Closing remarks

Only a bold man would dare attempt a synthesis of the wide-ranging discussions of the last two days, let alone distil from them any general principles of biological recognition. The only sentiment which could command general assent is one of gratitude to the Society, the organizers and the contributors for putting before us such a fascinating collection of papers and letting us sharpen our wits on so much new knowledge. The following remarks will therefore be nothing more than personal footnotes; if they omit reference to much of the work which has been presented, and only mention a few of the issues that have been raised, that is a matter of sheer necessity, not of personal prejudice. In our first session we began by reviewing the physical chemistry of molecular interactions in terms of quantum mechanics and statistical mechanics. Professor Buckingham warned us against the temptation of regarding molecular association energies as made up of additive contributions from pairs of atoms or ions, and Professor Symons stressed the highly individual structure-forming habits of water, which make it so difficult to interpret its solvation properties in terms of simplified models which treat water as a uniform dielectric, or as a mixture of monomers and high polymers. Professor Truter then introduced us to the structures of the complexes formed by ferrichrome A, nonactin and other biological agents with metal ions of various kinds. She drew attention to the large conformational changes which often accompany complex formation, and suggested that some at least of these agents owe their specificity to their high flexibility. Presumably one should interpret this generalization in terms of an ability of the complexing agent to meet very precisely the stereochemical needs of one particular ion, coupled with an inability to meet the precise needs of other ions without a certain amount of mechanical strain. To take a single example, it is not immediately obvious how the needs of T1+ differ from those of the alkali metal ions; but that is because we tend to overlook the effect of ionic polarization on the stability of the solvated ion. The T1 + ion, because of its outer electron pair, has a low-lying dipole transition (6s -> 6p) which confers on it a high electric polarizability. As a result, a set of negative ligands on one side induce a negative charge on the other side, and this will hinder the approach of further ligands on that side - as suggested by the curious stereochemistry of some of Professor Truter’s T1 + complexes.