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

A new approach to the data refinement of X-ray reflection measurements from lipid surface monolayers, applied to DMPA on pure water, reveals the struc tural organization of the lipid in unprecedented detail and provides new in sights into headgroup conformation and hydration as a function of lateral p ressure. While conventional box models are incapable of modeling the experi mental data at high momentum transfer satisfactorily, a quasimolecular comp osition-space refinement approach using distribution functions to map the s patial organization of submolecular headgroup fragments yields a much bette r description and overcomes inherent difficulties of box models. Upon going from the fluid liquid-expanded (LE) phase to the hexatic liquid-condensed (LC) phase, the orientation of the headgroup is tightly coupled to the orde ring of the acyl chains. Headgroups tilt toward the surface normal to accom modate for the large reduction in available area per lipid molecule. The sp read of the headgroup fragment distribution is considerably larger than the global interface roughness and increases slightly with compression. In dis tinction to earlier work on DMPE using the two-box approach, we find that t he phosphate hydration stays essentially constant across the whole isotherm . The discrepancy between the results observed with the different models is attributed to intrinsic deficiencies of the box model. (C) 2000 Elsevier S cience B.V. All rights reserved.