Hexoses

The objective of this article is to illustrate and visualize the structures and concepts of glucose (aldohexose^[1]) and fructose (ketohexose^[2]) that are difficult to visualize and illustrate by viewing two dimensional structures in textbooks.

Glucose

Fischer Projection Structure of D-Glucose.

The figure to the left contains D-glucose drawn as a Fischer projection structure. When drawing a Fischer projection the most oxidized group, in this case the aldehyde group, is positioned at the top, all horizontal bonds project to the front of the plane of the screen and all vertical bonds project behind the plane of the screen. The structure shown to the right in the Jmol applet is drawn in this same conformation, but the structure gives the appearance of being 3D. The applet shows the glucose molecule circling back on itself, so that carbon #6, C-6, (green) circles around to meet the aldehyde carbon, C-1 (orange). Projecting this 3D structure on to a 2D surface gives the Fischer projection structure. In order to observe that the hydroxyl groups on the chiral^[3] carbons project to the same sides of the carbon chain on the two structures, rotate the Jmol structure upward so that C-1 moves to the back of the screen. When you do that, you will see that the hydroxyl groups on the chiral carbons are on the same sides of the carbon chain as they are in the 2D structure. Compare the structure of the common saw-tooth conformation of D-glucose to that of the Fischer projection structure. Toggle off the spin and rotate the molecule so that the hydroxyl group on C-5 is on the right side of the carbon chain. Now, notice the differences in the orientations of the hydroxyl groups on the chiral carbons in the sawtooth conformation compared to those in the Fischer projection. This comparison shows that the saw-tooth conformation can not be used to make the enantiomeric^[4] assignment.

D-glucose in a conformation which positions the aldehyde carbon (yellow) so that it can react with the oxygen (green) bonded to C-5 to form a hemiacetal^[5] A result of this reaction is that C-1 becomes chiral, and one of two possible stereoisomers (anomers^[6]) is formed. One anomer, α-D-glucopyranose ^[7] is shown from the perspective of looking on the edge of the structure. This perspective or the Haworth^[8] projection is often shown in text books. The anomeric^[9] carbon, C-1 colored orange, is shown on the right side of the structure, its hydroxyl group is projecting down and C-6, not being in the ring, projects up. Notice that, unlike the Haworth projection, the pyranose ring is not planear. The other anomer, β-D-glucopyranose is shown, its structure so that it is positioned similar to α-D-glucopyranose - viewing the front edge of the ring, anomeric carbon is on the right and C-6 projects up. What is the one, and only one, difference in the 3D structures of these two molecules?

Fructose

The applet on the right shows D-fructose in a conformation in which the oxygen of C-5 is in position to react with C-2, the carbonyl carbon, forming a hemiketal^[10]. As in the case of glucose forming a hemiacetal, the carbonyl carbon becomes a chiral carbon and an anomeric carbon. The two possible anomers are called α-D-fructofuranose ^[11] and β-D- fructofuranose. The α anomer is shown with an edge-on-view, with the anomeric carbon (C-2) on the right side of the structure and with its hydroxyl group projecting down. C-1 is not part of the five membered ring and projects above the ring.