The Relative Hydrolytic Reactivities of Pyrophosphites and Pyrophosphates

The pH-rate profiles for the hydrolysis of pyrophosphate (PP(V)) and pyrophosphite (PP(III), pyro-di-H-phosphonate) are a complex function of pH, reflecting the different ionic species and their relative reactivities. PP(III) is more reactive than PP(V) at all pHs and only PP(III) shows a hydroxide-ion reaction at high pH, so it is 1010-fold more reactive than PP(V) in 0.1 M NaOH. The pKa2 of PP(III) [similar]0.44, so the dominant species at pH's > 1 is the di-anion PP(III)2−. Although there is no observable (NMR or ITC) binding of Mg2+ to the PP(III) di-anion there is a modest increase in the rate of hydrolysis of PP(III) by Mg2+. PP(III) is neither a substrate nor an inhibitor of pyrophosphatase, the enzyme that efficiently catalyses the hydrolysis of PP(V).