The question of whether DNA is able to transport electrons has attracted much interest, particularly as this ability may play a role as a repair mechanism after radiation damage to the DNA helix. Experiments addressing DNA conductivity have involved a large number of DNA strands doped with intercalated donor and acceptor molecules, and the conductivity has been assessed from electron transfer rates as a function of the distance between the donor and acceptor sites. But the experimental results remain contradictory, as do theoretical predictions. Here we report direct measurements of electrical current as a function of the potential applied across a few DNA molecules associated into single ropes at least 600 nm long, which indicate efficient conduction through the ropes. We find that the resistivity values derived from these measurements are comparable to those of conducting polymers, and indicate that DNA transports electrical current as efficiently as a good semiconductor. This property, and the fact that DNA molecules of specific composition ranging in length from just a few nucleotides to chains several tens of micrometres long can be routinely prepared, makes DNA ideally suited for the construction of mesoscopic electronic devices.

While holography truly constitutes an ingenious concept, ever since its invention by Gabor it has been troubled by the so-called twin-image problem limiting the information that can be obtained from a holographic record. For symmetry reasons there are always two images appearing in the reconstruction of a hologram and the unwanted out of focus twin-image obscures the object. Here we show a universal method of reconstructing a hologram completely free of twin-image disturbances while no assumptions about absorbing or phase shifting properties of the object need to be imposed. Thus, truthful amplitude and phase distributions are retrieved.

Here we present practical methods for simulation and reconstruction of in-line digital holograms recorded with plane and spherical waves. The algorithms described here are applicable to holographic imaging of an object exhibiting absorption as well as phase shifting properties. Optimal parameters, related to distances, sampling rate, and other factors for successful simulation and reconstruction of holograms are evaluated and criteria for the achievable resolution are worked out. Moreover, we show that the numerical procedures for the reconstruction of holograms recorded with plane and spherical waves are identical under certain conditions. Experimental examples of holograms and their reconstructions are also discussed.

The concept of holography with low energy electrons is described in view of its applications in molecular biology. The challenges and difficulties associated with Gabor type holography are outlined and the differences between coherent electron beams of high and low kinetic energy are discussed. The properties of the coherent electron point source for low energy electrons are reviewed as well as its application in the lens-less holographic microscope. Investigations of in-situ manipulation of objects with nanometer sized dimension will be discussed. Those experiments have recently been applied to DNA molecules and it has been discovered that DNA molecules are in fact electrically conducting biopolymers.

Imaging single proteins has been a long-standing ambition for advancing various fields in natural science, as for instance structural biology, biophysics, and molecular nanotechnology. In particular, revealing the distinct conformations of an individual protein is of utmost importance. Here, we show the imaging of individual proteins and protein complexes by low-energy electron holography. Samples of individual proteins and protein complexes on ultraclean freestanding graphene were prepared by soft-landing electrospray ion beam deposition, which allows chemical-and conformational-specific selection and gentle deposition. Low-energy electrons do not induce radiation damage, which enables acquiring subnanometer resolution images of individual proteins (cytochrome C and BSA) as well as of protein complexes (hemoglobin), which are not the result of an averaging process. low-energy electron holography | single protein imaging | preparative mass spectrometry | microscopy | structural biology M ost of the currently available information on structures of macromolecules and proteins has been obtained from either X-ray crystallography experiments or cryo-electron microscopy investigations by means of averaging over many molecules assembled into a crystal or over a large ensemble selected from low signal-tonoise ratio electron micrographs, respectively (1). Despite the impressive coverage of the proteome by the available data, a strong desire for acquiring structural information from just one individual molecule is emerging. The biological relevance of a protein lies in its structural dynamics, which are accompanied by distinct conformations. For a protein to fulfill its vital functions in a living organism, it cannot exist in just one single and fixed structure, but needs to be able to assume different conformations to carry out specific functions. Conceptually, at least two different conformations, just like in a simple switch, are needed. In view of oxygen transport to cells for example, binding oxygen in one specific conformation and releasing it again in a different conformation are needed. To address the "physics of proteins" as described by Hans Frauenfelder in his pioneering review (2), one needs to realize that proteins are complex systems assuming different conformations and exhibiting a rich free-energy landscape. The associated structural details, however, remain undiscovered when averaging is involved. Moreover, a large subset of the entirety of proteins, in particular from the important category of membrane proteins, is extremely difficult, if not impossible, to obtain in a crystalline form. If just one individual protein or protein complex can be analyzed in sufficient detail, those objects will finally become accessible.For a meaningful contribution to structural biology, a tool for single-molecule imaging must allow for observing an individual protein long enough to acquire a sufficient amount of data to reveal its structure without altering it. The strong inelastic scattering cross-section of high-energy ...

Field-ion techniques have been used to create physical point sources for ions and electrons with emission areas and angles orders of magnitude smaller than in any other available source. The monatomic pyramidal tips emit electrons or ionize noble-gas atoms originating from the single front atom. The angular divergence from the normal direction above the single W atom is less than 0.5° for ion beams produced by field ionization. The angular spread for emission from small clusters is somewhat larger for field ionization and electron emission. By employing tips as sources for free electrons in an STM-like setup, we were able to obtain high-resolution images of surfaces with an electron beam of only 15 eV primary energy. The image information is contained in the yield of the secondary electrons created at the sample surface.

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The phase problem is inherent to crystallographic, astronomical and optical imaging where only the intensity of the scattered signal is detected and the phase information is lost and must somehow be recovered to reconstruct the object's structure. Modern imaging techniques at the molecular scale rely on utilizing novel coherent light sources like X-ray free electron lasers for the ultimate goal of visualizing such objects as individual biomolecules rather than crystals. Here, unlike in the case of crystals where structures can be solved by model building and phase refinement, the phase distribution of the wave scattered by an individual molecule must directly be recovered. There are two well-known solutions to the phase problem: holography and coherent diffraction imaging (CDI). Both techniques have their pros and cons. In holography, the reconstruction of the scattered complex-valued object wave is directly provided by a well-defined reference wave that must cover the entire detector area which often is an experimental challenge. CDI provides the highest possible, only wavelength limited, resolution, but the phase recovery is an iterative process which requires some pre-defined information about the object and whose outcome is not always uniquely-defined. Moreover, the diffraction patterns must be recorded under oversampling conditions, a pre-requisite to be able to solve the phase problem. Here, we report how holography and CDI can be merged into one superior technique: holographic coherent diffraction imaging (HCDI). An inline hologram can be recorded by employing a modified CDI experimental scheme. We demonstrate that the amplitude of the Fourier transform of an inline hologram is related to the complex-valued visibility, thus providing information on both, the amplitude and the phase of the scattered wave in the plane of the diffraction pattern. With the phase information available, the condition of oversampling the diffraction patterns can be relaxed, and the phase problem can be solved in a fast and unambiguous manner. We demonstrate the reconstruction of various diffraction patterns of objects recorded with visible light as well as with low-energy electrons. Although we have demonstrated our HCDI method using laser light and low-energy electrons, it can also be applied to any other coherent radiation such as X-rays or high-energy electrons. ABSTRACTThe phase problem is inherent to crystallographic, astronomical and optical imaging where only the intensity of the scattered signal is detected and the phase information is lost and must somehow be recovered to reconstruct the object's structure. Modern imaging techniques at the molecular scale rely on utilizing novel coherent light sources like X-ray free electron lasers for the ultimate goal of visualizing such objects as individual biomolecules rather than crystals. Here, unlike in the case of crystals where structures can be solved by model building and phase refinement, the phase distribution of the wave scattered by an individual molecule mus...