Does phosphoryl protonation occurs in aqueous phosphoesters solutions

Ionisation of trimethylphosphate (TMP), dimethylphosphate (DMP) and diethylphosphate (DEP) is investigated by acidic titration in water by Raman (R), Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies. The vibrational frequencies of the PO(2)(-) ionic form and the neutral form were found in accord with the literature. While increasing further H(+) concentration, the PO band disappears in the benefit of new ones. These results, together with deuteration experiments indicate the presence of a new ionic form positively charged with general formula R(1)R(2)R(3)P(OH)(+) or R(1)R(2)P(OH)(+)(2). The pK of this phosphonium entities is lying in the range -2, -4. These results were confirmed by (31)P NMR titration. The occurrence of such a phosphonium ion in aqueous solutions might be of crucial importance for biochemical reactions and interactions, owing to the large spread of phosphoryl group in biomolecules and keeping in mind that intracellular compartments are more likely concentrated media with little free water than real aqueous solutions. Furthermore, pK's can be shifted by physical-chemical parameters like dielectric constant and electric field. This may involve at least fractional positive charge apparition that might be important in biochemical regulation by charge-charge and charge-dipole interactions. This finding will gain to be further explored on more complex molecules like phospholipids, nucleic acids and proteins.