Octaethylisobacteriochlorin, a Model of the Sirohydrochlorin Ring System of Siroheme Enzymes: Preparation and Spectroseopic and Electrochemical Properties of the Free Base and Its Zn(II) Complex

Abstract

As the first step in developing synthetic analogues of the siroheme prosthetic group of nitrite and sulfite reductases, octaethylisobacteriochlorin (H2(OEiBC)) and its Zn(II) complex (Zn(OEiBC)) have been prepared and certain physicochemical properties investigated. Details of the synthesis and purification of H2(OEiBC) are presented. From 360-MHz nuclear magnetic resonance evidence, the free base as obtained is most probably a mixture of two nearly equally abundant isomers with trans-reduced pyrroline rings. Quantitative spectral data for H2(OEiBC) and Zn(OEiBC) are reported. By absorption and fluorescence emission spectral criteria, H2(OEiBC) proves to be a viable model of sirohydrochlorin, the metal-free form of siroheme whose complete structure has recently been elucidated. H2(OEiBC) and Zn(OEiBC) undergo two one-electron oxidations to the radical cations and dications. Comparison of potentials for the two primary oxidations with those for corresponding octaethylporphyrin (OEP) and octaethylchlorin (OEC) species reveals that the ease of oxidation follows the order OEiBC > OEC > OEP. Successive potential intervals are 0.2-0.3 V. The electron paramagnetic resonance spectra of H2(OEiBC)+ and Zn(OEiBC)4+ exhibit no resolved hyperfine splittings. Absorption spectra of the radical cations were measured by a spectroelectrochemical technique using an OTTLE cell. When electrolyzed at potentials sufficient to generate the cation radical, solutions of H2(OEiBC) in acetonitrile and dichloromethane eventually develop an intense royal blue chromophore, whose formation is more rapid at potentials sufficient to generate H2(OEiBC)2+. The blue species was also obtained by reaction of H2(OEiBC) in dichloromethane with 2 equiv of the strong one-electron oxidant (p-BrC6H4)3N+. This species was identified as the protonated chlorin H3(OEC)+ and is proposed to be formed by dehydrogenation of H2(OEiBC)2+. © 1980, American Chemical Society. All rights reserved.