The reaction of tetranitromethane with bacteriorhodopsin under irradiation was separated into two stages by selecting wavelengths of the irradiation light. The first stage of the reaction was characterized by a blue shift of the absorption spectrum and the second stage was characterized by decrease of the absorption of the visible region. The blue shift at the first stage was not caused by nitration of tyrosine residues but a modification of the chromophore. This conclusion was confirmed by reconstitution experiments and chromophore analysis. Nitration of tyrosine residues progressed at the second stage of the reaction.
