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The channel has a large diameter of 0.8x1.1nm, and is non-selective for small ions. However, it has an upper exclusion size limit corresponding to molecular weights of about 600 Dalton of permeants. Because most metabolites have molecular weights lower than 600 Da and have been shown to pass through porin channels, porins are called general diffusion pores.
Typically one uses electronvolts, eV, as a unit of measurement for the mass of a subatomic particle. For size they are somewhere around an attometer, or 10-18 meters, though, again, the size of subatomic particles is difficult to talk about thanks to quantum effects. Measuring the size is difficult as well.