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 pK_a2 of PP(iii) ∼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).