Imaging biological structures with the cryo atomic force microscope.

It has long been recognized that one of the major limitations in biological atomic force microscopy (AFM) is the softness of most biological samples, which are easily deformed or damaged by the AFM tip, because of the high pressure in the contact area, especially from the very sharp tips required for high resolution. Another is the molecular motion present at room temperature due to thermal fluctuation. Using an AFM operated in liquid nitrogen vapor (cryo-AFM), we demonstrate that cryo-AFM can be applied to a large variety of biological samples, from immunoglobulins to DNA to cell surfaces. The resolution achieved with cryo-AFM is much improved when compared with AFM at room temperature with similar specimens, and is comparable to that of cryo-electron microscopy on randomly oriented macromolecules. We will also discuss the technical problems that remain to be solved for achieving even higher resolution with cryo-AFM and other possible applications of this novel technique.