The preferred solution conformation of warfarin at the active site of cytochrome P-450 based on the CD spectra in octanol/water model system

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Abstract
An octanol/water model system and circular dichroism (CD) spectroscopy have been used to study the solution conformation of warfarin in aqueous and lipid environments. Upon partitioning of (S)-warfarin from buffer pH 7.4 into octanol, the position of the absorption band due to the alpha, beta-unsaturated carbonyl chromophore shifts from 210 nm in the aqueous phase to 220 nm in the octanol phase. The shift is coupled to an increase in the molecular ellipticity of the band, suggesting the formation of a dissymmetric chromophore. Comparison of CD spectra of conformationally fixed analogues of warfarin to that of warfarin in solution suggests that the compound shifts from the open side chain keto form in the aqueous phase at pH 7.4 to the cyclic hemiketal form after partitioning into the lipid octanol phase. On the basis of these results, the hemiketal form is proposed as the preferred solution conformation of warfarin in the lipid environment of the active site of cytochrome P-450 and the relationship between solution conformation and stereoselectivity of warfarin metabolism by beta-naphthoflavone inducible cytochrome P-450 is discussed.

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