• NO-binding studies were also performed on iron tetraphenyl porphyrin (TPP-Fe). The XYZ files in the case of hemin, H-Tyra, H-Tyros, H-Styr and TPP-Fe are summarized in supplementary tables 2,5,8,11,14, respectively. After optimizing all geometries and reaching a stationary point for each, • NO was bound to the central metal via Fe-N*=O* bonding. The final systems were allowed to relax at the same level of theory. These atoms were starred to distinguish between the N and O atoms of • NO and those in the main porphyrin structures. For all the optimized geometries with [Fe(II)-NO] + (S = 0) ground state, the Fe-N*-O* units were linear with similar Fe-N* and N*-O* bond lengths (Table , supplementary tables 3,6,9,12). Moreover, the movement of the Fe atom out of the porphyrin-plane was significant only in hemin and H-Styr, which showed a slight enhancement over the Fe adducted to Cl in the main molecular structures. However, the Fe atom maintained its position in the other complexes. Some similarities were observed on comparing ∠(N-Fe-N), Fe-N* and binding energy between the different Fe(III)-based complexes before and after binding with • NO at the Fe(III)-NO (S = 3/2) state.