Ferriporphyrins in aqueous alkaline medium: The effects of salt and “ageing”

Abstract

The effects of NaCl and air on ferrideuteroporphyrin IX and ferrimesoporphyrin IX (1 · 10 −3–1 · 10 −4 M) in aqueous medium at pH 11–12 and at room temperature are compared with analogous systems containing ferriprotoporphyrin IX by using light absorption spectrophotometry in the 640–320 mμ range and analytical ultracentrifugation. At high NaCl concentrations, both ferrideuteroporphyrin IX and ferrinesoporphyrin IX show a shoulder near 600 mμ, a maximum at 565 mμ and higher intensity (Soret) bands around 380 and 340 mμ, respectively. Similar spectra have previously been obtained with ferriprotoporphyrin IX under analogous conditions, except for shifts to shorter wavelengths of most bands in the case of ferrideuteroporphyrin IX and ferrimesoporphyrin IX. In order to obtain similar spectral effects, progressively high concentrations of either the metalloporphyrin or salt are required in the series ferriprotoporphyrin IX < ferrimesoporphyrin IX < ferrideuteroporphyrin IX, indicating the stabilization of the complex by the porphyrin side chains. From the increase of the area under the refractive index gradient curve with increasing NaCl concentrations in sedimentation velocity experiments, aggregate formation by NaCl can be demonstrated for both ferrideuteroporphyrin IX and ferrimesoporphyrin IX, as has been shown previously for ferriprotoporphyrin IX. In contrast to solutions of ferriprotoporphyrin IX in air, no slow spectral changes (“ageing”) are recorded for ferrideuteroporphyrin IX and ferrimesoporphyrin IX in air, or for ferriprotoporphyrin IX in the absence of air. The fast process, due to the effect of salt, can be observed in all cases.