Turning World The Direction of Stirring

At the Biological Research Institute of Szeged in Hungary is the research team of Lajos Keszthelyi, which accumulated considerable international reputation through its contributions to the origin of biomolecular asymmetry. Here in Szeged, the idea became apparent that a chemical reaction starting with racemic or prochiral reac-tants performed in a right-stirred solution might give rise to contrary enantioen-richments in chiral products than the left-stirring of the same system. The following chemical reaction can be related to the prebiotic formation of (proto-) proteins and hence attracted the interest of the Szeged group: specific derivatives of amino acids called N-carboxyanhydrides (NCA) are known to form a polypeptide chain by a polymerization reaction. Kovacs and Keszthelyi (1981) studied the copoly-merization of these derivatives in the case of D,L-glutamic acid and in the case of D,L-alanine. Reactions were performed twice in 50 mL vessels stirred by calibrated precision magnetic stirrers at identical speeds of 540 rpm but with opposite directions of rotation. The experimental set-up is depicted in Fig. 4.6. Kovacs and Keszthelyi carefully followed the polymerization kinetics, characterized the obtained polymers by determining their molecular weight, and obtained chiroptical information by measuring circular dichroism spectra. Astonishingly, counterclockwise stirring of the solution produced longer polymers. And indeed, an effect of the direction of stirring was found so that clockwise stirring seemed to cause a preference for the incorporation of D-enantiomers into the polymers. The authors admitted, however, that the differences suggested a trend but were not convincing enough to conclude that stereoselectivity was due to the direction of stirring in these polymerizations.

So we had to wait until 2001, when Josep Ribo and his colleagues (2001) at the University of Barcelona in Spain discovered and published an experiment in which the vortices in stirred solutions induced chirality in a chemical system. One has to admit that the selected system was special: homoassociates of achiral diprotonated porphyrins formed helical agglomerates during an aggregation process. The direction of vortex motion during aggregation determined the chirality sign of these ho-moassociates, which was controlled by means of circular dichroism measurements.

Fig. 4.6 Stirred not shaken: Experimental set up for clockwise (right) and counterclockwise (left) stirring of appropriate chemical systems. The obtained vortices are magnified

Such experiments, however, are interesting for better understanding the influence of macroscopic forces on bifurcation scenarios, but they have not yet been taken serious enough to explain the origin of biomolecular asymmetry, since the selected systems seem too distant from both molecules of prebiotic importance and prebiotic physico-chemical conditions.

Even if - and what a big if! - one could elucidate appropriate chemical reactions that seem reasonable to have occurred during chemical evolution and showed molecular symmetry-breaking due to the direction of stirring, one would have to answer the question of where the right- or left-vortex originated. Was it just random or might the direction of spiral water turning have been linked e.g. to the rotation of Earth?

In this context, interesting information was published on the well-known Coriolis effect. There is no doubt that the Coriolis effect, named after the French engineer Gaspard Gustave de Coriolis, is responsible for the strong tendency of cyclones and tornados to spin counterclockwise in the northern half of the globe and spin in the other direction in the southern half. Moreover, the Coriolis effect plays a role in the flow of rivers, but the question remains whether the Coriolis force is strong enough to be a detectable influence on a water vortex, such as when it spirals down a drain. It is common knowledge that when water flows out of a bathtub it forms a vortex around the drain. Is there any scientific evidence that the bathtub's or washbasin's vortex in one hemisphere of the Earth turns opposite to the vortex in the other?

According to Gardner (2005), natives along the equator in Kenya are perpetrating an amusing demonstration to tourists. Here is how Robert Goldenburg10 described in Gardner (2005) what he saw when he visited Nanyuki in 1988:

"A standard feature of such stops is the demonstration of the Coriolis force by a local native equipped with a plastic dishpan, a hole punched out of its bottom. The man corks the hole, fills the pan with water and picks up a few twigs or pieces of straw, which he floats on the water surface. He then marches 20 paces south of the equator, followed by a dozen tourists, and pulls the plug. The twigs turn counterclockwise as the water runs out. He repeats the demonstration 20 paces north of the equator. The twigs swirl clockwise.

When we asked him to repeat the demonstration on the equator, he obliged, and the twigs did not move as the water drained out. Having bought trinkets and soda, I joined two other groups as he went through the entire demonstration twice more. If this 'Coriolis force' demonstration seems a matter of chance, I must report that the twigs behaved exactly the same way three times in a row. The man stated that the speed of the twigs' rotation correlates to 'latitude', the distance from the equator."

If this is a tricky swindle, how is it managed? Gardner (2005) guessed that in walking north or south the native secretly imparts a rotary motion to the water by a slight tipping of the dishpan. As for bathtub vortices, the question remains controversial.

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  • ELIZABETH
    What direction do we usually stir?
    2 months ago

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