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RobertDyck

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From: Winnipeg, Canada

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Muscular molecules

Nature: Muscular molecules

Making muscle is easy, Philip Ball reports. All it takes is a molecular needle and thread.
Engineers dreaming of making machines as small as molecules will be heartened by a report in the journal Angewandte Chemie1of a 'molecular muscle': an assembly of two molecules capable of stretching and contracting when prompted by chemical signals.

Synthetic materials that respond to some stimulus by growing or shrinking are well known -- 'piezoelectric' materials, for instance, change shape when stimulated by electric fields. But commonly this behaviour relies on the combined action of many billions of atoms or molecules. It is harder to make individual molecules show muscle-like behaviour -- those that do exist are typically chain-like polymers that grow or shrink like springs.

But the new molecular device devised by Jean-Pierre Sauvage and colleagues at the Université Louis Pasteur in Strasbourg, France, is different: it works in a similar way to real muscle.

When our muscles contract, no molecules get shorter. Muscle cells are long, thin tubes divided into segments, each containing filamentary molecules called actin and myosin. The two molecules interpenetrate each other like two toothbrush heads pushed together bristle to bristle. To contract muscle, the tips of the myosin filaments stick to the actin strands and pull themselves along like inchworms, making the 'bristles' interpenetrate more deeply.

Sauvage's team has devised twinned strands that can jump along one another to increase or decrease their combined length. This makes them like an actin and myosin pair -- except that these two molecules are identical. And they are cleverly held together: each molecule ends in a loop through which the tail of the other is threaded.

Such structures (hoops impaled on chain-like threads) are called rotaxanes. Over the past several years, Sauvage has become one of the world masters at threading the molecular needle. But his new rotaxanes are unusual because each of the paired molecules in the assembly acts as both hoop and thread.

The hoops are not there just to hold the two molecules together -- they also provide the mechanism for contracting the 'muscle'. The hoops contain units that stick to charged metal atoms (ions). The linear 'pole' portion of the molecules also contains two of these units: one, in the 'neck' closest to the hoop, prefers copper ions; the other, closest to the tail, favours zinc. Together, a metal-binding unit on one of the hoops and a unit on the poles will surround one of these metal ions and bind it tightly.

So when copper ions are present, the hoop on one molecule moves up near the neck of the other so that between them they can bind an ion. This leaves the tails poking out through the hoops. But if copper is substituted by zinc, the hoops move down towards the tails of their partner to capture one of these ions using the zinc-binding units. This pulls the tails up through the hoops so that the rotaxane contracts.

The researchers estimate that each contraction shortens the rotaxane by about 27 per cent, which is about the same as the maximum shortening in contracted muscle. And the similarity between man and molecule does not end there: an influx of metal ions -- in this case calcium -- triggers biological muscle contraction too.

tahanson43206

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Registered: 2018-04-27

Posts: 19,388

Re: Muscular molecules

For RobertDyck re new topic and Post #1

Impressive find!  Best wishes for success with this encouraging new topic.

(th)