Lysozyme entrapped within reverse hexagonal mesophases: Physical properties and structural behavior

A model protein (lysozyme) was incorporated into monoolein-based reverse hexagonal (H_II) mesophase and its structure effects were characterized by small angle X-ray scattering, ATR-FTIR spectroscopy, and rheological measurements. Modifications in molecular organization of the H_II mesophases as well as the conformational stability of lysozyme (LSZ) as a function of pH and denaturating agent (urea) were clarified. Up to 3 wt.% LSZ can be solubilized into the H_II. The vibration FTIR analysis revealed that LSZ interacted with OH groups of glycerol monooleate (GMO) in the outer interface region, resulting in strong hydrogen bonding between the surfactant and its environment. Simultaneously, the decrease in the hydrogen-bonded carbonyl population of GMO was monitored, indicating dehydration of the monoolein carbonyls. These molecular interactions yielded a minor decrease in the lattice parameter of the systems, as detected by small angle X-ray scattering. Furthermore, LSZ was crystallized within the medium of the hexagonal structures in a single crystal form. The α-helix conformation of lysozyme was stabilized at high pH conditions, demonstrating greater helical structure content, compared to D₂O solution. Moreover, the hexagonal phase decreased the unfavorable α → β transition in lysozyme, thereby increasing the stability of the protein under chemical denaturation. The rheological behavior of the hexagonal structures varied with the incorporation of LSZ, reflected in stronger elastic properties and pronounced solid-like response of the systems. The hydrogen bonding enhancement in the interface region of the structures was most likely responsible for these phenomena. The results of this study provided valuable information on the use of hexagonal systems as a carrier for incorporation and stabilization of proteins for various applications.