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

## TL;DR

This study uses Rheo-GIXD to investigate how lipid-peptide monolayers at the air-water interface respond structurally under shear flow, revealing phase behavior, jamming transitions, and crystallite merging.

## Contribution

It demonstrates the application of Rheo-GIXD to analyze in-situ molecular structural changes in biomolecular monolayers under shear stress, highlighting phase-specific responses.

## Key findings

- Flow jamming transition observed in Alamethicin and mixed monolayers.
- Pure DPPC monolayer shows increased crystallite size and tilt angle under stress.
- Mixed monolayer exhibits merging of crystallites without significant change in molecular area.

## Abstract

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.