Phosphate
Four forms of phosphate Phosphate exists as a mixture of four species, with relative amounts
dependent on the pH of the aqueous environment. At the lowest
pH, (most acidic), the predominant form is phosphoric acid. This
acid is moderately strong, stronger than acetic or citric acid
(the acids in vinegar and lemons), but much weaker than sulfuric
acid. Phosphoric acid gives a characteristic taste to soft drinks,
e.g. Coke. At pH 2, there is an equal amount of phosphoric acid and ionized
phosphate. In the Figure we have sodium as the counter ion. At pH 7, the approximate pH in living cells, there is an equal
amount of mono- and di- sodium phosphate. Phosphate buffers containing
these two phosphate salts (or equivalents) are frequently used
in biochemical and physiological studies. Tri-sodium phosphate (TSP), is quite basic, and is the major species
above pH 12. TSP is used as a cleaning agent, because the high
pH converts water insoluble fats and oils into water soluble soaps. The phosphodiester backbone Acids react with hydroxyl groups to form esters. In DNA each phosphate
has formed an ester with a hydroxyol group on two sugars. Each
sugar, a deoxyribose, is also linked to two phosphates. This chain,
alternating phosphates and sugars, is called the phosphodiester
backbone of DNA. While in this diagram the backbone looks symmetrical,
when we look at the structure of the deoxyribose, we will see
that it has a polarity.
The single remaining charged oxygen on the phosphates makes DNA
an acid (the A in DNA). These negative charges would strongly
repel each other and destabilize the DNA molecule, except for
the electrostatic screening by positive counter ions, mostly sodium,
potassium, and magnesium inside the cell.