AFM imagings of ferritin molecules bound to LB films of poly-1-benzyl-L-histidine

A water-soluble protein, ferritin, on a silicon surface has been imaged in pure water at room temperature with the atomic force microscope (AFM). The sample were prepared by binding ferritin molecules electrostatically to a charged polypeptide layer of poly-1-benzyl-L-histidine (PBLH). The hexagonal arrangement of ferritin molecules was imaged with high reproducibility, since the force between tip and the sample surface could be kept sufficiently lower than 10^-10 N. The applied force can be stabilized and weakened mainly due to a 'self-screening effect' of the surface charges of the ferritin-PBLH layer. We demonstrate that the electrostatic-binding sample preparation is one of the suitable methods for soft biological specimens to achieve the nondestructive low-force AFM imagings.