In solution, the 31 P isotropic chemical shift (iso) is highly sensitive to its local environment, particularly the protonation state of phosphate, as shown in Figure . Since the first report by Moon et al., P NMR has been a standard noninvasive method for determining intracellular pH in various biological models. A single sharp resonance is observed at each pH due to fast chemical exchange between phosphate species and rapid molecular tumbling, which averages out anisotropic interactions like dipolar couplings and chemical shift anisotropy (CSA). By plotting chemical shift versus the pH, a sigmoidal curve is typically observed for each protonation equilibrium (Figure ), where the inflection points correspond to the pKa values of the phosphate species, i.e., the pH at which two protonation states are present in equal proportions. However, the fast chemical exchange in solution prevents the individual analysis of different phosphate species involved in the exchange. Lowering the temperature slows down the exchange kinetics and would lead, in principle, to separate 31 P resonances for the various phosphate species. However, in the case of freezing, the reintroduction of anisotropic interactions such as CSA or dipolar couplings requires MAS to access high-resolution spectra.