Myoglobin retains iron heme and near-native conformation in DDAB films prepared from pH 5 to 7 dispersions

Abstract

A recent paper [M.T. DeGroot, M. Merkx, M.J.M. Koper, J. Am. Chem. Soc. 127 (2005) 16224] suggests that iron heme is released during casting films of myoglobin (Mb) and didodecyldimethylammonium bromide (DDAB) on pyrolytic graphite (PG) electrodes. Insoluble films of lipids and surfactants have become widely used for protein film electrochemistry, and protein conformation within them is a critical issue. The present communication examines the integrity of myoglobin structure in DDAB films by voltammetry, visible absorption and circular dichroism spectroscopy, and discusses these new data in context with previously reported studies. Voltammetry showed that peak potentials for Mb, horseradish peroxidase and hemin in DDAB films are significantly different, and that Soret band and CD spectra in the films are fully consistent with a near native Mb conformation within the films. The view that pyrolytic graphite surfaces are catalysts for denaturation and heme loss in Mb-DDAB films is discredited by consideration of key results in the literature. On the basis of all results and considerations, this communication confirms that when DDAB films are made from vesicle dispersions at pH 5 to 7 and used in neutral buffers containing NaBr, Mb in the film is in a near-native conformation.