Placing a sample in a magnetic field allows for magnetic circular dichroism (MCD) measurements to be obtained. In protein molecules, MCD can be used as a probe to monitor a chromophore’s local environment. Chromophores with large magnetic moments arising from either rotational symmetries (aromatics, porphyrins), unpaired spins (metal complexes), or both (hemes) are sensitive to electronic perturbations and therefore provide information regarding the molecule’s electronic state.