Calcium-ion induced color changes of a porphyrin solution

Interactions between the porphyrin-bearing multidentate ligands and group II metal ions in aqueous solution were studied. Changes in the visible spectra of 5 × 10−5 M solution of the porphyrin in 0.01 M 2-[4-(2-hydroxyethyl)-1-piperazinyl] ethanesulfonic acid (pH 7.4) containing 0.1 M KCl were observed upon titration of Ca2+ at 25 °C. From the analyses of NMR and visible spectral data, we proposed that the spectral changes are due to the formation of an associate between the porphyrin and Ca2+ via the following three interactions: (i) formation of a complex between the attached NTA groups and Ca2+; (ii) hydrophobic interaction among porphyrin rings; and (iii) interaction between the pyrrole nitrogens and Ca2+.