Anchoring of self-assembled hemoglobin molecules on bare indium-tin oxide surfaces

Abstract

The interfacial phenomena of spontaneous immobilization of a self-assembled monolayer (SAM) of hemoglobin (Hb) molecules on indium-tin oxide (ITO) surfaces are presented. Results from attenuated total reflectance Fourier transform infrared spectroscopy show the existence of heterogeneous interfacial ester bond formation between Hb molecules and ITO surfaces. An angle (θ) of ∼20° from the surface normal of a line connecting the two oxygen atoms of the interfacial carboxylates of native Hb was obtained from relevant spectral intensities, which reveals the angular conformation of the interfacial carboxylates. This angular conformation, nevertheless, was found to be different from that of self-assembled denatured Hb molecules on the same immobilized surface. The latter exhibits a wide range of θ values which, in general, were found to be more than 45°. The observed consistency in differences of the morphologies from results of scanning electron microscopy of both self-assembled native and denatured Hb molecules on ITO surfaces further suggests the angular conformation of the interfacial carboxylates to be closely related to the molecular conformation of the protein molecules on the anchoring nanocrystalline surfaces. High-resolution scanning tunneling microscopy further elucidates the conformation of SAMs of Hb molecules on ITO surfaces, revealing immobilization of lib molecules on ITO surfaces through exposed active binding sites via either of its two β chains. By use of a Hb variant, ferrous oxyhemoglobin (HbA0), its bioactivity while in a SAM configuration on ITO surfaces was studied and demonstrated to preserve remarkably well. Consistent results from self-assemblies of other proteins on the same ITO surfaces further prove the nonspecific binding nature of the immobilization. The present study may provide some fundamental insights into the heterogeneous interfacial information of SAMs of lib on ITO surfaces, which may serve as a model system for studying other self-assembled proteins on ITO surfaces and investigating various physicochemical related properties, etc.