Binding of long chain alkyl sulphates to equine ferric myoglobin.

Abstract

When ferric myoglobin reacts with alkyl sulphates, its absorption spectrum changes into one characteristic of a haemichrome or parahaematin. The reaction equilibrium was studied over a range of protein concentrations using dodecyl sulphate and other alkyl sulphates with different chain lengths. The results obtained from spectrophotometric measurements and equilibrium dialysis experiments are not in agreement with those of other authors who, from an analysis of spectral changes only, supported the hypothesis of an all-or-none type of reaction whereby 18 detergent anions were bound to one molecule of ferric myoglobin. From the results we conclude that the first stage of the association of dodecyl sulphate to ferric myoglobin at pH 8.0 involves the binding of one molecule of detergent without alteration of the myoglobin absorption characteristics. In the second phase a further 3 or 4 molecules of dodecyl sulphate become associated. This causes the ferric myoglobin to be converted into its ferric myochrome, which exhibits the characteristic haemichrome absorption spectrum. The binding of additional dodecyl sulphate molecules to the myochrome then proceeds in numerous discrete steps, so that a great number of complexes occur simultaneously in the reaction medium. The experimental evidence indicates that the ferric myoglobin retains its native configuration in these complexes, provided the number of bound dodecyl sulphate anions does not exceed 50. The association compounds can be described as low-spin coordination complexes, in which a second imidazole originating from histidine E7, occupies the sixth coordination site of the iron atom.