The Structure of D-Glucose

D-Glucose is the most common of the monosaccharides, occurring in the free state in the blood of animals and in the polymerized state, inter alia, as starch and cellulose. Tens of millions of tons of these polysaccharides are made by plants and photosynthetic microbes annually. A detailed study of the structure of glucose is justified on these grounds, and many of the structural features of all monosaccharides can be illustrated using glucose as an example. 

The Fischer projection formula for D-glucose (Example 2.3) is also known as the open- or straight-chain structure. This structure occurs only in solution. There are two crystalline forms of D-glucose, known as a and /3, which also have different optical activities when dissolved. X-ray diffraction studies have confirmed chemical evidence that a- and /3-D-glucose are structures containing a ring of five carbon atoms and one oxygen atom  

When a-D-glucose or /3-D-glucose is dissolved in water, the ring opens and the open-chain structure is formed. The reaction is reversible, and an equilibrium is established between the open form and the two ring forms. The chemistry of the process is understood in terms of the chemistry of the aldehyde group. In general, aldehydes react reversibly with alcohols to give hemiacetals and then, in the presence of an acid catalyst (Chap. 8), acetals  

The formation of a ring by the open-chain form of D-glucose can be considered to be the result of a reaction between the hydroxyl group on C-5 and the aldehyde group to give a hemiacetal. The aldehyde carbon becomes chiral as a result, thus giving rise to two hemiacetals, a- and /3-D-glucose. 

These hemiacetals are known as anomers, and C-l of the ring form is called the anomeric carbon. 

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A more detailed study on the structure of D-glucose based upon physical data and chemical reactivity has revealed that the open-chain formulation of the aldopentoses, aldohexoses, ketopentoses and ketohexoses is however an oversimplification. Thus, for example, in solution D-glucose exists as an equilibrium... 

Before we can go on to the pext aspect of the structure of D-(+)-glucose, determination of ring size, we must first learn a little more about the methylation of carbohydrates. 

The structure of D-glucose is shown. Draw its mirror image. 

Determine the structure of D-galactose, using arguments similar to those used by Fischer to prove the structure of D-glucose. 

Figure 11.3. Part I of the Fischer determiiiation of the structure of D-glucose. While in principle it might be any one of 16 possible isomers, once the penultimate carbon is written to the right, oniy eight possible isomers remain. And, since it was known that the Kiliani synthetic method could be used on pentoses of which there can be only four (once the penultimate carbon is written to the right ), the chemistry of the pentoses could be used to help define the structure of glucose.

Scheme 11.18. A representation of the Reichstein synthesis of L-ascorbic acid (Reichstein, T. Grussner, A. Helv. Chim. Acta, 1934,17,311). Note with regard to the structure of D-glucose as shown in the brackets, when the strncture is flipped 180°, the Rscher representation (where the groups on the horizontal are above the plane of the paper ) becomes reversed and would be below the plane. Thus, to preserve the absolute stereochemistry, the groups appear reversed in the drawing.

There are two possibilities for the structure of D-glucose, A and B. If D-glucose has structure A, then gulose, the sugar that gives the same aldaric acid on oxidation, must have the structure shown at the right in Figure 22.51. 

D-Glucose, D-mannose, and D-galactose are the most common aldohexoses in living systems. An easy way to learn their structures is to memorize the structure of D-glucose and then remember that D-mannose is the C-2 epimer of D-glucose and D-galactose is the C-4 epimer of D-glucose. 

D-Galactose is an aldohexose that does not occur in the free form in nature. It is obtained from the disaccharide lactose, a sugar found in milk and milk products. D-Galactose is important in the cellular membranes of the brain and nervous system. The only difference in the structures of D-glucose and D-galactose is the arrangement of the —OH group on carbon 4. 

For consistency the same disaccharides were evaluated here as were used in the evaluation of sensor 135. For the structures of D-glucose, melibiose and maltose, see Scheme 34 and for the structures of D-fructose, lactulose and leucrose, see Scheme 35. The fluorescence titrations of sensors 140( =3)-145( =g) and 146(pyrene) with different saccharides, were carried out in a pH 8.21 aqueous methanolic buffer solution, as described earlier. The fluorescence intensity of 140( =3)-145( =8) and 146(pyrene) (1-0 x 10 mol dm , A,ex=342 nm) increased... 

Further, when the two acids were separated and the Fischer-Kiliani synthesis was performed, (H-)-glucose and (+)-mannose were obtained. Fischer s proof then depended, in part, on the relationship between the structure of D-glucose and D-arabinose, and Fischer s problem was to determine the configurations of C-2 and C-3 of D-arabinose. 

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