Tobacco mosaic virus as an AFM tip calibrator.

The study of high-resolution topographic surfaces of isolated single molecules is one of the applications of atomic force microscopy (AFM). Since tip-induced distortions are significant in topographic images the exact AFM tip shape must be known in order to correct dilated AFM height images using mathematical morphology operators. In this work, we present a protocol to estimate the AFM tip apex radius using tobacco mosaic virus (TMV) particles. Among the many advantages of TMV, are its non-abrasivity, thermal stability, bio-compatibility with other isolated single molecules and stability when deposited on divalent ion pretreated mica. Compared to previous calibration systems, the advantage of using TMV resides in our detailed knowledge of the atomic structure of the entire rod-shaped particle. This property makes it possible to interpret AFM height images in term of the three-dimensional structure of TMV. Results obtained in this study show that when a low imaging force is used, the tip is sensing viral protein loops whereas at higher imaging force the tip is sensing the TMV particle core. The known size of the TMV particle allowed us to develop a tip-size estimation protocol which permits the successful erosion of tip-convoluted AFM height images. Our data shows that the TMV particle is a well-adapted calibrator for AFM tips for imaging single isolated biomolecules. The procedure developed in this study is easily applicable to any other spherical viral particles.