The photoreduction of metmyoglobin (metMb) and its effects on the changes of protein structure were investigated by UV-visible absorbance, fluorescence and circular dichroism (CD) spectra. MetMb in Na₂HPO₄-NaH₂PO₄ buffer were irradiated with xenon lamp at 430 nm and ultraviolet lamp. The results indicated that the absorbance intensity at Q band (545 nm and 580 nm) increased evidently after illumination. Through experimental verification, metMb had been reduced after the illumination. The verifying experiment included: adding sodium dithionite to metMb|adding potassium ferricyanide to the sample after the illumination|irradiating meanwhile adding CO to the sample. Through further discussion, it can be seen that the photoreduction mechanism of metMb with xenon lamp at 430 nm was photoinduced intramolecular electron transfer. In addition, the results of synchronous fluorescence and CD spectra of the sample after irradiation suggested that the polarity of microenvironment of tryptophan residues increased and the secondary structure of protein (CD data) varied with α-helices reduction from 63% to 57%, and β-sheet increase by 7%.

Key words: metmyoglobin, photoreduction, excited state, structure changes