Grazing incidence x-ray diffraction studies of lipid-peptide mixed monolayers during shear flow

Grazing Incidence X-ray Diffraction (GIXD) studies of monolayers of biomolecules at the air-water interface give quantitative information of in-plane packing, coherence lengths of the ordered diffracting crystalline domains and the orientation of hydrocarbon chains. Rheo-GIXD measurements revel quantitative changes in the monolayer under shear. Here we report GIXD studies of monolayers of Alamethicin peptide, DPPC lipid and their mixtures at the air-water interface under the application of steady shear stresses. The Alamethicin monolayer and the mixed monolayer show flow jamming transition. On the other hand, pure DPPC monolayer under the constant stress flows steadily with a notable enhancement of area/molecule, coherence length, and the tilt angle with increasing stress, suggesting fusion of nanocrystallites during flow. The DPPC-Alamethicin mixed monolayer shows no significant change in the area/DPPC molecule or in the DPPC chain tilt but the coherence length of both phases (DPPC and Alamethicin) increases suggesting that the crystallites of individual phases are merging to bigger size promoting more separation of phases in the system during flow. Our results show that Rheo-GIXD has the potential to explore in-situ molecular structural changes under rheological conditions for a diverse range of confined biomolecules at the interfaces.