Question

(a) Draw the more stable chair form of fucose, an essential monosaccharide needed in the diet and a component of carbohydrates on mammalian and plant cell surfaces. (b) Classify fucose as a D- or L-monosaccharide. (c) What two structural features are unusual in fucose?

Short answer

Answer

(a.)

(b) Fucose is an L-fructose.

(c) An absence of the OH group at the sixth carbon is observed in fructose. Another feature is that several six-membered carbohydrates comprise a D-configuration, but fucose has an L configuration.

Step-by-step solution

Fucose

Fucose comes under the category of monosaccharide comprising the formula ${\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{12}}{\mathbf{O}}_{\mathbf{5}}$ . It constitutes the primary sub-unit of a seaweed termed polysaccharide fucoidan.

Monosaccharide

Carbohydrates comprise several units, and the simplest one constitutes the monosaccharide. A few examples of monosaccharides include glucose and fructose.

Stable form and unusual features of fucose

(a) The structure of fucose can be given as:

Structure of fucose

The more stable chair conformation of fructose constitutes the form where only a few groups are situated at the axial position. The structure given below comprises two groups at the axial position and can be given as:

(b) The Fischer projection of fructose can be given as:

Fischer projection of fructose

Here, the chiral carbon of the OH group is situated on the left side. Hence, the given fructose is the L-fructose.

(c)One major difference between fucose and other sugar is the absence of hydroxyl group at the sixth carbon atom, . Another difference is that most of the six-membered carbohydrates possess a D-configuration, but fucose has an L-configuration.