Stark effect experiments in cytochrome c-type proteins: Structural hierarchies

We performed hole-burning Stark effect experiments on cytochrome c in which the iron of the heme was either removed or replaced by Zn. According to the experiments, the free-base compound has an effective inversion center, even in the protein. The Zn compound, on the other hand, shows quite peculiar features: in the low-frequency range of the inhomogeneous band, it definitely has a dipole moment, as indicated by a splitting of the hole in the external field. However, in the maximum of the inhomogeneous band, a severe charge redistribution occurs, as the experiments show. In addition to the Stark experiments, we performed calculations of the electrostatic fields at the pyrrole rings and at the metal site of the heme group. We interpret our findings with a model based on structural hierarchies: the protein can exist in a few subconformations, which can be distinguished through the structure of the heme pocket. The different pocket structures support different structures of the chromophore, which, in turn, can be distinguished through their behavior in an external field. These distinct structures, in turn, correspond to a rather broad distribution of protein structures, which leave, however, the pocket structure largely unchanged. These structures show up in inhomogeneous broadening.