GSK lecture: Nanotechnology in the real world
The GlaxoSmithKline International Achievement award for 2003 was presented to Professor Martyn Davies and colleagues from the biophysics and surface analysis group at the University of Nottingham and Molecular Profiles Ltd on 16 September. Christine
Clark reports on the group's work
Nanotechnology is a catch-all description of activities at the level
of atoms and molecules that have applications in the real world, Professor
Martyn Davies
said. The study of biology at the nano-level is likely to lead to the development
of better ways of giving drugs, the design of new sensors and the development
of new materials. “Far from being a remote theoretical science, nanotechnology
is inherent in everything that we do,” he explained.
Scanning probe microscopes (SPMs) are used to examine materials at the nano-level.
These have a sharp probe that scans across the surface of the material that is
being examined. The microscope is designed to detect one of a number of signals,
such as force (as in the atomic force microscope, AFM), light, magnetism or heat.
Figure 1: How scanning probe microscopes work
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The sharp tip or probe is positioned at the end of a soft spring (cantilever).
As the tip moves across a sample the cantilever moves up and down. A beam of
laser light is directed at the back of the cantilever and the reflected light
is detected by a suitable instrument and an image is created (see Figure 1).
This approach allows the visualisation of single molecules. It can be used in
a range of environments, such as in air or, preferably, in water.
Professor Davies said that one of the first biological applications of AFM
within the Nottingham group was the examination of DNA and, in particular,
the action
of drugs on DNA. One experiment concerned the addition of ethidium bromide
to DNA. As the concentration of ethidium bromide increased the conformation
of the
DNA changed from a relaxed form, at first, then to the super-coiled toroidal
form and finally to plectonemic (coiled around another molecule) forms. “These
changes are important because they affect the mechanical properties of DNA”,
Professor Davies said.
The behaviour of individual DNA molecules can be clearly visualised using AFM.
For example, his team has shown that a single enzyme can degrade DNA producing
dramatic structural changes as the molecule becomes uncoiled and, for practical
purposes, destroyed.
During imaging, the tip of the AFM probe has an interaction with the surface
or product that is being scanned and this feature can be exploited. For example,
the tip could be coated with a biological material and the surface of the sample
could be mapped for characteristics such as adhesion or hydrophobicity.
“A mechanical fingerprint of a single DNA molecule can be constructed by
picking up a single DNA molecule with the probe and stretching it. This produces
a characteristic
fingerprint and it is possible to show that the fingerprint changes when DNA
binds to drugs — each drug-DNA combination producing a unique fingerprint.” This
might offer a possibility for screening of future drugs, suggested Professor
Davies.
Another application for AFM lies in the understanding of physiological processes
at the molecular level. For example, the properties of titin, a substance integrated
into muscle cells that functions as a biological, molecular spring, have been
examined. The relationships between force and velocity of unfolding of individual
titin molecules have been studied and the effects of alterations in the amino
acid sequence have been investigated. “The real forces of nature are down
at this level,” he said. This method has made it possible to show the
mechanical properties of the titin molecule, which have subsequently been published
in Nature.
The application of SPM-based approaches to particle analysis in pharmaceutical
formulation has been particularly useful. For example, it is possible to map
two polymorphic forms of a substance in a sample to show how they are distributed.
Chemically functionalised probes have been used to examine different planes in
crystals and examine their surface chemistry. For example, one plane of the aspirin
crystal is dominated by phenyl and methyl groups and is strongly hydrophobic;
another plane comprises mainly oxygen and methyl groups. The AFM tip can sense
their different properties and detect the differences between the crystal faces.
AFM can be also used to observe dissolution or formation of crystals at molecular
level in different conditions — for example, steps on the surface of
an aspirin crystal that are each one molecule high (see Figure 2).
“Finally, AFM enables the examination of surfaces for potential biomedical
applications. It is possible to construct images of isolated biomolecules using
sensitive probes
and, as the technology improves, it should be possible to look at substructural
elements. AFM can also be used to look at the packing of molecules on surfaces
of importance in biotechnology, tissue engineering and sensors,” Professor
Davies concluded. |
