In particular, it involves the use of the alpha form of D-glucose and the beta form of D-fructose. In the diagrams below, note that the a-D-glucose is in the conventional orientation (the #6C is up). The b-D-fructose, however, is shown in two orientations. In the standard orientation the #6C is up on the left side and the b-2-OH is up on the right. Because it will be the b-2-OH group that bond to the -1-OH of the a-D-glucose, the b-D-fructose molecule must be inverted. Take a moment to study the diagrams and identify the location of the numbered carbon atoms in each diagram. (Similar diagrams are shown in Example 27 of your workbook for reference. The carbon atoms are numbered in those diagrams.)
As you look at these diagrams in the equation below (and in your workbook), be sure to note the numbering for fructose is opposite from the conventional orientation, and that is just because we want the two reacting OH's to be next to one another. When these two OH's react by an enzyme-catalyzed dehydration reaction, the resulting product is sucrose.
The glycosidic bond for the sucrose is sometimes referred to as an a-b-1-2 bond, because there's an alpha-OH from the glucose bonding to a beta-OH from the sucrose, and we're going from the #1 carbon on the glucose to the #2 carbon on the fructose. There are a few other ways of indicating this designation, (a-1)®(b-2) is probably the most descriptive, but they all try to say the same kind of thing, that we're dealing with the #1-OH in the alpha position bonded to the #2-OH in the beta position.
Sucrose is an unusual disaccharide in that it is a nonreducing sugar. This is because both of the hydrogen atoms removed in the dehydration reaction came from OH groups that were created during ring closure. Consequently, neither of the rings is able to open.
Structure and Lab Work
You are expected to be able to draw and recognize the structural formula for sucrose. I recommend that when you are in the lab, you practice with a-D-glucose and b-D-fructose by drawing their structures and also working with models. Then connect them together to become familiar with the structural formula for sucrose.
E-mail instructor: Sue Eggling
Clackamas Community College