||J.J.R. Fraústo da Silva and R.J.P. Williams, The Biological Chemistry of the Elements: The Inorganic Chemistry of Life, Oxford University Press, USA 2nd edition (November 1, 2001) Chapter 8 sodium, potassium, and chlorine: osmotic control, electrolytic equilibria, and currents Table 8.3 p. 236
||Relative permeabilities are ratios to Na+ = 1.0 for sodium channels and to K+ = 1.0 for potassium channels. The differences in selectivity between Na+ and K+ channels are remarkable. Note that: (1) only Tl+ approaches K+ in permeability in the K+ channel. No larger or smaller ion passes easily. (Tl+ is a useful probe of K+ channels). (2) The K+ channel excludes NH4+, which has the same radius, better than does the Na+ channel. (3) Large H-bonding spherical molecules, e.g., guanidinium, pass the Na+ channel. (4) No methylated compounds pass through either channel [p. 236].