Conformer, glucose

Figure 1 The ct-D-pyranose forms of the aldohexoses in their Cj conformations. Glucose is displayed on the top row with (from left to right) its 2-, 3- and 4-epimers. Idose is displayed on the bottom row with (from left to right) its 2-, 3- and 4-epimers. From the structures, the similarity between these monosaccharides, their affinity for water and the complexity derivedfrom their numerous stereogenic centres is apparent.

The following are representations of two forms of glucose The six membered ring is known to exist in a chair conformation in each form Draw clear representations of the most stable con formation of each Are they two different conformations of the same molecule or are they stereoisomers Which substituents (if any) occupy axial sites ... 

Haworth formulas are satisfactory for representing configurational relationships in pyranose forms but are uninformative as to carbohydrate conformations X ray crystal lographic studies of a large number of carbohydrates reveal that the six membered pyra nose ring of D glucose adopts a chair conformation... 

Cyclodextrins are macrocyclic compounds comprised of D-glucose bonded through 1,4-a-linkages and produced enzymatically from starch. The greek letter which proceeds the name indicates the number of glucose units incorporated in the CD (eg, a = 6, /5 = 7, 7 = 8, etc). Cyclodextrins are toroidal shaped molecules with a relatively hydrophobic internal cavity (Fig. 6). The exterior is relatively hydrophilic because of the presence of the primary and secondary hydroxyls. The primary C-6 hydroxyls are free to rotate and can partially block the CD cavity from one end. The mouth of the opposite end of the CD cavity is encircled by the C-2 and C-3 secondary hydroxyls. The restricted conformational freedom and orientation of these secondary hydroxyls is thought to be responsible for the chiral recognition inherent in these molecules (77). 

The amino acid compositions and sequences of the /3-strands in porin proteins are novel. Polar and nonpolar residues alternate along the /3-strands, with polar residues facing the central pore or cavity of the barrel and nonpolar residues facing out from the barrel where they can interact with the hydrophobic lipid milieu of the membrane. The smallest diameter of the porin channel is about 5 A. Thus, a maltodextrin polymer (composed of two or more glucose units) must pass through the porin in an extended conformation (like a spaghetti strand). 

Glycogen phosphorylase conforms to the Monod-Wyman-Changeux model of allosteric transitions, with the active form of the enzyme designated the R state and the inactive form denoted as the T state (Figure 15.17). Thus, AMP promotes the conversion to the active R state, whereas ATP, glucose-6-P, and caffeine favor conversion to the inactive T state. 

The chair conformation of cyclohexane has many consequences. We ll see in Section 1.1.9, for instance, that the chemical behavior of many substituted cyclohexanes is influenced by their conformation. In addition, we ll see in Section 2S.5 that simple carbohydrates such as glucose adopt a conformation based on the cyclohexane chair and that their chemistry is directly affected as a result. 

Galactose, a sugar related to glucose, contains a six-membered ring in which all the substituents except the -OH group indicated below in red are equatorial. Draw galactose In its more stable chair conformation. 

The conformational symbols for enantiomers are different. It is therefore important to state in the context whether the d or the L form is under consideration. Enantiomers have the same reference plane (see 2-Caib-7.3), and it should be noted that the mirror image of a-D-glucose-4Ci is a-L-glucose-C4. 

It functions by generating a gated pore in the membrane to permit passage of glucose the pore is conformationally dependent on the presence of glucose and can oscillate rapidly (about 900 times/s). 

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