news center

Science : Knotty problem emerges from the looking glass

Science : Knotty problem emerges from the looking glass

作者:鱼胫  时间:2019-02-27 02:14:00  人气:

By David Bradley THE CELTIC trefoil knot—a circle made up of three leaf-shaped lobes—has long fascinated chemists because it has left-handed and right-handed forms. This makes it a model for understanding many important molecules, such as certain drugs, DNA and amino acids, that also come in two mirror-image or “chiral” forms. Seven years ago, French chemists used an ingenious technique to synthesise the first molecular trefoil knot (New Scientist, Science, 19 August 1989). Now the same team has found a way to separate out the left-handed and right-handed forms of the molecule. This will allow them to study the way chiral molecules such as potential anticancer drugs interact with DNA, and could also lead to the development of new chiral catalysts that force a reaction down one of the two possible paths. The basic trefoil knot can be twisted so that it contains a double helix of strands down its centre (see Diagram). The strands can take either a left-handed or a right-handed thread. Jean-Pierre Sauvage and his team at the Louis Pasteur University in Strasbourg designed their trefoil knot by persuading two strand-like molecules to wrap themselves around two copper ions to form a double helix, and joining the ends. The problem, though, is that when the strands wind themselves round the copper ions they form left and right-handed spirals with equal ease, leaving an equal mixture of the two mirror-image trefoils. Sauvage and his colleagues have now managed to break the symmetry with another neat trick. To fish out one form of the trefoil, they add another chiral ion, called a dinaphthyldiylphosphate, in just one of its mirror-image forms, depending on which form of the trefoil is desired. This ion reacts preferentially with the corresponding form of the trefoil to produce a molecule that will crystallise out of solution in the form containing only the desired mirror image of the knot. The phosphate can then be removed,