Carbon-13 and proton nuclear magnetic resonance spectroscopy of water-soluble porphyrins and metalloporphyrins

Abstract

Carbon-13 and proton nuclear magnetic resonance (NMR) spectra have been recorded for porphyrins, zinc porphyrins, and iron(III) porphyrin complexes in aqueous media. Spectra of porphyrin- c and hemin- c confirm the structure with thioether linkages at positions α to the porphyrin ring. The pattern of NMR isotropic shifts has implications regarding electron transfer in cytochrome- c. Free-base porphyrin- c and meso-substituted porphyrins have been examined for pyrrole nitrogen-hydrogen tautomerism and possible aggregation in aqueous solution. Zinc porphyrin 13 C NMR spectra were recorded in order to provide diamagnetic references for paramagnetic iron(III) derivatives. Low-spin iron(III) porphyrin-biscyano complexes in aqueous solution exhibit NMR isotropic shift patterns similar to those previously observed for related compounds in non-aqueous media. The first 13 C NMR spectra are reported for μ-oxo-bridged iron(III) porphyrin dimers. A partilly resolved spectrum of a high-spin iron(III) porphyrin has also been obtained. Patterns of 13 C and proton isotropic shifts are compared, and unpaired spin delocalization mechanisms for 13 C resonances are discussed in a qualitative manner.