Question

(a) \((+)\) - Trehalose, \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\), a non-reducing sugar found in young mushrooms, gives only D-glucose when hydrolyzed by aqueous acid or by maltase. Methylation gives an octa-DMethyl derivative that, upon hydrolysis, yields only \(2,3,4,6-\) tetra-O-methyl-D- glucose. What is the structure and systematic name for \((+)\) -trehalose? (b) (-)-Isotrehalose and \((+)-\) neotrehalose resemble trehalose in most respects. However, isotrehalose is hydrolyzed by either emulsin or maltase, and neotrehalose is hydrolyzed only by emulsin. What are the structures and systematic names for these two carbohydrates?

Short answer

The structure of (+)-Trehalose is two α-D-glucose units connected by an α,α'-1,1'-glycosidic bond and has the systematic name α-D-Glucopyranosyl-(1→1)-α-D-glucopyranoside. The structures and systematic names for (-)-Isotrehalose and (+)-Neotrehalose are as follows: 1. (-)-Isotrehalose: α-D-Glucopyranosyl-(1→6)-α-D-glucopyranoside, with an α,α'-1,6' linkage. 2. (+)-Neotrehalose: β-D-Glucopyranosyl-(1→1)-α-D-glucopyranoside, with a β,α'-1,1' linkage.

Step-by-step solution

Determine the Trehalose structure

: It is given that Trehalose is a non-reducing sugar that yields only D-glucose when hydrolyzed. Also, the methylation of Trehalose results in an octa-D-Methyl derivative, which upon hydrolysis gives only 2,3,4,6-tetra-O-methyl-D-glucose. Therefore, Trehalose can be written as a combination of two glucose molecules, with all four free OH groups protected during methylation. To determine the linkage between the two glucose molecules, we note that the methylation product of Trehalose gave 2,3,4,6-tetra-O-methyl-D-glucose, meaning that positions 1 on both glucose molecules were not methylated. Thus, they are connected by a glycosidic bond: α,α'-1,1'-linkage. The structure of (+) -Trehalose will be two α-D-glucose units connected by an α,α'-1,1'-glycosidic bond.

Assign a systematic name to Trehalose

: Now that we have determined the structure of Trehalose, we can assign a systematic name. With an α,α'-1,1'-linkage between the two α-D-glucose units, the systematic name for (+) - Trehalose would be α-D-Glucopyranosyl-(1→1)-α-D-glucopyranoside.

Determine the structures of Isotrehalose and Neotrehalose

: We are given that (-)-Isotrehalose is hydrolyzed by emulsin or maltase, while (+)-Neotrehalose is hydrolyzed only by emulsin. As maltase affects the α-glycosidic bonds, we can assume that Isotrehalose has an α-glycosidic bond. Emulsin hydrolyzes β-glycosidic bonds, so Neotrehalose must contain a β-glycosidic bond. Knowing this, we can now deduce that (-)-Isotrehalose has an α,α'-1,6' linkage between two D-glucose units. (+)-Neotrehalose has a β,α'-1,1' linkage between two D-glucose units.

Assign systematic names to Isotrehalose and Neotrehalose

: Now that we have determined the structures of (-)-Isotrehalose and (+)-Neotrehalose, we can assign systematic names: 1. (-)-Isotrehalose: α-D-Glucopyranosyl-(1→6)-α-D-glucopyranoside 2. (+)-Neotrehalose: β-D-Glucopyranosyl-(1→1)-α-D-glucopyranoside In summary, we have determined the structures and systematic names for (+)-Trehalose, (-)-Isotrehalose, and (+)-Neotrehalose based on the information given and their enzymatic hydrolysis properties.

Key concepts

Non-Reducing Sugar

In the realm of carbohydrates, sugars can be classified based on their reactivity towards specific reagents like Benedict's or Fehling's solution, which test for the presence of reducing end-groups. Trehalose is termed a non-reducing sugar because it does not have a free aldehyde or ketone group capable of acting as a reducing agent.

Typically, a reducing sugar has a free anomeric carbon—a carbon that's part of a carbonyl group in the open-chain form and becomes a new stereocenter when the sugar cyclizes to form either an α or β anomer. In trehalose, however, both glucose units are involved in the glycosidic linkage, securing the anomeric carbons and preventing the open-chain form, hence no reduction occurs during tests. This particular feature influences the chemical behavior and biological functions of trehalose, separating it from other sugars that actively participate in redox reactions.

Glycosidic Bond

The glycosidic bond is a type of covalent bond that connects monosaccharide units in carbohydrates. It can form between any hydroxyl group on the glycoside (sugar molecule with the free anomeric carbon) and a hydroxyl group on another molecule (which can be another sugar).

In the case of trehalose, the bond is an α,α'-1,1'-linkage, entailing that the 1-position (anomeric carbon) of both glucose molecules are involved, creating a unique di-glucose disaccharide. What's interesting about a glycosidic bond is its configuration—either α or β—which impacts its digestibility by enzymes. For trehalose, since it is comprised of α-links, the orientation is such that the O-glycosidic bond faces the same direction as the CH2OH group on the anomeric carbon, defining its digestion pathway and structural conformation.

Hydrolysis of Carbohydrates

Carbohydrates undergo hydrolysis to break down into simpler units, typically through enzymatic or acid-catalyzed reactions. This hydrolytic process involves the addition of a water molecule, breaking the glycosidic bond and releasing the constituent monosaccharides.

For trehalose, hydrolysis under acidic conditions or by the enzyme maltase yields two glucose molecules, reflecting the enzyme's specificity for cleaving the α,α'-1,1'-linkage present in this sugar. Hydrolysis is a principal reaction in the metabolism of carbohydrates, enabling organisms to access the energy stored in complex sugars. The hydrolysis of non-reducing sugars like trehalose is particularly interesting, as they require specific conditions or enzymes to be cleaved, unlike reducing sugars that are more broadly reactive.