STM of folded and unfolded haemoglobin molecules electrochemically deposited on highly oriented pyrolytic graphite

Abstract

The structural characterization of folded and unfolded haemoglobin has been performed by scanning tunnelling microscopy (STM) for the first time. STM images show and oval-shaped pattern for the folded structure of this protein, and moreover two dimers consisting of one haemoglobin molecule can be clearly discerned. The dimensions of a folded molecule were determined as 6.4 × 5.4 × 0.7 nm³, which are in good agreement with the known size obtained from X-ray analysis. We have found that unfolding of haemoglobin molecules on the surface of highly oriented pyrolytic graphite (HOPG) can be achieved by electrochemical deposition. The STM analysis indicates clearly that the tertiary structure of the protein was lost by electrochemical deposition, and most of the haemoglobin molecules were almost fully extended and exhibited a twisted rope-like or a rod-like aggregated structure. Our investigation demostrates the capability of the electrochemical method in denaturing this redox protein and in preparing stable biological samples for use in STM imaging.