Nucleic Acid Acidity
Country: United States
Date: Fall 2010
My teacher's biology book says that nucleic acid
isn't actually acidic. Why is it called an acid?
DNA is acidic because it contains phosphoric acid, H3P04.
Ron Baker, Ph.D.
A little history might help explain this. The term 'nucleic acid' was
coined at time when the structure of biological molecules was unknown
-- proteins were known to exist, but were very mysterious, and real
knowledge of the nature of the chemicals of heredity was still 50
The term nucleic acid came to popularity in the late 19th century. In
the late 1890's, scientists were able to recover a material from
various biological sources (such as salmon sperm, yeast, plants) that
was not protein or carbohydrate or lipid, but they weren't sure of its
structure. Experiments with bacteria showed that this material plays a
role in heredity. Furthermore, this material could be broken down
ninto known bases such as purine or thymine. So they assumed it was an
acid that combined with those bases. Originally it was called nuclein,
but later, in 1899, a German chemist named Altmann coined the term
"nucleic acid", and the name stuck. It was called an 'acid' because it
combines with a base, and it was weakly acidic on its own, and
although it was insoluble natively, it could be solubilized in water
by forming a metal salt (such as with copper). It should be noted that
this was around the same time as modern definitions of acids (e.g.
Arrhenius, Bronsted, and Lewis) were being developed. As other
science-question sites have noted, there are acidic (e.g. 7 pKa)
groups on nucleic acids, but that's not the whole story.
Much later in the 20th century (and still ongoing today), scientists
elucidated the structure, chemistry, and function of nucleic acids,
such as DNA, RNA, and synthetic nucleic acids like PNA (you can read
more about all of these)
Can you send me the passage from the biology book? I'm curious how
they phrase it. It's possible the book is correct, and possible it's
mistaken, depending on how it's phrased. There are many definitions of
acids, and in this case the issue is complicated by the fact that the
term 'nucleic acid' was coined around 1899 before definitions were
Acids taste sour. We usually call our as acidic taste.
Based on the dissociation property of hydrogen, acids can be grouped
into strong acids (like sulfuric acid, hydrochloric acid, nitric acid
etc) and weak acids (generally organic acids & others like phosphoric
acids). Some of the weak acids have a weak sour taste.
Nucleic acids are composed of along chain of monomers called as
nucleotides. Each nucleotide is made up of a nitrogenous base (either
adenine or guanine or cytosine or thymine or uracil) a five carbon
sugar and a phosphate group (PO4). These three compounds are linearly
NITROGENOUS BASE-SUGAR-PHOSPHORIC ACID
Note that a single nucleotide has a base at one end and an acid at
another end – hence neutral!
But when these single units combine to form polymers (this is how they
normally exist in cells) the bases (hydrogen acceptors) are “buried”
and are involved in hydrogen bonding in DNA double helical structure.
The sugar (neutral) and phosphoric acid group form the backbone. The
acid phosphoric group gives “DNA” the negative charge. The polymeric
form is acidic and may not be having a sour/acidic taste (I have not
tasted DNA so far!).
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Update: June 2012