Evolution of myoglobin diffusion mechanisms: exploring pore and surface diffusion in a single silica particle.

ABSTRACT

This study elucidates the mass transfer mechanism of myoglobin (Mb) within a single silica particle with a 50 nm pore size at various pH levels (6.0, 6.5, 6.8, and 7.0). Investigation of Mb distribution ratio (R) and distribution kinetics was conducted using absorption microspectroscopy. The highest R was observed at pH 6.8, near the isoelectric point of Mb, as the electrostatic repulsion between Mb molecules on the silica surface decreased. The time-course absorbance of Mb in the silica particle was rigorously analyzed based on a first-order reaction, yielding the intraparticle diffusion coefficient of Mb (Dp). Dp-(1 + R)-1 plots at different pH values were evaluated using the pore and surface diffusion model. Consequently, we found that at pH 6.0, Mb diffused in the silica particle exclusively through surface diffusion, whereas pore diffusion made a more substantial contribution at higher pH. Furthermore, we demonstrated that Mb diffusion was hindered by slow desorption, associated with the electrostatic charge of Mb. This comprehensive analysis provides insights into the diffusion mechanisms of Mb at acidic, neutral, and basic pH conditions.