Protonation equilibria in bioinorganic systems

The molecules involved in many bioinorganic systems can be very complicated, they can interact not only with metal ions but also other species, such as protons, in the system. It is all too easy for the system to be so complicated that it appears that progress and understanding are scarcely possible. Fortunately, the situation is not as bad as this. In this section just one example is considered but it is sufficient to demonstrate the fairly self-evident conclusion that the more data there are available and, equally important, the more accurate these data, the greater the progress that can be made. 

However, these K values, whilst in principle obtainable by following pH changes as a function of added acid (or alkali), make no recognition of the various protonation sites. Let us assume that A, B and C behave as independent sites, that the protonation of the group A is independent of the protonation of the rest of the molecule, for example. Denote the protonation 

Not surprisingly, the relationship = I/2K2 = 3 is never accurately followed. The groups A, B and C are not totally independent. The protonation equilibrium at one site. A, say, is sensitive to the protonation at the others. Indicate the already protonated sites by suffixes so that instead of a simple k we now have k-, k, kc and /cgc (the dash indicates no protonated sites). We have been making the approximation that the group constant k is equal to all of the so-called microconstants, k-, k, k and k c- Although the move to microconstants has increased the number of unknowns by a factor of four, the situation is not as bad as it may seem. Because of the change, K, K2 and contribute three pieces of information, not just one. Further data are obtained from the detailed variation of the pH as acid is added in a titration. Similarly, the requirement that we reproduce not just one NMR spectrum but all its variants with pH, all provide more data, 

The important lesson to be learnt from this example is the fact that the microconstants are, indeed, rather similar. It is not unreasonable to hope that when there is no alternative to assuming that such microconstants are identical that the error introduced is not too great. 

As has been made clear in the text, this chapter does not attempt a balanced overview of bioinorganic chemistry. Almost any current inorganic chemistry text will contain a section which details some of the areas excluded. A good, broad-based and forward-looking series of articles which is still worth reading is to be found in the November (pages 916-1001) issue of J. Chem. Educ. (1985) 62. An overview of the subject is 

---