Metallo-porphyrins 212 advantages

Chelation with metal ions dramatically affects the reactivity of the porphyrin macrocycle, and this can be used to advantage in many cases. For example, for efficient electrophilic substitution of the ring, metals can be chosen (vide infra) which effectively release electron density to the organic porphyrin ligand, while reductions are best carried out on metallo-porphyrins in which the metal tends to deplete the porphyrin ligand of its electron density by way of back-bonding. 

Metallo-porphyrins can be adsorbed physically or attached chemically to active groups at the surface of carriers. Metallo-porphyrins heterogenized on high-surface oxides or porous polymers in principle have the following advantages over their homogeneous analogs (19-22) ... 

From these results, it is clear that chlorophyll adsorbed at an interface can catalyze redox reactions between solutes in the two liquid phases, and these reactions are accompanied by injection of negative charges into the low dielectric membrane interior. Metallo-complexes of porphyrins also have similar catalytic properties at the oil/water interface without illumination [13,14,43]. An advantage of porphyrins as catalysts in such experiments is their chemical stability in different media. 

Recent reconstitution experiments open the possibility of selectively introducing other metallo bacteriochlorins (or chlorins) at the acceptor BChl or BPheo sites. Zinc substitutions are obvious choices since the optical and coordination properties of Zn complexes resemble those of Mg derivatives but the former are easier to reduce (and harder to oxidize) than the latter. Perhaps a less obvious candidate for metal replacement is nickel which offers several advantages Ni complexes are also easier to reduce and harder to oxidize than Mg compounds but, in addition, redox potentials of Ni porphyrins and chlorins are close to those of the metal-free derivatives the optical properties of Ni chlorins are similar to those of Mg ones they possess shortlived excited states as a result of which they have proved to be valuable for resonance Raman studies. A more practical advantage of Ni complexes is their enhanced stability towards demetallation, compared to Mg or Zn complexes, a property of merit if the chromophores are to be used in reconstitution reactions. (Ni protoporphyrins IX have been successfully exchanged for the histidine-bound hemes in hemoglobin.) ... 

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