A comparison of the tetrapyrrole cofactors in nature and their tuning by axial ligands

This chapter illustrates how quantum chemical calculations can be used to elucidate structural and functional aspects of tetrapyrrole cofactors, focusing on porphyrins, cobalamins, coenzyme F430, and chlorophyll. A particular emphasis is put on the biochemical significance of axial ligands, which can tune the function of the tetrapyrroles. With the use of quantum chemical calculations, it is possible to draw important conclusions regarding aspects of tetrapyrroles that could not otherwise be accessed. The results show that the general reactivity is mainly determined by the metal and the tetrapyrrole ring system, whereas the electronic structure and reactivity are tuned by the choice of axial ligands, providing a unique insight into the design of cofactors in nature.