Synthesis and characterization of alkanethiolate-coordinated iron porphyrins and their dioxygen adducts as models for the active center of cytochromeP450: Direct evidence for hydrogen bonding to bound dioxygen

Two kinds of novel cytochrome P450 models, which have alkanethiolate axial ligands and hydroxyl groups inside molecular cavities, were designed and sy nthesized as functional O-2 binding systems. A superstructured porphyrin, d esignated as "twin-coronet" porphyrin, was used as the common framework of the model complexes. This porphyrin bears four binaphthalene bridges on the both sides and forms two pockets surrounded by the bulky aromatic rings. T hiobenzyloxy and thioglycolate moieties, which contain an alkanethiolate gr oup exhibiting various electron-donating abilities and degrees Df bulkiness , were covalently linked to twin-coronet porphyrin to yield thiolate-coordi nated hemes, TCP-TB and TCP-TG (twin-coronet porphyrin with thiobenzyloxy a nd thioglycolate groups), respectively. Both ferric complexes exhibited hig h stability during usual experimental manipulation under air and were chara cterized by MS, UV/vis, ESR spectroscopies, and CV. The ESR spectra exhibit ed low-spin signals (TCP-TB: g = 2.334, 2.210, 1.959; TCP-TG: a = 2.313, 2. 209, 1.966); The cyclic voltammogram of TCP-TB in CH3CN gave a quasi-revers ible wave which corresponds to the Fe-III/Fe-II redox couple: E-p/2 = -1.35 V (vs Fc/Fc(+)). On the other hand, TCP-TG showed a fine reversible wave: E-1/2 (Fe-III/Fe-II) = -1.12 V. The stable dioxygen adducts were formed in the reaction of the ferric complexes with KO2 under an oxygen atmosphere an d characterized by UV/vis and resonance Raman (RR) spectroscopies. In the R R spectra, the nu (O-O) bands of the dioxygen adducts were observed at 1138 cm(-1) (TCP-TB) and 1137 cm(-1) (TCP-TG). The hypothesis that hydrogen bon ding between the bound oxygen and the hydroxyl groups of the binaphthyl moi eties could increase their stability was verified by RR spectroscopy. When all hydroxyl groups were deuterated, only the frequencies of the nu (O-O) b ands were upshifted by 2 cm(-1) without any perturbation in the porphyrin s keleton. This work shows the first direct evidence for a hydrogen bond to d ioxygen in an oxy form of a thiolate-coordinated heme model system. These r esults are discussed in context of the process of dioxygen binding and acti vation in cytochrome P450.