Native and reconstituted myoglobin were prepared and their c.d. spectra recorded in the Soret region. Time-dependent changes in dichroism following reconstitution were observed and related to haem orientational disorder. Comparative c.d. studies, in agreement with n.m.r. studies, reveal that the degree and nature of this disorder are species-dependent.

Ligand-binding kinetics of native and reconstituted sperm-whale myoglobin were studied in relation to haem orientational disorder by rapid kinetic methods. In addition, native yellow-fin-tuna myoglobin with significant amount of haem disorder was also used. The O2 dissociation and association rates were found for the proteins with different degrees of haem disorder, and these results suggest that the isomers are characterized by almost identical kinetic parameters. Rates of CO recombination after photolysis were also identical for the two orientational isomers. The results clearly indicate that the rotation of the haem about the alpha-gamma meso axis has little or no effect on the ligand-binding properties of these myoglobins.