Synergistic Effects of Iron in the Presence of Physiological Monovalent Cosolute on Myelin Basic Protein (MBP) Drive Modification in the Structure and Interfacial Behavior at the Air-Water Interface.

Myelin basic protein (MBP), an intrinsically disordered protein responsible for membrane compaction in the central nervous system, exhibits a strong sensitivity to environmental ionic conditions. To elucidate how metal ions and ionic environments influence the MBP structure and interfacial behavior, Langmuir films of MBP were developed on a pure water subphase and on subphases modified with FeCl3 (40 μM), KCl (100 mM), FeCl3 + KCl (40 μM + 100 mM), and at a reduced subphase pH (≈ 4.0) without added ions. Films at the air-water interface were examined using surface pressure-area (Π-A) isotherms, the barrier oscillation method to determine dynamic viscoelasticity, and Brewster angle microscopy (BAM) to visualize in situ morphological evolution and corresponding film thickness. To validate BAM-derived thickness observations, all films were transferred onto solid substrates and characterized by using X-ray reflectivity and atomic force microscopy, enabling the extraction of detailed structural and height information. Results from these techniques clearly demonstrate that the unique ion-facilitated assembly of MBP at the air-water interface governs the variation in interfacial properties of the films. Notably, the coexistence of Fe3+, hydrolyzed Fe(III) species, and K+ ions produces a synergistic effect, as relatively more iron adsorption takes place in the presence of K+ ions, as evidenced from XPS analysis, thereby altering MBP assembly at the air-water interface under near-physiological concentration of KCl. These findings reveal how a specific ionic environment modulates the self-organization of MBP and may provide insights into myelin instability under ionic dysregulation.