Theoretical study of heme derivatives under DFT calculations

The molecular geometry of the heme derivatives was optimized by using DFT calculations at B3LP/3-21G* level and their HOMO-LUMO energies were calculated at B3LYP/6-31G* level. These calculations showed correlation with the reported experimental data. The LUMO density at the iron and sp3 carbons of the heme increases when its vinyl and propionic groups are substituted by electron withdrawing groups, while their HOMO density increases when electron donating groups are substituted in the same positions. When the propionic groups are modified with electron withdrawing or electron donating groups, the electronic density is maintained over their sp3 carbons, avoiding electron transfer from the substituent to the iron atom. Consequently, it is possible that different interactions also exist between the substrate and the sp3 carbons. With Cysteine as the fifth ligand, there is a change in the geometry of the heme derivatives, inducing a coplanarity among the four pyrrolic rings and impeding the interaction between the vinyl and nitrophenyl groups located at the C2 and C7 positions, respectively.