Determination of Protonation Constants of O-Phospho-l-serine in Aqueous Solution: Potentiometry, Microcalorimetry, NMR Spectroscopy and Quantum Chemical Calculations

O-Phospho- l -serine is one of the naturally occurring phosphorylated amino acids, having important pharmacological activity and bioactivity. The protonation constants of O-phospho- l -serine were determined by means of potentiometric titrations at 25 °C and ionic strength of 0.5 mol·L −1 (NaCl). The heat effects of the protonation reaction of the O-phospho- l -serine were measured by direct calorimetry. NMR spectroscopy has demonstrated that the first protonation site occurs at the nitrogen atom in the amino group, followed by one of the oxygen atoms in the phosphono group, and finally the carboxyl oxygen atom. This trend is in good agreement with the enthalpy of protonation and quantum chemical calculations. These data will help to predict the speciation of O-phospho- l -serine in physiological systems.