Spectral studies of magnesium porphyrin--apomyoglobin and apohemoglobin complexes

Complexes of magnesium protoporphyrin and magnesium mesoporphyrin with apomyoglobin and apohemoglobin have been prepared and studied by electronic, circular dichroism, and optical rotatory dispersion spectroscopy. The myoglobin complexes show prominent splittings and red shifts of the visible absorption bands, with respect to those for the hemoglobin analogs. Comparisons are made with other heme protein systems that display similar spectral features. Different reasons for the observations are considered, including multiple conformer formation, polarity effects of the protein environment, and the formation of six-coordinate magnesium species through H2O coordination for the Mb complexes (the latter explanation being favored).