SAC/SAC-CI study of the ground, excited, and ionized states of cytochromesP450CO

The SAC (symmetry adapted cluster)/SAC-CI method was used to calculate the ground, excited, and ionized states of carboxy P450 model complexes. The ex cited and ionized states were calculated up to 5.9 and 3.4 eV, respectively . Our initial calculations were with the D-4h-skeleton model structure. The Q-band was well described, but the BI band (lower of the split Soret bands ) was higher than the experimental value by 0.7 eV and the BIT band (higher of the split Soret bands) did not appear. Next, we used the distorted X-ra y crystallographic structure with side chains on porphyrin. The calculated results reproduced not only the Q-band but also both the BI and BIT bands i n reasonable agreement with experimental findings with regard to both absor ption positions and intensities, thus demonstrating the importance of using the actual molecular geometry. In particular, the BIT band is attributed t o excitation from the pi bond of the porphyrin side chain. We examined the effect of proteins on the spectrum by using the point charge model for envi ronmental proteins. We then examined the effect of the double bond of the p orphyrin side chain by saturating it with the thiohydrate bond and clarifie d that the experimental geometry is important for reproducing the BI band a nd that the double bond of the porphyrin side chain is important for reprod ucing the BIT band. We also performed calculations for ionized states as a model of compound I (oxo-Fe(IV) porphyrin pi -cation radical complex) of P4 50s. The lowest energy state was the porphyrin pi cation radical with A(2u) -like symmetry.