The Relative Stability of Internal Metal Complexes. II. Metal Derivatives of 8-Hydroxyquinoline 5-Sulphonic Acid and a Series of Monocarboxylic Mono-α-amino Acids including Histidine

Stability constants of ct series of metal complexes derived from the series of α-amino acids, glycine, alanine, leucine, valine, histidine, and 8-hydroxyquinoline 5-sulphonic acid have been determined by an electrometric method. The metals fall in the same order, as regards stability of their complexes, as was found for the chelating molecules studied in Part I of this series. As the length of the a chain of the amino acid increases, there is a small decrease in the stability of the amino acid complexes of any one metal. The cobalt II complex of histidine is much more stable than cobalt II complexes derived from any of the other α-amino acids. It is suggested that this explains why histidine is able to nullify the inhibiting action of cobalt on the growth and respiration of various aerobic and anaerobic organisms. Though there is no great difference in the stability of corresponding metal complexes derived from 8-hydroxyquinoline and 8-hydroxyquinoline 5-sulphonic acid (when allowance is made for different solvents used in making the measurements), there is a marked difference in the bacteriostatic action of these two chelating molecules. A reason for this difference is suggested. The stability constants of any chelating reaction involving the liberation of any number of hydrogen ions can be evaluated by using the generalized mathematical treatment which has been set out in this series of papers.