) and widely distributed biopol5miers in living organisms. (However, there are some plankton in which the lipid concentration exceeds that of carbohydrates (Wakeham et al, 1997).) The fundamental monomeric rmits of most carbohydrates are five- (pentose) and six-(hexose) carbon sugars, of which there are roughly ten common forms (Fig. 8.9). These structural units usually occur as either aldoses or ketoses, in which either the first (aldose) or second (ketose) carbon... 

Monosaccharides can exist as aldehydes or ketones and are called aldoses or ketoses, respectively. For example, (Figure 9.3) shows the structures of glyceraldehyde, an aldo-triose, and dihydroxyacetone, a keto-triose. Glyceraldehyde and dihydroxyacetone have the same atomic composition, but differ only in the position of the hydrogens and double bonds. Moreover, they can interconvert via an enediol intermediate (Figure 9.4). When the structures of molecules are related in these ways, the molecules are called tautomers. 

Formose is composed of Ca to Cg aldoses and ketoses, each chiral compound existing in the n and the l form. Whenever the reaction is stopped at the yellowing point, the composition of the sugar members is rather... 

Aldopentoses - olive green aldohexoses - blue-grey deoxypentoses - pink to violet deoxyhexoses - yellow to green ketoses - yellow to red for disaccharides colors depend on both composition and linkage... 

The name carbohydrates comes from their composition which is represented by the general formula C (H20) which resembles the combination of carbon and water. This composition implies that a molecule of monosaccharide consists of a hydrocarbon chain with attached hydroxyl groups. More detailed analysis shows that most of the monosaccharide molecules have hydrocarbon chains which are from five to seven carbon atoms long. From the study of the chemical behavior of different monosaccharides it follows that some of them exhibit reactions typical for aldehydes and others show reactions typical for ketones. The monosaccharides can therefore be classified into two groups aldoses and ketoses. In their nomenclature, the names of carbohydrates are formed from the root based on the number of C-atoms, the fundamental functional group (ketone or aldehyde) and the suffix -ose. While the aldehyde group always contains the first carbon atom of the chain, keto-group in all known carbohydrates appears at the second carbon atom of the chain. Some monosaccharides named in accordance with these rules are represented in the scheme below. 

On the basis of the usual methods of analysis, which involve the determination of reducing sugars, ketoses, and mannose, Wolfrom and Lewis (80) concluded that very few side reactions take place if the glucose is treated with lime water saturated at 35°C. After about five days, the equilibrated mixture had the following composition ... 

The compositions of reducing sugars (aldoses, ketoses, mono- and di-saccharides) in solution (76 lefs.) and the hydrophobic behaviour of sugar molecules in aqueous media (33 refs.) have been reviewed. 

According to our work [42], when glucose and mannose are treated with 66.7 mM of aqueous NaOH solution at 65 C for a period as short as even 5 min, the composition of the products Glc/Man/Fru are 74 3 23 and 4 87 9, respectively. The corresponding ketose,i.e. fructose, was preferentially formed rather than the corresponding epimers, mannose and glucose. 

Table 4.6. Equilibrium composition of aldoses and ketoses in aqueous solution Compound T (°C) a-Pyranose p-Pyranose a-Furanose p-Furanose...

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