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.