On the photosensitivity of liganded hemoproteins and their metal-substituted analogues.

Abstract

The photosensitivity of low-spin liganded Hb, myoglobin and peroxidase, and their metal-substituted analogues was examined, using 3 different metals (Fe, Mn, Co) in several oxidation states and employing a variety of diatomic or pseudo-diatomic ligands (L). A number of photosensitive systems were discovered and an overall stereo-electronic classification scheme for these photodissociation reactions is presented. Linear, formally d6, metal-ligand fragments [e.g., Fe(II) + CO; Mn(II) + NO] are relatively photolabile, but systems with a bent fragment, and higher electron occupancy [e.g., Fe(II + O2; Co(II) + NO] are relatively photoinert. Photostability appears to correlate with the occurrence of long-wavelength features in the optical absorption spectra and the classification scheme is explained by considerations of electronic structure. The discussions are further applied to d5 systems and to low-spin d6 metalloporphyrins with nitrogenous bases as axial ligands.