A Model of Polar Group Statics in Lipid Bilayers and Monolayers

Abstract

We have constructed a simple model of uncharged phospholipid polar groups which takes into account details of conformational states. The model represents the polar group dipoles as point dipoles which possess magnitude and orientation determined by the conformational states. The available states in gel or fluid phases of lipid bilayers are determined by the cross section areas of the hydrocarbon chain region which reflect their van der Waals interactions and steric restrictions. The electrostatic interactions between the polar group dipoles, located in an aqueous medium near the interface with an oily dielectric, the hydrocarbon chain region, are described by a nonlocal model which takes into account average spatial correlations of hydrogen-bonded water clusters in the aqueous medium. We carried out computer simulation studies of gel and fluid phases of pure phosphatidylcholine (PC) and phosphatidylethanolamine (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 plane of ∼0−3° (PC fluid), ∼30° (PC gel), ∼1−4° (PE fluid) and ∼4−7° (PE gel). These results are in accord with some experiments and some molecular dynamics simulations, where they are known. The last result shows that 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 positions of the P, O, CH2(α), CH2(β) and N moieties as well as the fluctuations that they undergo perpendicular to the bilayer plane. We found, for example, that PC polar groups in a fluid phase exhibit large fluctuations which essentially entirely disappear in the gel phase. Fluctuations 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 (∼0.37) direction for a PC polar group to point in is ∼30° toward the hydrophobic plane, while the corresponding probability (∼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 ∼0.84, while a PC group points at an angle of ∼30° away from the hydrophobic plane with probability ∼1. We calculated the 2H NMR quadrupole splittings of the α and β CH2 groups and found them to be similar to the values measured. The thickness of the polar region was found to change by ∼2.4 Å between the fluid and gel phases, while the corresponding change for PE bilayers was ∼0.8 Å. We considered various approximations and showed the importance of accounting adequately for hydrogen-bonding effects in the aqueous medium. We studied the effects of a tethered “polypeptide”, modeled as a flexible polymer containing dipoles, interacting 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 interface and somewhat more strongly to a gel phase than to a fluid phase, in accord with measurements.