Probing single flavoprotein molecules on graphite in aqueous solution with scanning tunneling microscopy

Single flavoprotein molecules on graphite were studied by electron-transfer (ET) reactions at the electrode/electrolyte interface in an aqueous environment at the single-molecule level. The height of adsorbed GOx molecules as measured by scanning tunneling microscopy (STM) is found to be mush smaller than that given in the X-ray crystallographic, data due to the structural and electronic properties of proteins and induced compression of proteins by the tip. The effect from the interaction of the redox center with the surrounding solvent molecules and with the substrate is estimated to be 0.34 eV. The adsorbed GOx molecules are found to become clear when the electrode potential is stepped from -0.36 to -0.46V, which shows that GOx on highly oriented pyrolytic graphite (HOPG)depends on the substrate potential.