Fraunhofer in-line electron holography of frozen-hydrated DNA superhelices
We have visualized DNA molecules in solution by Fraunhofer in-line electron holography. So far, applications of the technique at high resolution have been limited to gold particles. This report is the first application of Fraunhofer in-line electron holography to a nanometer-sized biological specimen.The phase shift of an electron beam due to DNA is so small, estimated as only 2π/33 for 100-keV electrons, that it has been only possible to visualize them with either heavy-atom staining or metal shadowing techniques. However, these methods give only low-resolution images of the molecules, and worst of all, preservation of specimens is poor. The frozen-hydrated specimen preparation improved the preservation of specimen, and images of naturally supercoiled DNA in solution have been visualized using underfocusing of 1-2 μm. Such a defocusing is large enough for such a small object as DNA (diameter ∽ 2 nm) to be in a Fresnel condition; z ≈ a2/λ, where z is the defocusing, a is the diameter of an object, and λ is the wavelength.