Ask a Chemist
Associate Professor Allan Blackman from the University of Otago is our in house chemist. He’ll be answering your questions about all things chemical so if you have something you’ve always wondered about email us here
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January 31- Why is DNA called an acid, especially since it contains a ‘base’ and doesn’t appear to have any dissociating hydrogen?
DNA is short for deoxyribonucleic acid, and, as you point out, it does seem rather unusual that it is named as an acid when it is best known for containing four different types of bases, namely adenine, cytosine, guanine and thymine. However, DNA does actually have acidic properties, a fact first recognised by Richard Altmann in the late 19th century who introduced the name ‘nucleic acid’, and these stem from the fact that it is formally a derivative of phosphoric acid. If you compare the structures of phosphoric acid (Figure 1) and a short strand of DNA (Figure 2), you’ll see that in the latter, two protons of phosphoric acid are replaced by carbon atoms either in, or attached to, the five-membered ring. In chemical terms, such a group is called a phosphate diester. The remaining proton is now quite acidic, and is relatively easily lost, thereby giving DNA its acidic character. Indeed, under neutral conditions, DNA is deprotonated at this site, and the oxygen atom bears a negative charge. Despite the fact that DNA does contain many basic groups, their basic properties are masked somewhat because of the fact that they hydrogen bond with each other to form base pairs. Hence it’s the acidic part of the molecule that dominates, and that is why we know DNA as an acid.
Figure 1: Phosphoric acid
Figure 2: Short strand of DNA