A model of hydrogen bond formation in phosphatidylethanolamine bilayers

Abstract

We have modelled hydrogen bond formation in phospholipid bilayers formed, in excess water, from lipids with phosphatidylethanolamine (PE) headgroups. The hydrogen bonds are formed between the NH + 3 group and either of the PO − 2 or the ( sn2 chain) CO groups. We used a model that represented the conformational states accessible to a PE headgroup by 17 states and modelled lipid dipole–dipole interactions using a non-local electrostatics theory to include the effects of hydrogen bonding in the aqueous medium. We used Monte-Carlo simulation to calculate equilibrium thermodynamic properties of bilayers in the fluid ( T = 340 K) or gel ( T = 300 K) phases of the bilayer. We defined E h to be the difference in free energy between a hydrogen bond formed between a pair of lipid groups, and the energy of hydrogen bonds formed between water and those two groups, and we required its average value, 〈 E h〉, to be ∼−0.3 kcal/mol (∼−0.2×10 −13 erg) as reported by T.-B. Shin, R. Leventis, J.R. Silvius, Biochemistry 30 (1991) 7491. We found: (i) E h = −0.9×10 −13 erg gave 〈 E h〉 = −0.21×10 −13 erg (gel phase) and 〈 E h〉 = −0.19×10 −13 erg (fluid phase). (ii) The relative number of CO groups on the sn2 chain calculated to take part in interlipid hydrogen bonding in the fluid phase compared to the gel is 1.06 which compares well with the experimental ratio of ∼1.25 (R.N.A.H. Lewis, R.N. McElhaney, Biophys. J. 64 (1993) 1081). The ratio of such groups taking part in interlipid hydrogen bonding compared to water hydrogen bonding in each phase was calculated to lie between 0.16 and 0.17. (iii) We calculated the distribution of positions of the headgroup moieties, P, O, CH 2(α), CH 2(β) and N, and found that, in both phases, the O lay furthest from the hydrocarbon chain layer (average ∼5.3 Å) with the PO 2 and NH 3 groups lying at ∼ 5 Å. This results in the P–N dipole lying nearly parallel to the bilayer plane in both phases. The thickness of the headgroup layer underwent essentially no change on going from the gel to the fluid phase. The 2 H NMR quadrupole splittings for the α and β CH 2 groups were 4.9 and 5.7 kHz (fluid phase) and 7.1 and 7.3 kHz (gel phase), respectively, on the assumption of sufficiently rapid rotation around the z-axis. (iv) In both phases, the location of the NH + 3 group exhibited a strong peak around 5.2 Å into the aqueous medium, with much smaller peaks around 2.6 and 7.8 Å, the two CH 2 groups exhibited narrower, double-peaked distributions and the O and the PO − 2 each exhibited a narrow single peak. (v) PE headgroups, in a homogeneous gel phase, exhibited dipolar orientational long-range order in the plane of the bilayer. The distribution of orientation angles exhibited a full width at half height of between ∼40° and ∼50°. In a fluid phase no such order was observed. (vi) The number of hydrogen bonds did not differ substantially between the fluid and gel phases. This model is unlikely to display any significant effect of hydrogen bonding upon the “main” hydrocarbon chain melting phase transition at T m, except, possibly, a broadening of any hysteresis, compared to the case of PC bilayers where interlipid hydrogen bonding is absent.