Sugar molecules

A major trend in organic synthesis, however, is the move towards complex systems. It may happen that one needs to combine a steroid and a sugar molecule, a porphyrin and a carotenoid, a penicillin and a peptide. Also the specialists in a field have developed reactions and concepts that may, with or without modifications, be applied in other fields. If one needs to protect an amino group in a steroid, it is advisable not only to search the steroid literature but also to look into publications on peptide synthesis. In the synthesis of corrin chromophores with chiral centres, special knowledge of steroid, porphyrin, and alkaloid chemistry has been very helpful (R.B. Woodward, 1967 A. Eschenmoser, 1970). 

Disacchandes are carbohydrates that yield two monosaccharide molecules on hydroly SIS Structurally disaccharides are glycosides m which the alkoxy group attached to the anomeric carbon is derived from a second sugar molecule... 

The separation of fmctose from glucose illustrates the interaction between the framework stmcture and the cation (Fig. 5) (50). Ca " is known to form complexes with sugar molecules such as fmctose. Thus, Ca—Y shows a high selectivity for fmctose over glucose. However, Ca—X does not exhibit high selectivity. On the other hand, K—X shows selectivity for glucose over fmctose. This polar nature of faujasites and their unique shape-selective properties, more than the molecular-sieving properties, make them most useful as practical adsorbents. 

All known eight-stranded a/p-barrel domains have enzymatic functions that include isomerization of small sugar molecules, oxidation by flavin coenzymes, phosphate transfer, and degradation of sugar polymers. In some of these enzymes the barrel domain comprises the whole subunit of the protein in others the polypeptide chain is longer and forms several additional domains. An enzymatic function in these multidomain subunits, however, is always associated with the barrel domain. 

Figure 4.21 The polypeptide chain of the arabinose-binding protein in E. coli contains two open twisted a/P domains of similar structure. A schematic diagram of one of these domains is shown in (a). The two domains are oriented such that the carboxy ends of the parallel P strands face each other on opposite sides of a crevice in which the sugar molecule binds, as illustrated in the topology diagram (b). [(a) Adapted from J. Richardson.)...

CAP controls a number of operons, all of which are involved in the breakdown of sugar molecules and one of which is the lac operon. When the level of the breakdown products of lactose is low, the concentration of cyclic AMP in the cell increases and CAP is switched on, binds to its specific operators, and increases the rate of transcription of adjacent operons. 

Glucose A 6-carbon sugar molecule, which is the building block of natural substances like cellulose, starch, dextrans, xanthan, and some other biopolymers and used as a basic energy source by the cells of most organisms. 

Expression (4) is applicable to the solubility equilibria of some substances in water, but not of all. Contrast, for example, water solutions of sugar, salt, and hydrochloric acid. Sugar forms a molecular solid and, as it dissolves in water, the sugar molecules remain intact. These molecules... 

Sugar is added to a cup of coffee until no more sugar will dissolve. Does addition of another spoonful of sugar increase the rate at which the sugar molecules leave the crystal phase and enter the liquid phase Will the sweetness of the liquid be increased by this addition Explain. 

For most exopolysaccharides their shape is determined by the angle of bonds which governs the relative orientations of adjacent sugar residues in the chain. However, die range of relative orientations of adjacent sugar molecules is limited by steric interactions between molecules along the chain. 

Olestra is a nondigestible plastic, made by adding eight fatty acid molecules to the sugar molecule sucrose. The fatty acids block access to the sugar, so enzymes can t break it down. The result is a molecule that behaves like a fat, but cannot be digested by humans. 

Glycoprotein A protein containing sugar molecules attached to its polypeptide chain. 

Glycosylation The incorporation of a sugar molecule into a protein. 

However, the pattern is complicated by several factors. The sugar molecules to be hydrogenated mutarotate in aqueous solutions thus coexisting as acyclic aldehydes and ketoses and as cyclic pyranoses and furanoses and reaction kinetics are complicated and involve side reactions, such as isomerization, hydrolysis, and oxidative dehydrogenation reactions. Moreover, catalysts deactivate and external and internal mass transfer limitations interfere with the kinetics, particularly under industrial circumstances. 

Stereospecific dependencies of the nonselective relaxation-rates for other protons in a sugar molecule may be seen in the data for a set of 3,4,6-tri-O-acetyl-l-0-benzoyl-2-deoxy-2-bromo-D-hexopyranose derivatives (40-42), as well as other 2-halogeno derivatives. A comparison of their... 

If a semipermeable membrane separates two identical solutions, solvent molecules move in both directions at the same rate, and there is no net osmosis. The two sides of the membrane are at dynamic equilibrium. The situation changes when the solutions on the two sides of the membrane are different. Consider the membrane in Figure 12-14a. which has pure water on one side and a solution of sugar in water on the other. The sugar molecules reduce the concentration of solvent molecules in the solution. Consequently, fewer solvent molecules pass through the membrane from the solution side than from the pure solvent side. Water flows from the side containing pure solvent to the side containing solution, so there is a net rate of osmosis. 

Sugar dissolves In water to give a solution that contains individual sugar molecules distributed uniformly among the water molecules. The aqueous sugar solution is stable and remains uniform indefinitely. Recall from Chapter 1 that a solution is a homogeneous mixture. On the microscopic scale, one microscopic portion of a solution looks the same as every other microscopic portion. 

One of the characteristic differences between a solution and a heterogeneous mixture is the sizes of the particles composing each of them. Molecule-sized particles such as sugar molecules, with dimensions in the nanometer range, tend to form solutions. Particles such as cornstarch granules, with dimensions larger than micrometers, tend to form heterogeneous mixtures. 

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