Submolecular organization of DMPA in surface monolayers: beyond the two-layer model

Area of Science:

• Surface science
• Biophysics
• Materials science

Background:

• X-ray reflection measurements are crucial for studying lipid monolayers.
• Conventional box models struggle to accurately describe experimental data at high momentum transfer.
• Understanding lipid headgroup conformation and hydration is key to monolayer organization.

Purpose of the Study:

• To develop and apply a new data refinement approach for X-ray reflection measurements of lipid monolayers.
• To investigate the structural organization, headgroup conformation, and hydration of DMPA on pure water.
• To compare the new approach with conventional box models.

Main Methods:

• Application of a quasimolecular composition-space refinement approach using distribution functions.
• Analysis of X-ray reflection data from dimyristoylphosphatidic acid (DMPA) monolayers on pure water.
• Mapping the spatial organization of submolecular headgroup fragments.

Main Results:

• The new refinement approach provides a superior description of experimental data compared to box models.
• Headgroup orientation becomes tightly coupled to acyl chain ordering when transitioning from the liquid-expanded (LE) to the liquid-condensed (LC) phase.
• Headgroups tilt towards the surface normal with increasing lateral pressure, and headgroup distribution spread exceeds interface roughness.

Conclusions:

• The quasimolecular refinement approach overcomes limitations of box models for X-ray reflection data.
• Lipid headgroup conformation and hydration are strongly influenced by lateral pressure and phase transitions.
• Phosphate hydration in DMPA monolayers remains constant across the isotherm, contrasting with previous findings for DMPE.