FT-Raman, FT-IR and normal-mode analysis of carcinogenic polycyclic aromatic hydrocarbons. Part I - a density functional theory study of benzo(a)pyrene (BaP) and benzo(e)pyrene (BeP)

Abstract

In the mechanistic study of the oxygenation of polycyclic aromatic hydrocarbons (PAHs) by mixed‐function oxidase, we employed hemoglobin (Hb) as a model compound for cytochrome P450. We found that the relative intensities of several Raman peaks of benzo(a)pyrene (BaP) and benzo(e)pyrene (BeP) in the region between 600 and 1600 cm−1 are significantly enhanced by the oxygenated Hb but not by the deoxygenated Hb. This seems to indicate that these affected vibrations could be the modes involved in PAH epoxidation by the heme‐bound oxygen. Density functional theoretical calculations were carried out using a commercially available software package. The DN** basis set appears to give the most satisfactory results. The difference in wave numbers between observed and calculated wave numbers is estimated to be <20 cm−1. Peak assignment through animation shows that all these heme‐bound oxygen‐affected modes are in‐plane vibrations. This seems to indicate that BaP molecules, in forming the activated complexes, are more likely vibrating in‐plane than out‐of‐plane. In other words, BaP molecules may diffuse out of the heme pocket without interaction if their vibrations do not enhance orbital overlapping with the heme‐bound oxygen molecules.