Chemical exchange in tissue extracts revisited: bicarbonate and deuterium isotope effects on 31P resonances of phosphoethanolamine and phosphocreatine.

It is not sufficiently appreciated that chemical exchange can markedly affect the appearance of 31P tissue extract NMR spectra. In addition to the commonly recognized 31P chemical shift effects of divalent metal cation (e.g. Mg2+) binding upon ATP resonances, multiple resonances for phosphoethanolamine (PE) and phosphocreatine (PCr) are observed under certain conditions of pH, temperature, and D2O and bicarbonate concentrations. In the presence of bicarbonate ion (commonly used to neutralize acidic extractions) carbamate formation causes a second 31P resonance for PE to appear. This effect has been described previously for 13C and 1H amino acid resonances in tissue extracts [Sherry et al. J. Magn. Reson. 89, 391-398 (1990)]. The observation of a splitting of the PCr 31P resonance in aqueous solutions containing D2O has been recently ascribed to proton scalar coupling but was described earlier in an underappreciated report [Kupriyanov et al. Biochem. Biophys. Res. Comm. 114, 1117-1125 (1983)] as due to a deuterium isotope effect. These effects, carbamate formation and deuterium isotope shift, are verified herein to cause marked shifts in PE and PCr 31P resonances. The dependence upon experimental parameters is explored.