Binding of Ca2+ and Mg2+ to phosphatidylserine vesicles: different effects on P-31 NMR shifts and relaxation times.

Phosphorus-31 NMR lines corresponding to inner and outer surfaces of sonicated phosphatidylserine vesicles can be distinguished by the effects of added Ca²⁺ or Mg²⁺ at low bulk concentrations (millimolar or less). The changes in chemical shift and relaxation times indicate that Ca²⁺ binds directly to the PS phosphate, neutralizing at least a portion of the negative charge and restricting the motion of this group. Mg²⁺ ion also binds to the head group, but apparently not as strongly as Ca²⁺, nor is the mobility of the headgroup affected as much.