Dielectric-Dependent Strength of Interlipid Hydrogen Bonding in Biomembranes: Model Case Study
Atomistic aspects of the structural organization, dynamics, and functioning of hydrated lipid bilayers - model cell membranes - are primarily governed by the fine balance of intermolecular interactions between all constituents of these systems. Besides the hydrophobic effect, which shapes the overall skeleton of lipid membranes, a very important contribution to their behavior is made by hydrogen bonds (H-bonds) between lipid head groups. The latter determine crucial phenomena in cell membranes, such as dynamic ultrananodomain organization, hydration, and fine-tuning of microscopic physicochemical properties that allow the membrane to adapt quickly when binding/insertion external agents (proteins, etc.). The characteristics of such H-bonds (strength, spatial localization, etc.) dramatically depend on the local polarity properties of the lipid-water environment. In this work, we calculated free energies of H-bonded complexes between typical donor (NH3+, NH, OH) and acceptor (Câ•O, OH, COO-, COOH) groups of lipids in vacuo and in a set of explicit solvents with dielectric constants (ϵ) from 1 to 78.3, which mimic membrane environment at different depths. This was done using Monte Carlo simulations and an assessment of the corresponding potential of mean force profiles. The strongest H-bonded complexes were observed in the nonpolar environment, and their strength increased sharply with decreasing ϵ below 17. When ϵ changed, the largest free energy gain (10.8 kcal/mol) was observed for pairs of acceptors Câ•O and O(H) with donor NH3+. The complexation of the same acceptors with NH donor in this range of ϵ values was rather less sensitive to the environmental polarity, by ∼1.5 kcal/mol. Dielectric-dependent interactions of polar lipid groups with water were evaluated as well. The results explain the delicate balance that determines the unique pattern of H-bonds for a particular lipid bilayer. Understanding the factors that regulate the propensity for H-bonding in lipid bilayers provides a fundamental basis for the rational design of new membrane nano objects with predefined properties.