Electrostatic forces, potentials and charges play a key role in determining the structure and function of proteins, DNA and larger biomolecular structures. For example, the negative charge on DNA is a crucial factor in the packing of genetic material in the cell nucleus and in the interaction of DNA with various proteins during the transcription of genetic information. To understand these interactions and their effect on biomolecular structure and complexes, we need access to both the structure and the electrostatics at the nano-scale.
Scientists at the LCN (Hoogenboom's group), the UCL Chemistry Department (Howorka's group) and King's College London (Mesquida's group) have now imaged the molecular structure of DNA and simultaneously visualised the negative charges on the DNA. To this end, they have adapted a technique called Kelvin Probe Force Microscopy to image molecules on atomically flat mica surfaces, which are ideal for the adsorption of biological molecules. It is an important step forward with respect to earlier measurements of the same groups (Nano Lett., 2009), in which the molecular structures appeared in an unnaturally stretched conformation due to interaction with the underlying substrate.
The structural images were obtained by following the contours of the molecules with a miniaturised probe, acting as a nano-scale blind man's stick. In the Kelvin Force Probe Microscopy experiments, the probe also simultaneously measured the forces due to the electrical charges of the molecules. Using customised instrumentation to minimise the distance at which this electrostatic signal was recorded, the researchers have demonstrated a significant improvement in sensitivity for surface charges, which facilitated the DNA measurements. Although the instrumentation has been developed for studying biomolecules, the same principles can also be applied to image local structure and electronic properties of semiconductors in miniaturised electronic circuits.
This work, funded by the Biotechnology and Biological Sciences Research Council (BBSRC, grant references: BBG0117291, BBE00959X1 and BBE0104661) and the Royal Society (RG080161) has recently been published in Applied Physics Letters (Carl Leung et al., Appl. Phys. Lett., 97, 203703 2010).
Journal link: http://apl.aip.org/resource/1/applab/v97/i20/p203703_s1
Figure: (a) Topographic structure and (b) map of negative (black) charges of DNA molecules. (c) Overlapping topography and Kelvin Probe Force Microscopy (KPFM) line profiles extracted at the position indicated by the arrows in (a) for a single DNA molecule.