Contribution of phosphate groups to the dipole potential of dimyristoylphosphatidylcholine membranes

Phloretin, a molecule which is known to decrease the dipole potential of lipid membranes, has a little effect on the C=O frequencies of dimyristoylphosphatidylcholine bilayers (DMPC) in comparison to that observed on the phosphates. In the first case, the frequency is displaced very slightly to higher values, while a pronounced downward shift is observed on the asymmetric vibration frequencies of the phosphates. The effect of phloretin on the phosphate groups is correlated with a decrease in the monolayer potential of DMPC in the gel and in the liquid crystalline state. From the comparison with monolayers spread on water and on 0.15 M trehalose, it is concluded that the P=O bonds of the phosphates can contribute to the dipole potential of the membrane in addition to the contribution given by the orientation of the P--N+ dipole.