Factors controlling the reactivity of divalent metal ions towards pheophytin a.

In this study, we evaluate the factors which determine the reactivity of divalent metal ions in the spontaneous formation of metallochlorophylls, using experimental and computational approaches. Kinetic studies were carried out using pheophytin a in reactions with various divalent metal ions combined with non- or weakly-coordinative counter ions in a series of organic solvents. To obtain detailed insights into the solvent effect, the metalations with the whole set of cations were investigated in three solvents and with Zn2+ in seven solvents. The reactions were monitored using electronic absorption spectroscopy and the stopped-flow technique. DFT calculations were employed to shed light on the role of solvent in activating the metal ions towards porphyrinoids. This experimental and computational analysis gives detailed information regarding how the solvent and the counter ion assist/hinder the metalation reaction as activators/inhibitors. The metalation course is dictated to a large extent by the reaction medium, via either the activation or deactivation of the incoming metal ion. The solvent may affect the metalation in several ways, mainly via H-bonding with pyrrolenine nitrogens and the activation/deactivation of the incoming cation. It also seems to affect the activation enthalpy by causing slight conformational changes in the macrocyclic ligand. These new mechanistic insights contribute to a better understanding of the "metal-counterion-solvent" interplay in the metalation of porphyrinoids. In addition, they are highly relevant to the mechanisms of metalation reactions catalyzed by chelatases and explain the differences between the insertion of Mg2+ and other divalent cations.

Quantum Chemical Description of Oxygen Activation Process on Co, Mn, and Mo Porphyrins.

The aim of the present theoretical study is to examine the dioxygen activation process occurring at the metalloporphyrin complexes as the first step of the selective oxidation of hydrocarbons, with the stress put on how this may be affected by the type of the central metal. In order to do so, the properties of the porphyrin complexes of Mn, Mo, and Co are discussed by means of quantum chemical calculations within Density Functional Theory (DFT). As a first step, the dioxygen binding by the above-mentioned systems is considered, followed by the study of the interactions of one and two hydrogen atoms with different types of the adsorbed O2 molecule onto the porphyrin complex. Finally, the stability of the formed oxo species is discussed.