Shape factors small molecule diffusion

The prototype of a small pore-forming toxin is the S. aureus a-toxin, also called ct-hemolysin, that has been extensively investigated hy Bhakdi and coworkers. Monomers of ct-hemolysin (33 kDa) hind to the surface of erythrocytes, and after lateral diffusion within the lipid hilayer, seven monomers oligomerize to form pores in the cell membrane. The ct-hemolysin forms mushroom-shaped pores with an outer diameter of lOnm and an inner diameter of approximately 2.5 nm. Small molecules can pass through the pore and diffuse into/out of the cytosol, along with water. As a consequence of such movement, cell homeostasis is greatly disturbed and pushed into an unhealthy state. In animals, the a-hemolysin represents a major virulence factor of S. aureus which causes hemolysis as well as tissue destruction. ... 

Molecular-sieve membranes can yield high separation factors by permitting small molecules to diffuse while essentially excluding or severely restricting the accessibility of larger molecules (Figure 9.2(d)). This type of diffusion, where the pores are of molecular size, has been referred to as shape selective or configurational diffusion. 

A decrease of the solubility is expected in nanostructured polymer blends due to the reduced polymer matrix volume, as well as a decrease in diffusion due to a more tortuous pathway for the diffusing molecules. The reduction of the diffusion coefficient is higher than that of the solubility coefficient. Indeed, the volume fraction of nanoplatelets is low and, thus, the reduction of the matrix volume is small. The major factor, then, is the tortuosity, which is connected directly to the shape and the degree of dispersion of nanoplatelets. Better dispersed clay systems increase the tortuosity path of the diffusing molecules whereas larger aggregates decrease the aspect ratio of the nanoparticles and can act as a low-resistance pathway for the gas transport. 

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