Spectroscopy and Electrochemistry of Cytochrome P450 BM3-Surfactant Film Assemblies
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We report analyses of electrochemical and spectroscopic measurements on cytochrome P450 BM3 (BM3) in didodecyldimethylammonium bromide (DDAB) surfactant films. Electronic absorption spectra of BM3-DDAB films on silica slides reveal the characteristic low-spin FeIII heme absorption maximum at 418 nm. A prominent peak in the absorption spectrum of BM3 FeII-CO in a DDAB dispersion is at 448 nm; in spectra of aged samples, a shoulder at ~420 nm is present. Infrared absorption spectra of the BM3 FeII-CO complex in DDAB dispersions feature a time-dependent shift of the carbonyl stretching frequency from 1950 to 2080 cm-1. Voltammetry of BM3-DDAB films on graphite electrodes gave the following results: FeIII/II E1/2 at -260 mV (vs SCE), ~300 mV positive of the value measured in solution; S rc, S , and H values for water-ligated BM3 in DDAB are -98 J mol-1 K-1, -163 J mol-1 K-1, and -47 kJ mol-1, respectively; values for the imidazole-ligated enzyme are -8 J mol-1 K-1, -73 J mol-1 K-1, and -21 kJ mol-1. Taken together, the data suggest that BM3 adopts a compact conformation within DDAB that in turn strengthens hydrogen bonding interactions with the heme axial cysteine, producing a P420-like species with decreased electron density around the metal center.