Question

Why are deoxyribonucleic acids called acids? What are the acidic groups in their structure?

Short answer

Answer: DNA is called an acid due to the presence of phosphate groups in its structure, which contain the phosphoric acid group (\textrm{PO}_4^{3-}). This functional group can donate protons (H^+ ions) to a solution, increasing its acidity.

Step-by-step solution

Overview of DNA composition

DNA is a complex molecule made up of subunits called nucleotides. Each nucleotide contains three components: a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, or guanine). Nucleotides are connected to each other through a sugar-phosphate backbone with the nitrogenous bases projecting outwards.

Identifying the acidic functional group

The acidic property of DNA comes from the phosphate group in the nucleotides. A phosphate group consists of one phosphorus atom bonded to four oxygen atoms, with one of the oxygen atoms forming a bond with the deoxyribose sugar. The remaining oxygen atoms in the phosphate group form a functional group called the phosphoric acid group, which has the following chemical formula: \textrm{PO}_4^{3-}.

Explanation of the acidic nature

The reason DNA is called an acid is because the phosphoric acid group in the phosphate has the ability to donate protons (H^+ ions) to a solution, thereby increasing the acidity (lowering the pH) of the environment. The release of protons by the phosphate group occurs when the oxygen atoms form hydrogen bonds with water molecules or other molecules in the solution. This ionization of the phosphoric acid group gives DNA its acidic properties.

Conclusion

Deoxyribonucleic acids are called acids because the phosphate groups present in their structure contain the phosphoric acid group (\textrm{PO}_4^{3-}). This functional group can donate protons (H^+ ions) to a solution, increasing its acidity.