Fig. 4. MD simulations identify the regulatory mechanism of microenvironmental pH in a PAA-modified channel of PCN-222-Fe@PAA.

a Cross-sectional and (b) side views of the system. The centre of mass of the MOF was placed at (0, 0, 0), and the channel was aligned along the Z-axis. The Z-direction length of the whole system is 120 Å, of which −43 Å~43 Å is the range of MOF, and the rest is the water layer with a thickness of ~34 Å. The five PAA chains are in different colours. The blue and purple spheres represent the nitrogen atom of the Tris group and the chloride ions. Hydronium is shown using the space-filling model. c Distribution of charge and mass of PAA in the 22/5 system. The light pink and grey backgrounds represent the error bands for molar mass and total charge, respectively. d 22/5 PAA chains show a relatively narrow distribution within the confined environment with less overlap between chains. e Permeation free energy of hydronium through the pore. The shape of the curve is highly associated with the population of PAA. Two snapshots show representative binding modes of hydronium with PAA. The information for the 27/0 system is shown in (f‒h).