Question

Another D-2-ketohexose, psicose, yields a mixture of allitol and altritol when reduced with \(\mathrm{NaBH}_{4} .\) What is the structure of psicose?

Short answer

D-psicose is a C-3 epimer of D-fructose, with the OH on the left at C-3.

Step-by-step solution

Identify Psicose

Psicose is a ketohexose, which means it has six carbon atoms and a ketone group. It belongs to the family of sugars similar to fructose. To determine its structure, note that it is a C-3 epimer of fructose.

Draw the Structure of Fructose

Fructose is a ketohexose with the ketone group on the second carbon. The structure of D-fructose is: 1. - Carbon 1: CH2OH - Carbon 2: C=O - Carbon 3: OH on the right - Carbon 4: OH on the left - Carbon 5: OH on the right - Carbon 6: CH2OH Write this down and understand that in D-fructose the R-group at the third carbon is directed to the right.

Modify the Structure for Psicose

Since D-psicose is a C-3 epimer of fructose, alter the configuration at the third carbon (switch the OH group). For D-psicose: 1. - Carbon 1: CH2OH - Carbon 2: C=O - Carbon 3: OH on the left - Carbon 4: OH on the left - Carbon 5: OH on the right - Carbon 6: CH2OH This gives you the structure of D-psicose.

Explanation for Reduction Products

D-psicose, when reduced by \(\text{NaBH}_4\), forms allitol and altritol as stereoisomers. These polyols imply symmetric reduction which reduces the carbonyl group to a hydroxyl group, forming a mixture indicating different configurations at the previously ketone carbon which, in turn, confirms psicose's ketohexose structure.

Key concepts

Ketohexose

Ketohexoses are a fascinating group of sugars that play a crucial role in carbohydrate chemistry. These sugars are unique because they possess six carbon atoms, as indicated by the 'hex-' prefix, and contain a ketone group, denoted by 'keto-'.
Because of this specific structure, ketohexoses, like psicose, are part of the broader monosaccharide family often compared to fructose.
Understanding the structure of ketohexoses involves recognizing their carbon chain, where the ketone functional group is typically found on the second carbon atom:

• This structure allows ketohexoses to act as reducing sugars, meaning they can donate electrons, often resulting in significant reactions in biological systems and food chemistry.
• Additionally, the presence of multiple hydroxyl (OH) groups along with a ketone group distinguishes them from their aldose counterparts, where an aldehyde group is present.

Sugar Epimers

In the world of sugars, epimers are molecules that differ only in the configuration around one carbon atom. This minor variation, although subtle, can lead to important differences in the properties and reactions of sugars.
Psicose and fructose provide a perfect example of such a relationship, where psicose is the C-3 epimer of fructose.
This means:

• The hydroxyl group on the third carbon in psicose is oriented differently than in fructose.
• Such a small change influences the sugar's overall chemical behavior, including how it interacts during metabolic processes.
• Moreover, this particular epimerization at C-3 explains why psicose and fructose have distinct reduction products when they are subjected to chemical reactions, like reduction by sodium borohydride.

Reduction Products

When sugars undergo reduction, particularly by a reagent like sodium borohydride (\( \mathrm{NaBH_4} \)), they transform from their carbonyl-containing form into sugar alcohols or polyols.
This is a crucial process that demonstrates the versatility and dynamic nature of sugar chemistry.

• In the case of D-psicose, the reaction with \( \mathrm{NaBH_4} \) leads to the creation of two polyols: allitol and altritol.
• These are stereoisomers formed when psicose's ketone group is reduced to a hydroxyl group, a process that is made possible due to the specific orientation of its molecular structure.
• The formation of these products highlights how a single sugar can give rise to multiple products through reduction, each with slight differences in their carbon configurations and hydroxyl group orientations.
• Understanding these dynamics helps in exploring the potential uses of such sugar alcohols in the food industry and pharmaceuticals.

Stereochemistry

Stereochemistry involves the study of the spatial arrangement of atoms within a molecule, which is vital in understanding the function and reactivity of chemical compounds. In sugars, like psicose, stereochemistry is key to its role and interaction in biochemical pathways.
For ketohexoses:

• Stereochemistry determines the position of each hydroxyl group around the carbon atoms, which can affect how these sugars taste and behave biologically.
• It dictates whether a sugar acts as a preferable food source for certain organisms or influences its sweetness level.
• In the reduction reaction, the specific stereochemistry at the former ketone carbon in psicose allows the formation of distinct sugar alcohols.
• The three-dimensional arrangement also guides how these molecules interact in complex biochemical pathways, providing a foundation for understanding their role in health and disease.

This knowledge is essential for chemists and biochemists as they synthesize and utilize sugar-based compounds.