Biological atomic force microscopy: from microns to nanometers and beyond.

In recent years, atomic force microscopy (AFM) has become the most rapidly developing imaging method. We summarize its recent applications in structural biology, with emphasis on high-resolution imaging, and illustrate these applications with images of cell membranes, DNA, and soluble and membrane proteins. With present technology, and at room temperature, nanometer resolution of DNA and soluble proteins is achievable, although the resolution attained on cell surfaces is more limited (10-50 nm). We suggest that high-resolution imaging of cell surfaces and very high (sub-nanometer) resolution molecular imaging requires an approach that increases specimen rigidity. Operation at cryogenic temperatures, the most promising approach, is discussed, and a cryo-AFM designed for operating at ambient pressure and liquid nitrogen temperature is described. The results include high-resolution images and evidence of increased molecular rigidity at low temperatures.