Optical absorption band III of deoxyheme proteins as a probe of their structure and dynamics Original Research Article

Theory of the temperature dependence of a non-Frank–Condon optical absorption band in the case of weak anharmonicity is developed and applied to the interpretation of band III properties of deoxyheme proteins. It is obtained that dependence of the transition dipole moment on the displacement of the iron out of the heme plane, the weak anharmonic coupling between the heme doming and the iron–histidine vibration, and the glass–liquid phase transition cause notable and very specific temperature and pressure dependence of band III. It follows from this study that intensity of band III is sensitive to the iron–porphyrin distance, whereas its position can be affected by both the changes in this distance and protein electric field on the heme. Because the iron–porphyrin distance is affected by the protein, the band intensity and position (and not only width) must be sensitive to the protein structure and dynamics. In particular, intensity of band III can be used as a probe of protein dynamics, its dependence on the solvent viscosity, and protein and heme relaxation in the flash-photolysis experiments.