The effect of ligand size and stereochemistry on the reactivity of the alpha and beta chains within hemoglobin.

The reactions of human hemoglobin A with methyl, ethyl, n-propyl, n-butyl, iso-butyl, sec-butyl, and tert-butyl isocyanide were examined in the presence and absence of inositol hexaphosphate. As the size and bulk of the aliphatic side-chain increases, the relative association rates and affinities of the beta-chains for isonitriles increase compared to those of the alpha chains. This result indicates that the beta heme pocket within hemoglobin is more open and accessible to ligand molecules than the alpha heme pocket.