Comparison of demetalation properties between zinc chlorin and zinc porphyrin derivatives: Effect of macrocyclic structures

Metalloporphyrins and metallochlorins are important biological molecules in nature. To examine the effect of macrocyclic structures on removal of central metals from these cyclic tetrapyrrole molecules without peripheral substitution effects, demetalation of zinc methyl pyropheophorbide a (zinc chlorin 1) and zinc methyl protopyropheophorbide a (zinc porphyrin 2) was kinetically analyzed under acidic conditions. Both 1 and 2 exhibited gradual spectral changes from zinc complexes to free-base forms with several isosbectic points in acetone/water (10:1) at the proton concentration of 6.1 × 10−3 M. Demetalation of zinc porphyrin 2 was slower than that of zinc chlorin 1 in the temperature range between 15 and 35 °C. This indicates that the porphyrin macrocycle provides tolerance to removal of the central metal from cyclic tetrapyrrole ligands compared with the chlorin macrocycle.