A MODEL OF POLAR GROUP STATICS IN LIPID BILAYERS AND MONOLAYERS

We have constructed a simple model of uncharged phospholipid polar gro ups which takes into account details of conformational states. The mod el represents the polar group dipoles as point dipoles which possess m agnitude and orientation determined by the conformational states. The available states in gel or fluid phases of lipid bilayers are determin ed by the cross section areas of the hydrocarbon chain region which re flect their van der Waals interactions and steric restrictions. The el ectrostatic interactions between the polar group dipoles, located in a n aqueous medium near the interface with an oily dielectric, the hydro carbon chain region, are described by a nonlocal model which takes int o account average spatial correlations of hydrogen-bonded water cluste rs in the aqueous medium. We carried out computer simulation studies o f gel and fluid phases of pure phosphatidylcholine (PC) and phosphatid ylethanolamine (PE) systems as well as PC-PE homogeneous fluid phases, and we omitted any consideration of PE-PE hydrogen bonding. We found that the P-N dipole makes an average angle with the local bilayer plan e of similar to 0-3 degrees (PC fluid), similar to-30 degrees (PC gel) , similar to 1-4 degrees (PE fluid) and similar to 4-7 degrees (PE gel ). These results are in accord with some experiments and some molecula r dynamics simulations, where they are known. The last result shows th at in both PE gel and fluid phases the polar group is oriented to take part in PE-PE hydrogen bonding. We calculated both the average positi ons of the P, O, CH2(alpha), CH2(beta) and N moieties as well as the f luctuations that they undergo perpendicular to the bilayer plane. We f ound, for example, that PC polar groups in a fluid phase exhibit large fluctuations which essentially entirely disappear in the gel phase. F luctuations of PE polar groups are small in the gel phase and larger, though not as large as for PC groups, in the fluid phase. In the fluid phase, the most probable (similar to 0.37) direction for a PC polar g roup to point in is similar to 30 degrees toward the hydrophobic plane , while the corresponding probability (similar to 0.8) for a PE polar group is to be oriented parallel to the bilayer plane. In a gel phase the PE group points parallel to the plane with probability similar to 0.84, while a PC group points at an angle of similar to 30 degrees awa y from the hydrophobic plane with probability similar to 1. We calcula ted the H-2 NMR quadrupole splittings of the alpha and beta CH2 groups and found them to be similar to the values measured. The thickness of the polar region was found to change by similar to 2.4 Angstrom betwe en the fluid and gel phases, while the corresponding change for PE bil ayers was similar to 0.8 Angstrom. We considered various approximation s and showed the importance of accounting adequately for hydrogen-bond ing effects in the aqueous medium. We studied the effects of a tethere d ''polypeptide'', modeled as a flexible polymer containing dipoles, i nteracting with a PC bilayer and compared it to a polymer (a) without dipoles and (b) with dipoles but with the ''polypeptide''-lipid polar group electrostatic interaction switched off. We found that case b is attracted to the interface but that, when the interaction is switched on, a tethered ''polypeptide'' is attracted only weakly to a PC interf ace and somewhat more strongly to a gel phase than to a fluid phase, i n accord with measurements.