Spectroscopic studies in protein films with highly sensitive single-mode guided-wave plataforms

In this work we report experimental results on the molar absorptivity of cytochrome c adsorbed at different submonolayer levels onto an aluminum oxide waveguide surface. The spectra was acquired using the broadband, single-mode, integrated optical waveguide spectroscopic technique, which is an extremely sensitive tool able to reach submonolayer levels of detection required for this type of studies. For a protein surface density of 2.3 pmol/cm² the molar absorptivity measured at 695 nm was 335 M^-1 cm^-1, and for a surface density of 14.6 pmol/cm² was 720 M^-1 cm^-1 which is much closer to the value of cyt c dissolved in an aqueous neutral buffer (830 M^-1 cm^-1). Our data show a clear dependence of the protein optical properties on its surface density. The modification of the protein molar absorptivity can most likely be attributed to conformational changes of the surface-adsorbed species.