Potassium-39 and sodium-23 NMR studies of cation binding to phosvitin

The binding of Na+ and K+ to hen egg-yolk phosvitin has been studied by 23Na and 39K NMR. A transition in the binding behaviour of these cations is shown to accompany deprotonation of the phosphate groups on this highly charged protein. At lower pH the data are well described by a purely mass-action binding model, whereas at higher pH significant polyelectrolyte effects are apparent. From single ion as well as competition experiments K+ is shown to bind more strongly to phosvitin than does Na+. The temperature and magnetic field strength dependences of the 23Na and 39K relaxation rates provide a picture of phosvitin as a highly flexible macromolecule. This work demonstrates the potential of 39K NMR as a useful tool in the study of protein-cation interactions.