Electric fields and the reducing ability of Cytochrome C Hemes Cytochrome C (Cyt C) is important as a redox partner in biological systems where it transfers electrons to Complex IV in mitochondria . It contains an iron heme center that acts as a reductant in its ferrous (Fe 2+ ) form. Prior studies underscored the role of electrostatic preorganization in heme function and catalysis , and here we use PyCPET to probe 3-D electric field at the heme Fe in a human Cyt C (PDB 2N9I). PyCPET allows us to specify the orientation, location, and volume where we computed the electric field (Fig. ). At each representative frame obtained from clustering the MD trajectory of ferrous Cyt C, we visualized the 3-D 𝐸𝐸 "⃗ (𝑟𝑟) using the visualization tool in PyCPET. This utility generates files for the ChimeraX software (Fig. ).