Differences in ionization that can bring about selectivity

2 Differences in ionization that can bring about selectivity 

Students often ask, How can the degree of ionization be relevant where biological action is concerned Surely poorly ionizing substances produce at least a few ions and, as these are removed, fresh ones are generated from the neutral molecules according to the mass-action law, as in Equation (i)  

This objection is intelligently argued, but it must be remembered that ions are not bound to their receptors by covalent bonds, and hence they can easily leave the receptors. In short, to keep a receptor saturated with a given ion, there must be an excess of this ion in the solution bathing the receptor. For example. 

Many bacteria are much more sensitive than E. coli to crystal violet. Thus Streptococcus pyogenes is inhibited by a dilution of 1 in 320000 (though not by 1 in 640000) and Staphylococcus aureus is killed by 1 in 2000000. Although these differences may be partly due to differences in sites of loss, there are different values for different species and (in the hosts) for different tissues in the one species. This affords a basis for the selective action of cationic drugs. 

Other types of selectivity depend on properties in which ions, as a class, differ from molecules. These differences will now be discussed under three headings covalent reactivity (the making and breaking of covalent bonds), adsorption at surfaces, and penetration of membranes. 

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