PORPHYRINS-CYCLODEXTRIN .2. DISSOCIATION, REDUCTION, AND PROTON RELAXIVITY OF AN IRON(III) PORPHYRIN MU-OXO DIMER IN CYCLODEXTRIN SOLUTIONS

Spectrophotometric evidence for the dissociation of an Fe(III) porphyrin mu-oxo dimer induced by porphyrin-cyclodextrin complexation in aqueous solution is reported. Depending on the pH, the monomer may be in a diaquo monohydroxo or diaquo dihydroxo form with pK values of 6.4 or 8.5, respectively. In the absence of beta-cyclodextrin (beta-CD), the mu-oxo dimer exists as mono- or dihydroxo species with a pK of 11.7. gamma- and pulse radiolysis techniques are used to study the Fe(III)/Fe(II) reduction. Fe(III)TSPP(H2O)2 (TSPP is tetrakis(4-sulfonatophenyl)porphyrin) is reduced by hydrated electrons and CH2OH. radicals into Fe(II)TSPP(H2O)2 whereas Fe(III)TSPP(OH-)2 and (H2O)FeTSPP-O-FeTSPP(OH-) are both reduced to the monohydroxo Fe(II)TSPP(H2O)(OH-) monomer. The Fe(II)-O-Fe(II) dimer may be formed in strongly alkaline solution. Measurements of the proton (NMR) relaxivity values of this Fe(III) porphyrin in the absence and in the presence of beta-CD indicate the possibility of using Fe(III)TSPP in conjunction with beta-CD as a paramagnetic contrast agent for in vivo imaging of tumors and tissue.