Comparison of solution and crystalline state protein structures. Photoacoustic study of horse and human hemoglobins.

In an effort to assess the influence that crystallization may have on protein conformations, optical absorption spectra of crystalline state hemoglobin derivatives have been examined. These spectra were obtained from photoacoustic spectra using a computer-assisted analysis. Comparisons of crystal and solution state hemoglobins using crystal minus solution state difference spectra indicate that the conformations of these proteins are similar in both states. Crystallization does not change the absorption properties of horse oxyhemoglobin or the cyanide and azide adducts of horse and human methemoglobin. Spectra of crystalline methemoglobins, which are prepared by ligand exchange in the crystalline state, are identical to the spectra of the final crystalline state adduct prepared without ligand exchange. Further, the allosteric effector, inositol hexaphosphate, causes the same spectral changes in solution and crystalline state hemoglobins. However, differences between crystal and solution state spectra of both fluoride and aquo methemoglobin are observed. These differences suggest that small but perhaps functionally significant changes in the heme regions of these derivatives accompany crystallization.