PROTEIN fibres in muscles are probably nature鈥檚 best molecular springs. But
chemists in Austria now claim to have made the world鈥檚 most intricate synthetic
version of the same thing. Their spring measures 1.11 nanometres from tip to tip
when compressed, and stretches to 1.41 nanometres.
鈥淚t鈥檚 the smallest structurally characterised spring in the world,鈥� says
Bernhard Kr盲utler of the University of Innsbruck. The spring forms part of
a molecule called a rotaxane, containing a string of 12 carbon atoms.
Kr盲utler and his colleagues 鈥渟toppered鈥� each end of the chain with a
vitamin B12 molecule, with cobalt atoms bolting the stoppers to the
spring.
In its relaxed form, the chain makes a regular zigzag pattern. Kr盲utler
compresses the spring simply by dissolving it in water. The water molecules
clustering around the chain apply the 鈥減ressure鈥� that makes it kink and
contract. It then takes on an irregular pattern of humps and troughs.
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To make the spring 鈥渞elax鈥� again, Kr盲utler adds a cylindrical sugar-like
molecule called a cyclodextrin. This forms a molecular collar around the chain,
which shields it from the pressure of the water molecules and allows it to relax
back to the regular, zigzag shape. The researchers describe their results in the
Journal of the American Chemical Society (vol 119, p 2313).
Though the spring is something of a curiosity, Kr盲utler is confident
that molecules like it could help to unravel the complex forces at work when
compounds dissolve. Knowledge of these forces might be useful in the synthesis
of drugs. Important biological proteins are often sheathed in water molecules,
which dictate their shape and function. Without the water, the proteins don鈥檛
work.
