Structural transitions of monoolein bicontinuous cubic phase induced by inclusion of protein lysozyme solutions

TL;DR

This study investigates how the inclusion of lysozyme proteins induces a structural transition in monoolein cubic phases, revealing the role of protein concentration, salt effects, and frustration in phase stability using SAXS analysis.

Contribution

It demonstrates the protein-induced transition between cubic phases and relates it to minimal surface transformations, providing new insights into protein-lipid interactions in cubic phases.

Findings

Lysozyme inclusion causes a transition from Pn-3m to Im-3m structure.

NaCl lowers the concentration threshold for the phase transition.

High lysozyme and NaCl concentrations lead to phase disappearance due to crystallization or phase separation.

Abstract

Inclusion of protein lysozyme molecules in lipidic monoolein cubic phase induces a transition from a $\rm Pn\bar{3}m$ structure to $\rm Im\bar{3}m$ one. Small-angle X-ray scattering (SAXS) method with high intensity synchrotron radiation enabled us to follow closely the transition depending on the conditions of lysozyme solutions. We showed that concentrated lysozyme solutions induced the appearance of the $\rm Im\bar{3}m$ structure coexisting with the $\rm Pn\bar{3}m$ structure. From the relation between the lattice parameters of these two structures it was shown that they were related by the Bonnet transformation of underlying triply periodic minimal surfaces. We found that the transition also occurred at lower lysozyme concentration when NaCl induced attraction between lysozyme molecules. The origin of the transition was considered as a frustration in the cubic phase where lysozyme molecules were highly confined. A simple estimation of the frustration was given, which took into account of the translational entropy of lysozyme molecules. At the highest concentration of lysozyme and NaCl the $\rm Im\bar{3}m$ structure was found to disappear and left only the $\rm Pn\bar{3}m$ structure. This was probably either due to the crystallization or phase separation of lysozyme solutions ongoing microscopically, which absorbed lysozyme molecules from channels of the cubic phase and thus removed the frustration.