Protein (Lysozyme) Concentration-Dependent Structure, Morphology, and Hysteresis Behavior of a Three-Component (Lysozyme-DMPA-Cholesterol) Protein–Lipid Langmuir Monolayer

Protein (lysozyme)-lipid (DMPA and cholesterol) three-component mixed films (LDC) with varied lysozyme concentration (i.e., LDC_L<i>_x</i>) are investigated at the air-water interface. Elastic modulus-surface pressure (C_s^-1-Π) curves derived from Π-A isotherms show that mechanical behavior is strongly dependent on the monolayer composition, and for the same reason, the hysteresis behavior modifies. It is evidenced that the LDC_L_0.3 monolayer (lysozyme: 0.3 mg/mL) has significant hysteresis, which is reversible in nature, while the other mixed monolayers do not show such hysteresis behavior. Morphology at the air-water interface via Brewster angle microscopy (BAM) and at the air-solid interface via atomic force microscopy (AFM) shows that the presence of protein in the LDC_L<i>_x</i> monolayer modifies the lateral distribution of molecules, thereby forming a stripe-like pattern at the air-water interface (in optical length scale) with barrier compression or root-like structure on the solid surface at higher Π (in micron length scale), which is not observed in the case of lipid films. Moreover, lysozyme-added LDC_L<i>_x</i> films show an increase in thickness with compression, which is not observed for lipid films, as evidenced from the electron density profiles (EDPs). The morphology modification and thickness variation of LDC_L<i>_x</i> films with compression are most probably due to the reorientation of lysozyme molecules. This structural modification in LDC_L<i>_x</i> films with Π, however, seems to be reversible under expansion, as can be evidenced from the similar in situ morphology observation and similar thickness of the films deposited during both first and second compression. A variation in the strength of interaction forces among film-forming molecules depending on the monolayer composition basically affects the lateral distribution and organizational orientation with surface pressure, thus ultimately influencing macroscopically the monolayer properties such as elastic, hysteresis, morphological, and structural on water and solid surfaces.