ADVANCES in microscopy mean that scientists can, for the first time, watch biological processes in action at the molecular level. Using a new type of instrument, the atomic force microscope, Paul Hansma and his colleagues at the University of California in Santa Barbara have taken images of the blood-clotting protein fibrin as it polymerises (Science, vol 243, p 1525).
The microscope, known as the AFM, is one of the latest developments in what are called scanning probe microscopes. It uses a splinter of diamond to 'feel' the surface of a material. A system of delicately sensitive cantilevers transmit and measure optically the deflection produced when the needle bobs up and down as it traces the uneven surface of the molecules.
The AFM is ideal for imaging biological molecules, such as proteins because it can scan surfaces directly and can also scan in water, fibrin's natural habitat.
Hansma recorded what happened to a solution of fibrinogen on a mica surface when he added a few drops of the clotting enzyme thrombin. Thrombin converts fibrinogen into the monomer of fibrin by removing sets of peptides from its centre. The monomers immediately start to join up to form polymers.
Image A shows a molecule of fibrinogen. These molecules were difficult to see because they would not stick to the surface of the mica. It took nearly 13 minutes from adding the thrombin to build up one chain of polymer on the mica (B to G). Image H shows two chains after another two minutes, and image I shows that a third chain has formed after a further minute or so. It took just over half an hour to form the network of protein (J), which in blood would trap blood cells to produce a clot.
Hansma believes that scanning probe microscopes such as the AFM will revolutionise the study of biological and chemical processes. One problem is that the protein moves about in water. Hansma hopes to improve the quality of the images by lowering the temperature enough to keep the protein still, or by imaging the protein in more viscous fluids.
'From the beginning, we hoped it would be possible to use scanning probe microscopes to image biological and medical samples. It is becoming clear that they will be useful. We look forward to a very rapid growth in applications,' said Hansma.
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