Experimental Determination of Internal Electric Fields in Ordered Systems: Myoglobin and Cytochrome C

We have developed methods to determine internal molecular electric fields at the sites of probe molecules which were doped into various host matrices from the Stark effect on persistent spectral holes. These internal electric fields are of fundamental importance in charge separation and transfer processes especially in biological systems (e.g. photosynthetic reaction center). Two interesting systems to demonstrate the potential of our methods are the globular proteins myoglobin and cytochrome C, both of which contain a heme (iron porphyrin) chromophore as a suitable probe molecule. The porphyrin π-electron system serves as a detector for the two in-plane components of the internal electric field. The orientation and magnitude of the in-plane field provide structural information. We conclude that in myoglobin the dominant field sources are the deprotonated propionic acid side chains of the porphyrin, but that in cytochrome C additional field sources close to the porphyrin also contribute significantly.