The Crystal-Solution Problem of Sperm Whale Myoglobin

The central question to be discussed in this paper is whether the structure established for sperm whale myoglobin in the crystalline state is the same as that of the protein in solution. As judged by its ultraviolet optical rotatory dispersion, the helical content of metmyoglobin in solution does not differ from that in the crystal, 77 per cent. Although an uncertainty of about ±5 per cent must attach to this result, it excludes many alternative arrangements of the polypeptide chain. The folding of the chain may be further restricted to the basic form seen in the crystal if the dimensions of the molecule in solution and the interactions of specific chemical groups are taken into account. Since the rotatory dispersion of metmyoglobin is constant with respect to ionic strength, and since the dispersions of reduced and oxymyoglobin reveal no change in helical content upon their formation from metmyoglobin, one may infer that the structure of the protein is largely maintained both as it dissolves and during its reversible combination with oxygen. The crystallographic model of myoglobin thus offers a structural basis for attempting to explain its physiological function in solution. The relevance of this conclusion to the crystal-solution problems presented by other species of protein is then best seen in the light of common factors that govern the equilibrium of all proteins between crystal and solution.