Incommensurate Lamellar Phase from Long Chain Mannosides: Investigation by X-Ray Scattering and Replica Exchange Molecular Dynamics (REMD)

We report the synthesis and phase behaviour of two anhydrous mannosides (αManC18 and αManC18:1) with stearyl and monounsaturated oleyl hydrocarbon chains that can be obtained from vegetable sources. These mannosides have been characterised by differential scanning calorimetry, polarised optical microscopy and small- and wide-angle X-ray scattering. The two compounds exhibited multiple d-spacing bilayer structures. αManC18:1 gives a distinct incommensurate lamellar that persists until high temperature. On the other hand, αManC18 forms an incommensurate gel phase only at low temperature but undergoes a transition into the lamellar (Lα) liquid crystal phases on heating. The results were confirmed by replica exchange molecular dynamics (REMD). The average hydrocarbon chain/headgroup tilt angles 〈ϕ〉/〈θ〉, and average chain bending angle 〈ψ〉 were calculated for the simulated bilayer systems at ∼25°C. The multiple averaged headgroup tilt angles 〈θ〉 explain the observed incommensurate bilayer structures with different bilayer thicknesses in these long hydrocarbon chain of mannosides. Moreover, different tilting angles of the headgroup result in different surface areas per lipid. Relating the incommensurate phase properly to molecular parameters is important especially in multi-component membranes, where the interplay of different molecule types is difficult to predict on intuitive grounds.