Optical spectroscopic observation of a metastable form of sperm whale myoglobin generated by reconstitution

The optical spectrum of Sperm Whale myoglobin, which has been freshly reconstituted with iron protoporphyrin-IX, is shown to be different from that obtained from the native myoglobin, and from the reconstituted, incubated myoglobin (These last two have equivalent absorption spectra). The effect is immediately evident as a shift of about +1 nm in the Soret band during incubation of freshly reconstituted metMb. Difference spectroscopy can be used to deconvolute changes in optical spectra occurring during and after Mb reconstitution into two components. The initial phase reflects incorporation of hemin into the protein matrix; this is already known to produce two forms, differing by relative hemin orientation. The rate of the second process follows the known pH dependence of iron protoporphyrin-IX reorientation. Presence of the second process indicates that the absorption spectrum of each of the two hemin-insertion Mb forms is unique, so interconversion between the two forms is monitored. Thus iron protoporphyrin-IX reorientation in proteins may be studied by visible spectroscopy.