Effects of a thiolate axial ligand on the π→π* electronic states of oxoferryl porphyrins: A study of the optical and resonance Raman spectra of compounds I and II of chloroperoxidase

Optical absorption and resonance Raman spectra have been investigated for enzymatic intermediates, compounds I and II, of chloroperoxidase (CPO) which contains a thiolate-ligated iron porphyrin. Compound I of CPO (CPO-I), an oxoferryl porphyrin π cation radical, gave an apparently asymmetric single-peaked Soret band at 367 nm, for which band fitting analyses revealed the presence of two transition bands around 365 and 415 nm. Compound II of CPO (CPO-II), an oxoferryl neutral porphyrin, gave a split Soret spectrum with two bands (blue and red Soret bands) at 373 and 436 nm. Thus both CPO-I and CPO-II can be categorized as hyperporphyrins. The maximum extinction coefficients (ε_b and ε_r) and energies (E_b and E_r) of the blue and red Soret bands of CPO-II were found to fall on an ε_b/ε_r versus E_b-E_r correlation line derived from data reported for six-coordinate ferrous derivatives of cytochrome P450 and CPO. Corresponding data for CPO-I did not fall on the correlation line. Resonance enhancement of the Fe^IV=O stretching (ν_FeO) Raman band was found for CPO-I when Raman scattering was excited at wavelengths within both transition bands around 365 and 415 nm, while the ν_FeO Raman band was not identified for CPO-II at any of the excitation wavelengths examined here. These findings suggest that the thiolate axial ligand causes Soret band splitting of CPO-II through configuration interaction between the sulfur→porphyrin e_g* charge transfer and porphyrin a_1u,a_2u→e_g* transitions, while the FeO portion is important in determining the shape of the Soret band of CPO-I.