Factors controlling the reactivity of divalent metal ions towards pheophytin a

Ł Orzeł,M Witko,G Stochel,A Kania,J Waś,L Fiedor,A Susz,J Staroń,D Rutkowska-Zbik

doi:10.1007/s00775-017-1472-1

Abstract

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.

Highlights

Metalloporphyrinoids play key roles as photocatalytic and catalytic centers, and as carriers of electrons and small molecules in biological systems
To account for the effect of residual water in the reaction medium, the kinetics of Z n2+ insertion was determined in hydrated MeOH containing between 0 and 10% (v/v) water
The addition of water to an organic solvent slows down the metalation [Fig. S1 in the Electronic Supplementary Material (ESM)]

Summary

Introduction

Metalloporphyrinoids play key roles as photocatalytic and catalytic centers, and as carriers of electrons and small molecules in biological systems Many such metallocomplexes, especially those with transition metal ions, are important catalysts in industry and the laboratory [1], and can be used as photosensitizers in various kinds of phototherapy and diagnostics. The impact of metal ion reactivity on metalation of porphyrins has frequently been discussed Most of these studies, carried out in aqueous solutions, showed analogies between the rates of metalation and water exchange both in simple aquacomplexes [14, 15] and in organic media [16,17,18], pointing to the solvent release step as an important factor in the kinetics of metalloporphyrin formation.

Results

Discussion

Conclusion