Studies of the conformation of apomyoglobin in aqueous solutions and denaturing organic solvents

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Abstract
The properties of apomyoglobin were examined in aqueous solutions and various helix‐ and random‐coil‐forming solvents by solvent perturbation, optical rotation, circular dichroism, and viscosity measurements. The solvent perturbation data obtained in neutral aqueous solutions suggest 25–40% exposure of the two tryptophyl residues and 50–60% exposure of the three tyrosyls. The estimates of burial of these groups are in the ranges expected for myoglobin based on its X‐ray structure. In the helicogenic alcohols, methanol, ethanol, 2‐chloroethanol, trifluoroethanol, and 1‐propyl alcohol, as well as in acidic solutions, 8 M urea and 6M guanidine hydrochloride, essentially all the tryptophyl and tyrosyl residues are found to be exposed to solvent based on this method. Analysis of the ORD and CD data indicates that in the alcohols the α‐helix content of apomyoglobin has in most cases changed from 58–59% to about 80–95%. Analysis of the intrinsic viscosity data based on the equations of Simha and Kirkwood and Auer indicates that the polypeptide chain in these solvents has the dimensions of fully extended α‐helical rods, with lengths of 221–251 Å and mean diameters of 12.8–13.6 Å. It is concluded that apomyoglobin in the various alcohols must have an extended but somewhat irregular rodlike structure, having a few bend or irregular sequences between the α‐helical segments due largely to the presence of the four proline residues, 37, 88, 100, and 120 in the amino acid sequence.