Interaction of Polycyclic Hydrocarbons with Cytochrome P-450. II. Flash Photolysis Studies on Various Hydrocarbon-Bound P-4481 CO Compounds1

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Abstract
Effects of aromatic hydrocarbon binding on the photodissociability of CO from ferrous carbonmonoxy P-448 1 and their subsequent recombination reactions were studied with the aid of the flash photolysis technique. We found that both size and structure of the bound hydrocarbon are sensitively reflected in the flash photolysis parameters. The CO aduct of hydrocarbon-free P-448 1 was relatively insensitive to light giving a quantum yield of approximately 0.1. The binding of a relatively small hydrocarbon with 2 to 3 fused benzene rings, such as naphthalene and anthracene, had no influence on the photosensitivity, while additions of a larger molecule such as benz[a]anthracene and dibenzanthracenes resulted in a considerable increase in the quantum yield. Such a trend is also seen with the binding of benzoflavone derivatives but not with terphenyls. Among 21 hydrocarbons examined, 7,8-benzoflavone was most effective in increasing the photodissociability giving a quantum yield of near 1.0. When the effects of hydrocarbons on the rate of CO recombination were examined, they could be classified into the following three groups. (1) Tricyclic hydrocarbons such as phenanthrene and anthracene, and one of the tetracyclic hydrocarbons, benzo[c]phenanthrene, which enhanced the rate over ten times. (2) Pentacyclic hydrocarbons and their structural analogues such as dibenzanthracenes, 3-methyl-cholanthrene and 7,8-benzoflavone, which decreased the rate to below one tenth. (3) Other hydrocarbons of various structures, which did not affect the rate significantly. In the case of pyrene and triphenylene complexes of which photodissociation was not detectable, the CO binding rate was 3–5 times higher than that of the hydrocarbon-free form as judged by the stopped flow method. Based on these findings, the relationship between the effects and the structures of hydrocarbon molecules was discussed.