Glucopyranose chair form

Figure 9.9 Conformation of D-glucose. The pyranoses adopt a non-planar ring conformation and the chair form with the highest number of equatorial rather than axial hydroxyl groups is favoured. It should be noted that a-D-glucopyranose, in contrast to /3-D-glucopyranose, has an axial hydroxyl group.

All of the crystalline pyranoses thus far examined adopt a chair conformation. A boat conformation has not yet been found for crystalline monocyclic compounds of sugars. Fused-ring systems seem to be required for part of the molecule to adopt a boat form, as in sedo-heptulosan (5) (where a chair form is also a part of a boat form (fused to the boat form)27 and l,6-anhydro-/3-D-glucopyranose. 

Figure 9.4 Chair forms of glucose anomers. Note that the -OH group on the anomeric carbon (carbon 1) is axial (less stable) in a-D-glucopyranose, whereas it is equatorial in (3-D-glucopyranose. Mutarotation therefore favors the latter.

Figure 11.7. Chair and Boat Forms of P - d -glucopyranose. The chair form is more stable because of less steric hindrance as the axial positions are occupied by hydrogen atoms.

However, two chair forms can be drawn for each of the two anomers of D-glucopyranose, as shown in Figure 9-6 for -D-glucopyranose. The structure on top is designated the Ci form because it contains carbon 4 and carbon 1 above and below the plane of the molecule, respectively. The designation of 4 for the structure on the bottom conveys the opposite sense. In the Ci conformation, all the large substituent groups are in equatorial positions, whereas in the 4 conformation, they are in axial positions. is the more stable and therefore the preferred chair conformation. 

A mathematical method has been described for determining, from independent variables that include the contribution of valence-angle deformation, the potential barrier for conformational inversion in j8-D-glucopyranose. The energy required for the conversion of a chair form to a boat form was calculated to be 13.5 kcal.mole , and the energy required for the reverse process, 6.5 kcal.mole . Ex-... 

Draw the chair forms of both P-D-glucopyranose and a-D-glucopyranose from Fig. 3-9. It is easy to see that for each of the a and P forms the C1 conformation would be favored as most of the functional groups lie in... 

C(l)-chair conformation, but instead adopts a half-chair form with torsion angles in the range 0—82°. The five-membered anhydro ring has an envelope conformation. The drastic effect of the three-membered-ring formation on the geometry of (42) is emphasized by an examination of l,6-anhydro-/J-D-glucopyranose (43), the conformation of which, although distorted, is still... 

Glucose binds in both the BII and Bill crystal forms in the deep cleft separating the two lobes of each subunit. Glucose appears to bind in the C-l (chair equatorial) a-D-glucopyranose conformation (73). All of the hydroxyls, except the 1-hydroxyl, are thus in an equatorial position. Protein side chains are hydrogen bonded to the 3-, 4-, and 6-hydroxyls and possibly to the 1-hydroxyl of glucose. The 2-hydroxyl appears to be pointing toward open space (73). This corresponds nicely with the observed substrate specificity. 

X-ray crystallographic studies have shown that crystalline D-glucose as commonly isolated exists in the a-D-glucopyranose form. Furthermore the stable chair conformation (1), in which the hydroxyl groups on C-2, C-3 and C-4, and the hydroxymethyl group on C-5 are equatorial, is preferred to the alternative chair conformation (6) in which these groups occupy axial positions. 

Fig. 2-11. Chair conformations of jS-D-glucopyranose. Form 1, in which the OH and CH2OH substituents are equatorial, is favored.

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