Question

The trisaccharide raffinose occurs principally in cottonseed meal. (a) Name the three monosaccharide units in raffinose. (b) Describe each glycosidic bond in this trisaccharide. (c) Is raffinose a reducing sugar? (d) With how many moles of periodic acid will raffinose react?

Short answer

Question: Identify the three monosaccharide units that compose raffinose, describe the glycosidic bonds linking these units, and determine whether raffinose is a reducing sugar or not. Finally, find out the number of moles of periodic acid that raffinose will react with. Answer: Raffinose is composed of three monosaccharide units: D-galactose, D-glucose, and D-fructose. It has an α(1→6) glycosidic bond connecting D-galactose to D-glucose, and an α(1→2) glycosidic bond connecting D-glucose to D-fructose. Raffinose is not a reducing sugar because all of its anomeric carbons are involved in glycosidic linkages, leaving no free functional groups to act as reducing agents. Raffinose will react with two moles of periodic acid due to the presence of two vicinal diols in its structure.

Step-by-step solution

(a) Identifying monosaccharide units in raffinose:

Raffinose is a trisaccharide, meaning it is composed of three monosaccharide units. These units are D-galactose, D-glucose, and D-fructose. All three are hexose sugars containing six carbon atoms.

(b) Describing glycosidic bonds:

In raffinose, the glycosidic bonds link the monosaccharide units together. The bonds are as follows: 1. An α(1→6) glycosidic bond connects D-galactose to D-glucose. 2. An α(1→2) glycosidic bond connects D-glucose to D-fructose.

(c) Determining if raffinose is a reducing sugar:

A reducing sugar is a sugar that can donate a free aldehyde or free ketone functional group in its open-chain or cyclic form. Raffinose is not a reducing sugar because all of its anomeric carbons that could form aldehyde or ketone groups are involved in glycosidic linkages, leaving no free functional groups to act as reducing agents.

(d) Moles of periodic acid reacting with raffinose:

Periodic acid (HIO_4) is an oxidizing agent that reacts with vicinal diols (two adjacent hydroxyl groups) on carbohydrates. Raffinose has two such vicinal diols in its structure (one between C2 and C3 of D-galactose, and another between C3 and C4 of D-glucose). Thus, raffinose will react with two moles of periodic acid to cleave these vicinal diols.