What is Aldaric Acid? 

One of the groups of sugar acids is an aldaric acid. This is a group of acids where the carbonyl groups and terminal hydroxyl groups of the sugars are replaced by carboxylic acids. The formula that is representative of the aldaric acid is HOOC-(CHOH)n-COOH. 

Synthesis of Aldaric Acid 

When nitric acid is used to oxidize the aldoses, aldaric acid can be synthesized. However, during this process, the open-chain polyhydroxy aldehyde form of sugar undergoes reaction. This is because in aldose a terminal carbon has hydroxyl group in addition to the aldehyde group of the primary carbon that gets fully oxidized into the carboxylic acid groups. When the oxidation happens only in the terminal hydroxyl group, it results in uronic acid. When the oxidation happens only in the aldehyde group, it results in aldonic acid and when both the groups get oxidized then it results in the aldaric acid. 

Oxidation by Nitric Acid 

Nitric acid is a strong oxidizing agent. The oxidation of the secondary hydroxyl group found in the aldonic acid molecule by the nitric acid produces aldaric acid as a product compound. This kind of oxidation reaction is very essential as it can aid in finding a latent symmetry in the regions of the molecule which are left out. When nitric acid is allowed to react with ribose, xylose, allose, and galactose, achiral aldaric acids can be synthesized. These aldaric acids are optically inactive molecules. However, if nitric acid is allowed to react with diastereomeric aldose sugar, then chiral aldaric acids are synthesized which have the same configuration. For instance, when lyxose and arabinose are oxidized by utilizing nitric acid as an oxidizing agent, then chiral aldaric acid is formed as they have distereomeric centers. . 

Structure of Aldaric Acid

Depending on the sugars they are derived from, the sugars have obtained their nomenclature. For instance, the oxidized form of glucose is glucaric acid and xylose is xylaric acid. However, these sugars will not be similar to the parent sugars and have a carboxylic functional group at either end of the carbon chain. Hence, two different sugars can be derived from the same aldaric acid. Fischer projections are representative of this phenomenon as they represent sugar in an open chain form. For instance, L-glucaric acid and D-glucaric are the same compounds but they are represented as D and L due to difference in their configurations. Many forms of aldaric acids do not exhibit any optical activity due to the arrangement of several atoms in their structure in such a way that an imaginary plane could pass in between the structure. They are said to exist in meso forms and thus, do not have any optical activity in spite of them having multiple chiral centers. This usually happens when the sugar and its enantiomer are oxidized to form the same aldaric acid. D-galactose is an example as it has four chiral centers but L-galactaric acid and D-galactaric acid contain the opposite configuration at every chiral center. Hence, they can be expected to be an enantiomer which is of the same compound. So, the galactaric acid can be represented as an achiral meso form and does not fall under the category of optically active molecules. This can once again be understood by considering the fischer projection to know how the carbons have switched positions through the upside-down configuration. A compound different from aldaric acid is the adipic acid which is represented as HOOC-(CH2)4-COOH. It is structurally similar to aldaric acid, therefore, it was figured out that aldaric acids can be tetra hydroxyl derivatives of adipic acid. 

Use of Aldaric Acid 

There are four secondary hydroxyl groups present in the aldaric acid and two carboxlic groups that can be utilized for various used. Due to the presence of two carboxylic acid groups, they can be utilized as base chemicals to synthesize the chelating agents, polyanhydrides, polycations, polyesters, metal-organic frameworks, macromolecules, medicines, cross-linked hydrogels, polyamides, and coordination polymers. Aldaric acid can be utilized as monomers that can be utilized for the production of polymers that are biodegradable, condensed, non-toxic, and hydrophilic when compared to the polymers that are made out of petrochemicals. 

These acids can be utilized as carriers during the drug delivery process. When the polyamides that are composed of repeating units of secondary amides and aldaric acids are utilized, viruses can be replaced using them easily.  These viruses were used as vectors for nucleic acid medicine products. Numerous other macromolecules which contain the aldaric acid can function as a drug delivering system. 

Symmetry of Aldaric Acid  

The symmetry of the aldaric acid and the properties associated with it will generate information about the possible configurations of the secondary hydroxyl groups. If the oxidation of nitric acid has been used to synthesize the aldonic acid, it will be optically inactive. This is because the hydroxyl groups assemble in a symmetrical arrangement. For instance, d-galactose provides an optically inactive aldaric acid. The reaction of monosaccharides with the periodate provides a mix of oxidation products that can degrade the entire molecule. The products involve formic acid, formaldehyde, and carbon dioxide. It is important to find out if the monosaccharide is a ketose or an aldose by calculating the number of equivalents of every oxidation. When glucose is allowed to react with a periodate of five moles, it provides five moles of formic acid and one mole of formaldehyde. 

Context and Application 

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for ; Bachelors and Masters Genetics, Biochemistry and Molecular biology Biological science Masters in Biotechnology bachelor of medicine, bachelor of surgery. 

Related Concepts 

Sugar acids, Dicarboxylic acids, Carboxylic acids.