Question

D-Arabinose and D-ribose give the same phenylosazone. D-Ribose is reduced to the optically inactive penta- hydric alcohol, ribitol. D-Arabinose can be degraded by the Ruff method, which involves the following reactions: The tetrose, D-erythrose, so obtained can be oxidized with nitric acid to mesotartaric acid. What are the configurations of D-arabinose, D-ribose, ribitol, and D-erythrose?

Short answer

The configurations of the given molecules are: D-ribose: CH2OH, (CHOH)3, CH=O; Ribitol: CH2OH, (CHOH)4; D-arabinose: CH2OH, (CHOH)3, CH=O; and D-erythrose: CH2OH, (CHOH)2, CH=O.

Step-by-step solution

Configuration of D-ribose

According to the given information, D-ribose and D-arabinose form the same phenylosazone. For two sugars to give the same phenylosazone, their configurations must be the same at the second and third carbon atoms. Therefore, D-ribose has the same configuration at these carbons as D-arabinose. The configuration of D-ribose is as follows: D-ribose: CH2OH, (CHOH)3, CH=O

Configuration of ribitol

D-ribose is reduced to form the optically inactive penta-hydric alcohol, ribitol. During the reduction, the aldehyde functional group (CH=O) at the first carbon atom is reduced to an alcohol (CH2OH). Since the new molecule is optically inactive, it does not have any chiral centers. The configuration of ribitol is as follows: Ribitol: CH2OH, (CHOH)4

Configuration of D-arabinose

During the Ruff degradation, D-arabinose forms D-erythrose. We do not know the configuration of D-erythrose or D-arabinose yet, but we know that both sugars have the same configuration at the third and fourth carbon atoms. The configuration of D-arabinose is as follows: D-arabinose: CH2OH, (CHOH)3, CH=O

Configuration of D-erythrose

D-erythrose is formed from D-arabinose by the Ruff degradation. We know that the configuration of D-erythrose at the third and fourth carbon atoms is the same as that of D-arabinose. The configuration of D-erythrose is as follows: D-erythrose: CH2OH, (CHOH)2, CH=O In conclusion, the configurations of the given sugars and sugar alcohol are as follows: - D-ribose: CH2OH, (CHOH)3, CH=O - Ribitol: CH2OH, (CHOH)4 - D-arabinose: CH2OH, (CHOH)3, CH=O - D-erythrose: CH2OH, (CHOH)2, CH=O

Key concepts

D-ribose Structure

D-ribose is a five-carbon sugar (pentose) that is a crucial component of RNA (ribonucleic acid). The structure of D-ribose is pivotal since it helps define the molecule's biological role.

In its linear form, the D-ribose molecule has an aldehyde group at one end, represented as CH=O, and a hydroxyl group (CH2OH) at the other end. Between these, there are three more carbon atoms each bearing their own hydroxyl group (CHOH). Understanding the orientation of these hydroxyl groups is essential, as it determines the sugar's properties and interactions.

The molecule can be depicted as follows in its linear form (from the first carbon at the aldehyde to the last carbon bearing the CH2OH group):
CH=O
(CHOH)₃
CH2OH

In a solution, D-ribose tends to form a ring structure for stability, which is more commonly found in biological systems. This ring form is crucial for the construction of the ribose-phosphate backbone of RNA.

Ribitol Configuration

Ribitol is a sugar alcohol derived from the reduction of ribose. When D-ribose is reduced, the aldehyde group (CHO) at the first carbon is converted into an additional hydroxyl group, resulting in four CHOH groups and one CH2OH group in ribitol.

The structure of ribitol as a sugar alcohol can be represented as (from one end of the molecule to the other):
CH2OH
(CHOH)₄
Because ribitol essentially has the same arrangement of carbon and its substituents as D-ribose without the aldehyde group, it lacks the characteristic chiral centers that impart optical activity. For students to visualize this, thinking of ribitol as a 'flattened' version of ribose where the sides are the same can be helpful, leading to no optical activity.

D-arabinose Configuration

D-arabinose is another pentose sugar with a configuration similar to D-ribose, differing only at the second carbon's hydroxyl group orientation.

Following the same depiction used for D-ribose, D-arabinose has a linear structure that can be written as:
CH2OH
(CHOH)₃
CH=O

The arrangement of the hydroxyl groups is particularly important because if two sugars form the same phenylosazone, it indicates they have identical configurations at the second and third carbon atoms—a key concept in sugar chemistry. Therefore, D-arabinose and D-ribose form the same phenylosazone since their configurations agree at the relevant carbon atoms. This is a classic case in carbohydrate chemistry that students should take note of, illustrating how minor structural differences can profoundly influence chemical behavior and reactivity.

Ruff Degradation

Ruff degradation is a process that sequentially shortens the carbon chain of a sugar, transforming a pentose into a tetrose. This sequence of reactions is an illustrative example of chemical transformations that sugars can undergo.

In the case of D-arabinose, the Ruff degradation converts it into D-erythrose. It retains the same configuration at the third and fourth carbon atoms due to the nature of the degradation process. This demonstrates significant concepts in carbohydrate chemistry, such as the retention of stereochemistry in certain reactions.

Students should note that understanding degradation processes like Ruff not only demonstrates the ability to convert sugars into different forms but also emphasizes the stereochemical relationships between these forms. The configuration of D-erythrose as a result of Ruff degradation from D-arabinose is expressed as:
CH2OH
(CHOH)₂
CH=O

The process and its implications are powerful tools in the field of carbohydrate chemistry for deducing structures and understanding the behavior of these biomolecules in nature.