Basic Sugar Structure

The simplest carbohydrate, called a monosaccharide, is composed of a structure that can not be hydrolyzed to simpler polydroxylic compounds. A disaccharide is a carbohydrate that contains two of these basic units, and a polysaccharide contains many polyhydroxylic monomers. 

The aldehyde or ketone group of monosaccharides can undergo an intramolecular reaction with one of its own hydroxyl groups to form a cyclic, hemiacetal or hemiketal structure, respectively (Fig. 20). In aqueous solutions, this cyclic structure actually predominates. The open-chain aldehyde or ketone form of monosaccharides is in equilibrium with the cyclic form, but the open structure exists less than 0.5% of the time in aqueous environments. It is the open form that reduces Fehling s or Tollen s 

Since in aqueous solutions the cyclic form of monosaccharides is in equilibrium with their corresponding open forms, the ct and p structures continually interconvert. At equilibrium, one form usually predominates. For instance, glucose dissolved in water consists of about a 2 1 ratio of p-D-glucose to a-D-glucose. Although their chemical constituents are identical, the biochemical properties between the a and the P forms can be quite different. Monosaccharides linked together to form disaccharides and polysaccharides cannot continue to interconvert and are therefore frozen in the a or p forms. Changing one monosaccharide in a complex carbohydrate to its opposite 

Since in aqueous solutions the cyclic form of monosaccharides is in equilibrium with their corresponding open forms, the a and P structures continually interconvert. At equilibrium, one form usually predominates. For instance, glucose dissolved in water consists of about a 2 1 

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Figure 3.5 The basic polynucleotide structure as shown in (a) outline form and (b) in chemical detail. The 5 end of the chain is defined by lacking a nucleotide attached to the first sugar s carbon number 5 the 3 end lacks a nucleotide attached to the carbon number 3 of the last sugar in the backbone...

Most aminoglycosides are complexes of several almost identical components differing either in the degree of methylation of one amino sugar unit, as in the case of gentamicin, or in the stereochemistry of the disaccharide unit, as in the case of neomycin. Differences in the substitutions on the basic ring structures between the various aminoglycosides account for the relatively minor differences in antimicrobial spectra, patterns of resistance, and toxicity. 

The significance of n.m.r. spectroscopy for structural elucidation of carbohydrates can scarcely be underestimated, and the field has become vast with ramifications of specialized techniques. Although chemical shifts and spin couplings of individual nuclei constitute the primary data for most n.m.r.-spectral analyses, other n.m.r. parameters may provide important additional data. P. Dais and A. S. Perlin (Montreal) here discuss the measurement of proton spin-lattice relaxation rates. The authors present the basic theory concerning spin-lattice relaxation, explain how reliable data may be determined, and demonstrate how these rates can be correlated with stereospecific dependencies, especially regarding the estimation of interproton distances and the implications of these values in the interpretation of sugar conformations. 

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