Interrelation between hydration and interheadgroup interaction in phospholipids.

The stability and the ionic conductivity of biological membranes and of lipid bilayers depend on their hydration. A small number of water molecules adhere strongly to the different residues of the lipid headgroups and are oriented by them. An additional number of water molecules adhere more weakly, preserving their freedom of rotation, but are essential for bestowing the thermodynamic properties of hydrated bilayers and of biological membranes. Around six water molecules are attached so strongly to the headgroups of different phospholipids (PL) that they are rendered unfreezable, or their freezing is extended over such a wide range of temperatures that it cannot be detected by differential scanning calorimetry (DSC). If cholesterol is added to the PL above the concentration at which phase separation of the cholesterol phase occurs, the number of unfreezable water molecules per PL increases, indicating that the PL molecules on the border line between the two phases attach nearly twice as many water molecules as those in the middle of the phase. The orientation of about seven or eight water molecules attached to PL headgroups (seven to phosphatidyl serine (PS)) can be detected by polarized FTIR. The dichroic ratio of the successively adhering water molecules to the headgroup of PS fluctuates between 2.6 and 2.9, with the cumulative value of about 2.8 for the seven water molecules adhering to the headgroup of PS. In addition, in this case, the number of water molecules oriented by PL molecule residues on the border line of the two phases is much larger ( approximately 13 for PS). Interaction between two opposite negatively charged layers containing PS approaching each other may lead, after correlated electrostatic attraction, to change in the conformation of the headgroups with concomitant dehydration. This process is enhanced by Ca(+) and by Li(+), but it may also occur with Na(+) and K(+) as counter-ions if the layers are mutually aligned. This process may be important in the fusion mechanism of biological membranes, and its molecular modeling has been carried out.

Effect of electric fields on the structure of phosphatidyl choline in a multibilayer system.

Polarized attenuated total reflection (ATR)-FTIR measurements were carried out on aligned multibilayers of dipalmitoyl phosphatidyl choline (DPPC) under the influence of high electric fields. The electric fields varied from 0 up to 5.5 x 10(6) V/cm in the hydrocarbon layer and up to 1.1 x 10(6) V/cm in the polar layer of the aligned multibilayer, when the applied potential across the 1-microm-thick multibilayer plus the 0.5-microm-thick air gap reached the value of 1000 V. At relatively low applied potentials of less than 100 V, when the electric fields in the hydrocarbon and in the polar layer were below 5.5 x 10(5) and 1.1 x 10(5) V/cm, respectively, the inhomogeneous field between the two layers is adequate to start driving the polar groups into the hydrocarbon layer, exerting a pressure and penetrating them. This results in distortion of the orientation of the hydrocarbon chains. Only at much higher potentials above 600 and 700 V starts the direct reorientation of the dipoles of the different polar residues by the electric field.