Molecular complexes of water-soluble porphyrins

Water-soluble porphyrins tend to form self-aggregates under certain conditions. The structure of the self-aggregate (H-type dimer, H-type higher aggregate, or J-aggregate) sensitively depends on the structure of the peripheral substituents at the meso-positions of the porphyrin. Water-soluble porphyrins also form relatively stable π-stacked complexes with various aromatics. Polar effects are important in complexation of anionic porphyrins with anthraquinonesulfonates. The results suggest that the main attractive interaction for formation of π-stacked complexes of water-soluble porphyrins is the σ-π interaction as claimed by Hunter and Sanders. Meanwhile, the aryl groups at the meso-positions of the water-soluble porphyrins are included by per-O-methylated β-cyclodextrin through van der Waals interactions to form extremely stable 1:2 complexes (porphyrin:cyclodextrin). Such a character of the water-soluble porphyrins can be applied to convenient preparation of supramolecular hetero-porphyrin arrays.

A Myoglobin Functional Model Composed of a Ferrous Porphyrin and a Cyclodextrin Dimer with an Imidazole Linker

A 1:1 inclusion complex (Fe(II)PImCD) of 5,10,15,20-tetrakis- (4-sulfonatophenyl)porphinatoiron(II) (Fe(II)P) and an O-methylated beta-cyclodextrin dimer with an imidazole linker (ImCD) was found to bind dioxygen in aqueous solution. The half-saturation pressure of dioxygen (P(1/2)O2) is 1.7 torr at 25 degrees C, which is 10 times lower than that for a previous myoglobin functional model (hemoCD) with a pyridine linker. Meanwhile, the half-life of oxygenated Fe(II)PImCD is 3 h, which is 10 times shorter than that of oxygenated hemoCD. The covering of the iron(II) center by a microscopic environment is essential for preventing autoxidation of oxygenated ferrous porphyrin, which is promoted by nucleophilic attack of H2O and/or nucleophiles such as inorganic anions. Due to structural requirements, covering of the Fe(II) center of Fe(II)PImCD is insufficient compared with the case of hemoCD. As a result, water molecules can penetrate more easily the cleft of the O2-Fe(II)PImCD complex and act as an autoxidation inducer. This structure also causes poorer selectivity against carbon monoxide (M = 1040). In contrast, the dioxygen affinity of Fe(II)PImCD is much higher than that of hemoCD because the imidazole moiety is a stronger electron donor than pyridine.