The 31P nuclear magnetic resonance spectrum of cows' milk

The 31P nuclear magnetic resonance spectrum of liquid milk was examined. Of the three peaks observed, the two larger were assigned to inorganic phosphate (Pi) and the seryl phosphate (SerP) residues of casein; the third peak was assigned to a phosphodiester, which is probably glycerophosphoryl choline. The pH-dependences of the chemical shifts of the Pi and SerP were measured with and without added EDTA, and the results confirm the assignments. The width of the Pi peak in milk is significantly greater than in similar solutions lacking casein, probably because of binding to, or chemical exchange with, the casein micelle. Most of the SerP residues in milk are not sufficiently mobile to have been detected in these experiments but a significant fraction of SerP residues is able to move freely and can be titrated.