Studies on the reaction of isocyanides with haemproteins

Abstract

This paper reports the results of an investigation of the binding of methyl ethyl-, isopropyl-, n-propyl- and tertiary-butyl-isocyanides with normal human haemoglobin, meso- and deutero-reconstituted haemoglobins, several modified forms of the protein and the artificial CN-met intermediates. Consideration of the experimental data allows some conclusions to be drawn.

• (a) The functional properties of haemoglobin appear to be independent of the nature of the ligand, since both homotropic and heterotropic interactions are maintained, to a large extent, in the case of the bulky isocyanides. In addition, it is observed that any chemical modification which alters the affinity or the co-operativity in the reaction of haemoglobin for oxygen, also results in a similar change in the reaction with any other ligand.

• (b) For normal haemoglobin the value of n in the Hill equation is uniformly lower for the isocyanides (2.4 to 2.2) than for oxygen or carbon monoxide under comparable conditions, and the value of the partition constant between isopropylisocyanide and oxygen is variable with the extent of the replacement. These facts may be rationalized on the basis of a functional heterogeneity between the α and β chains in their reactions with bulky ligands. A fit of the experimental data indicates that in the assembled haemoglobin molecule one of the two chains may have an affinity three to four times higher than the other.

• (c) The over-all equilibrium constant ($\text{I = (C}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)}{}^{\mathrm{-1}}$) for the binding of haemoglobin to the isocyanides depends on the molecular dimensions of the ligand in a complex manner. An explanation of the pattern can be provided on the basis of the knowledge that the aliphatic side-chain of the ligand tends to be packed toward the interior of the molecule, in anon-polar environment. Therefore, binding would be favoured by the transfer of the hydrocarbon chain from water to the nonpolar environment, and hindered by steric effects which become more and more serious as the size of the ligand increases.