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.

Spectroscopic and kinetic evidence for redox cycling, catalase and degradation activities of Mn(III)(TPPS) in a basic aqueous peroxide medium.

Mn(III)(TPPS) was found to react rapidly with hydrogen peroxide in basic aqueous solution to form intermediate (TPPS)Mn(V)[double bond, length as m-dash]O and (TPPS)Mn(IV)[double bond, length as m-dash]O species which, in the presence of excess H2O2, are reduced fully back to Mn(III)(TPPS) with clear evidence for redox cycling of Mn(III)(TPPS). The system shows very strong catalase and degradation activities.