Panel A: Representative low temperature MCD spectra for (from L to R) 6C octahedral, 5C square pyramidal, 5C trigonal bipyramidal, and 4C distorted tetrahedral non-heme Fe (NHFe) complexes. Panel B: MCD spectra for resting PAH (black) overlaid with spectra for (from L to R) PAH-L-Phe (blue), PAH-pterin (green), and PAH-L-Phe-pterin (red). Only coordination of both cosubstrates leads to a 5C site. Adapted from ref 2.