The kinetics of the reactions of sheep haemoglobin with iso cyanides

Abstract

The kinetic basis for the decrease in affinity of haemoglobin and myoglobin for successive members of the series of alkyl isocyanides has been investigated. The initial rate of combination of ethyl, n-propyl, isopropyl, isobutyl and tertiary butyl isocyanides with haemoglobin and with myoglobin decreases with increasing chain-length by about 500-fold in the case of haemoglobin and about 40-fold in the case of myoglobin. The rate of combination of the fourth isocyanide molecule with haemoglobin (i.e. I + Hb4I3[long dash][forward arrow] Hb4I4) is, however, only about 14 times smaller for tertiary butyl as compared with ethyl isocyanide; this finding is in line with Pauling''s suggestion that combination of successive ligand molecules with haemoglobin decreases the steric hindrance for combination with further ligand molecules. The latter effect, however, varies considerably from ligand to ligand and is not only a property of the haemoglobin molecule. The velocity of dissociation of the first molecule of isocyanide from saturated haemoglobin, on the other hand, is the same within experimental error for all members of the series. With myoglobin the rate constants for dissociation show differences of two- to three-fold but there is no systematic trend. Thus the burden of accounting for the differences in affinity between the isocyanides falls upon the combination velocity constants. This finding is in contrast to that for the gaseous ligands O2, CO and NO, where the affinity is primarily determined by the dissociation velocity constants. The bearing of the results on the "crevice" hypothesis of haemoglobin structure is discussed. In line with the well-known fact that the first member of an homologous series is often anomalous it was found that the rate of dissociation of methyl isocyanide from haemoglobin was about 50 times greater than that for all the higher homologues of the series. Preliminary experiments were carried out on the rate of combination of free haem with several of the isocyanides, but were not pursued further on account of the difficulties mentioned in the text.