I don't know if I have linked to this paper before, but if I haven't I definitely should. It's a seminal Science paper by Frank Westheimer in which he describes his thoughts on why nature chose phosphates as the most important signaling and building blocks in living organisms.
The entire paper is eminently readable and among other things, Westheimer compares the properties of phosphates with other possible groups such as arsenates and sulfates. Basically the bottom line is that phosphates possess the right value of pKa to be doubly ionized at physiological pH. The singly ionized form in the phosphodiester linkages of nucleic acids prevents the group from being easily hydrolyzed from nucleophilic attack by water, without making the group so stable that it won't undergo enzymatic hydrolysis.
As in other chemical schemes that Nature has developed, it's the right combination of stability and lability that makes phosphates ideal for being among the most fundamental chemical entities in life's inventory. And all essentially because of the right acid-base chemistry of phosphoric acid.
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