ANALYSIS OF A SIROHEME MODEL-COMPOUND - CORE-SIZE DEPENDENCE OF RESONANCE RAMAN BANDS AND THE SIROHEME SPIN STATE IN SULFITE REDUCTASE

Resonance Raman (RR) and infrared spectra are reported for nickel(II) octaethylisobacteriochlorin (OEiBC) and its N-15(4) and meso-d3 isotopomers. The bands are assigned to porphyrin-like modes with the aid of a normal-coordinate analysis by using a force field developed for nickel octaethylporphyrin (OEP) and by scaling the force constants to bond lengths expected on the basis of the crystal structure of nickel meso-tetramethylisobacteriochlorin. The eigenvectors show a relationship between OEP and OEiBC modes, despite some localization to one or the other half of the OEiBC ring caused by the unequal lengths of the methine bridge bonds adjacent to the pyrrole and pyrroline rings. Excitation at wavelengths near the B and Q absorption bands show Q-resonant RR spectra to be dominated by a single Franck-Condon active mode, nu-29, for NiOEiBC, in contrast to the vibronic scattering from many modes seen in porphyrins. On the other hand, B-resonant spectra that are dominated by Franck-Condon active modes for porphyrins show equally strong enhancement of vibronic modes in NiOEiBC. These differences are attributed to the energy separation of the a1u and a2u HOMO's, brought about by pyrrole ring reduction, and the resultant diminution of effects which are associated with configuration interaction. When the vibrational assignments are used to track the RR bands of a series of iron-OEiBC complexes, a distinctive dependence of the high-frequency bands on the ring core size of the porphyrin analogues is observed, although the slopes of the correlations differ from those reported previously for porphyrin or chlorin. Direct comparison of the iron-OEiBC high-frequency bands with those of the Eschericha coli sulfite reductase hemoprotein confirms the previous inference that the iron(II) siroheme is intermediate spin in the semireduced protein but high spin in the fully reduced protein.