Optical properties of Yeast Cytochrome c monolayer on gold: An in situ spectroscopic ellipsometry investigation

The adsorption of Yeast Cytochrome c (YCC) on well defined, flat gold substrates has been studied by Spectroscopic Ellipsometry (SE) in the 245–1000 nm wavelength range. The investigation has been performed in aqueous ambient at room temperature, focusing on monolayer-thick films. In situ δΨ and δΔ difference spectra have shown reproducibly well-defined features related to molecular optical absorptions typical of the so-called heme group. The data have been reproduced quantitatively by a simple isotropic optical model, accounting for the molecular absorption spectrum and film-substrate interface effects. The simulations allowed a reliable estimate of the film thickness and the determination of the position and the shape of the so-called Soret absorption peak that, within the experimental uncertainty, is the same found for molecules in liquid. These findings suggest that YCC preserves its native structure upon adsorption. The same optical model was able to reproduce also ex situ results on rinsed and dried samples, dominated by the spectral features associated to the polypeptide chain that tend to overwhelm the heme absorption features.

Spectroscopic ellipsometry allows to control the state of Yeast Cytochrome c molecules adsorbed on gold (left) through the detection of absorptions related to the heme group (right).Figure optionsDownload high-quality image (43 K)Download as PowerPoint slideHighlights
► In situ spectroscopic ellipsometry of Cytochrome c monolayer on ultraflat gold.
► Sharp spectral features related to Soret band typical of heme group are identified.
► A simple isotropic optical model reproduces ellipsometric spectra quantitatively.
► Accurate estimate of film thickness and of optical properties of molecules.
► Adsorbed molecules under wet conditions preserved their native structure