Effect of Lysozyme Adsorption on the Interfacial Rheology of DPPC and Cholesteryl Myristate Films

Abstract

A model tear film lipid layer composed of a binary mixture of cholesteryl myristate (CM) and 1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DPPC) was characterized using surface tension measurements, Brewster angle microscopy (BAM) and interfacial stress rheology (ISR). Isotherms showed that films containing >or=90 mol % CM have a 17-fold greater % area loss between the first and second compressions than the films with less CM. BAM images clearly showed that CM films did not expand after compression, and solid-like regions extending 1-2 mm were observed at low pressures (1 mN/m). Lipid films with <or=70 mol % CM expanded after compression in BAM indicating that the films were primarily fluid. ISR data confirmed these results, as DPPC and 30:70 CM/DPPC films were fluid at all pressures and more compliant than films with higher CM content. Films with >or=50 mol % CM became elastic at higher surface pressures. Increasing CM content reduced the surface pressure at which the mixed film became elastic. Lysozyme adsorption into a CM film increased the compressibility and resulted in a more expanded film. Lysozyme increased the ductility of the CM/DPPC films with no film breakdown occurring up to the highest pressure measured (40 mN/m). In summary, CM increased the elasticity of the lipid films, but also caused them to become brittle and incapable of expansion following compression. Lysozyme adsorption increased the ductility and decreased the isotherm hysteresis for CM/DPPC films.

Rheological properties of protein films

“…Tears contain lysozyme as a bactericide and it has been shown [103] that lysozyme interacts with membrane lipids at the A-W interface to form films that are more ductile and less likely to fracture.…”

Formation of ultrathin protective films for ophthalmological use from aqueous solutions of polymers and taurine

“…1), particularly the existence of area II, provides evidence that protective films formed from solutions of polymers and their mixtures with taurine form and are stable. The maximum achievable values of p range from 25 to 35 mN/m, which is less than the value of p for native lipid films (60 mN/m [8]). We note that addition of substances with high surface activity to tear fluid can lead to dispersal (break-up) of both the lipid film and the mucin layer, which limits their use.…”

Lens-care-solution-induced alterations in dynamic interfacial properties of human tear-lipid films

“…In Fig. 3, we report examples of dynamic surface pressure as a function of film-thickness curves called isocycles [9][10][11]. As seen from this figure, a maximum surface pressure of 50 ± 2 mN/m was subject-independent; however, the shape of iso-cycles varied between human tear-lipid samples collected from two individuals.…”

“…The expanded (i.e., less than 2 nm thick) Meibomian-lipid monolayer with relatively low surface pressures (up to 30-35 mN/m) was explored quite extensively [9][10][11][12][13]31]. In contrast, our work focused on interfacial properties of condensed tear-lipid multilayers with initial thickness close to 90 ± 20 nm.…”

“…These lipid films play a vital role in maintaining ocular surface homeostasis by facilitating spreading of the tear aqueous over a cornea [1][2][3][4][5][6][7]. The interactions between tear-lipid films and tearaqueous constituents (e.g., natural tear proteins, or ingredients added in topical ophthalmic medications or lens care solutions) are important and may influence overall tear-film stability [8][9][10]. It has been established that the tear-lipid layer plays a crucial role in tear-film stabilization, and surface physical properties such as interfacial elasticity and viscosity of tear-lipid films are the key factors determining thin-film stability [11][12][13].…”