Elucidation of the differences between the 430-and 455-nm absorbing forms of P450-isocyanide adducts by resonance Raman spectroscopy

Alkylisocyanide adducts of microsomal P450 exist in two interconvertible forms, each giving the Soret maximum around 430 or 455 nm. This is demonstrated with a rabbit liver P450 2B4. Resonance Raman spectra of the 430- and 455-nm forms were examined for typical P450s of the two types as well as for P450 2B4 because the 430-nm form of P450 2B4 is liable to change into P420. P450cam and P450nor were selected as a model of the 430- and 455-nm forms, respectively. For the n-butyl isocyanide (CNBu) adduct, the Fe(Il)-CN-Bu stretching band was observed for the first time at 480/467 cm(-1) for P450cam and at 471/459 cm(-1) for P450nor with their (CNBu)-C-12/(CNBu)-C-13 derivatives. For P450cam, but not P450nor, other C-13 isotope-sensitive bands were observed at 412/402, 844/835, and 940/926 cm(-1). The C-N stretching mode was identified by Fourier transform IR spectroscopy at 2116/2080 cm(-1) for P450cam and at 2148/2108 cm(-1) for P450nor for the C-12/C-13 derivatives. These findings suggest that the binding geometry of isocyanide differs between the two forms bent and linear structures for P450cam-CNBu and P450nor-CNBu, respectively. In contrast, in the ferric state, the Raman C-13 isotopic frequency shifts, and the IR C-N stretching frequencies (2213/2170 and 2215/2172 cm(-1)) were similar between P450cam and P450nor, suggesting similar bent structures for both.