Experimental surface pressure (π) and electric surface potential (ΔV) isotherms were measured for membrane lipids, including the following phosphatidylcholines (PCs)-1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC); 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC); 1,2-diarachidoyl-sn-glycero-3-phosphocholine (DAPC); and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). In addition, other phospholipids, such as phosphatidylethanolamines (represented by 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE)) and sphingolipids (represented by N-(hexadecanoyl)-sphing-4-enine-1-phosphocholine (SM)) were also studied. The experimental apparent dipole moments (μAexp) of the abovementioned lipids were determined using the Helmholtz equation. The particular contributions to the apparent dipole moments of the investigated molecules connected with their polar (μ⟂p) and apolar parts (μ⟂a) were theoretically calculated for geometrically optimized systems. Using a three-layer capacitor model, introducing the group's apparent dipole moments (calculated herein) and adopting values from other papers to account for the reorientation of water molecules (μ⟂w/εw), as well as the for the local dielectric permittivity in the vicinity of the polar (εp) and apolar (εa) groups, the apparent dipole moments of the investigated molecules were calculated (μAcalc). Since the comparison of the two values (experimental and calculated) resulted in large discrepancies, we developed a new methodology that correlates the results from density functional theory (DFT) molecular modeling with experimentally determined values using multiple linear regression. From the fitted model, the following contributions to the apparent dipole moments were determined: μ⟂w/εw=-1.8±1.4 D; εp=10.2±7.0 and εa=0.95±0.52). Local dielectric permittivity in the vicinity of apolar groups (εa) is much lower compared to that in the vicinity of polar moieties (εp), which is in line with the tendency observed by other authors studying simple molecules with small polar groups. A much higher value for the contributions from the reorientation of water molecules (μ⟂w/εw) has been interpreted as resulting from bulky and strongly hydrated polar groups of phospholipids.
Keywords: Langmuir monolayers; dipole moments; electric surface potential; phospholipids.