The Role of Axial Ligand in Determining the Mg²⁺/TM²⁺ (TM = Fe, Mn, Cu, Zn) Selectivity in Chlorophyll

Life on Earth is sustained due to plants, algae, and cyanobacteria's exquisite ability to store energy from light and convert it into a form that can be utilized by living organisms. In this process, the compound of immense significance, chlorophyll, actively participates. It consists of a chlorin unit with different substituents, depending on the type of chlorophyll molecule. The main element, however, is the magnesium cation bound to the center of the tetrapyrrole macrocycle. One additional axial ligand of diverse nature is often attached to the cation to complement the structure of the complex. The present study aims at elucidating the role of the additional ligand for determining the metal selectivity in chlorophyll molecules by implementing a widely applied methodology based on Density Functional Theory. The obtained results provide information about the thermodynamic outcome of the competition between Mg²⁺ and a series of transition biogenic metal cations such as Fe²⁺, Mn²⁺, Cu²⁺, and Zn²⁺ in model chlorophyll metal binding sites in two environments of different polarity, delineating the key factors that contribute to the process in the greatest extent. The results of calculations are discussed in light of known experimental data and hence shed light on the metal selectivity process in this system.