Characterization of methylphosphonate as a 31P NMR pH indicator

The 31P NMR pH indicator, methylphosphonate, has been extensively characterized, and the uncertainty in pH determination by its chemical shift has been analyzed. The pKa decreases by 0.003 pH unit/degrees C and 0.06 pH unit/100 mM ionic strength. The pKa appears not to be sensitive to Ca2+ but is sensitive to Mg2+, resulting in an uncertainty of +/- 0.05 pH unit. Substituting 300 mM Na+ for 300 mM K+ causes the pKa to decrease by 0.1 pH unit. Taking the effects of temperature, ionic strength, and cation identity into account, the overall estimated uncertainty in pH determination can be as high as +/- 0.1 pH unit. Methylphosphonate was tested as a pH indicator in Ehrlich ascites tumor cells. Our data indicate that both the unchanged and monoanion forms of methyl phosphonate are very permeable, rendering this compound unsuitable as a pH indicator in this system. However, the sensitivity of this compound's chemical shift to pH and the relative insensitivity to other parameters suggest that phosphonates, as a group, may be applicable as pH indicators by 31P NMR.