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Applications and Analogies
Ron DeLorenzo Middle Georgia College Cochran, GA 31014
The World’s First “Pastarimeter”: An Analogous Demonstration of Polarimetry Using Pasta Fusilli Claire Saxon, Scott Brindley, Nic Jervis, Graeme R. Jones,* E. David Morgan, and Christopher A. Ramsden School of Chemistry and Physics, Lennard-Jones Laboratories, Keele University, Staffordshire, ST5 5BG, UK; *
[email protected] Communication Chirality, the phenomenon of left- and right-handedness, is a fundamental property of many molecules essential for life (such as sugars and amino acids) and an attractive scientific topic to present to the general public. Demonstrations can lead members of the public into the science by discussing everyday objects such as hands, clocks, nuts, and bolts which simply demonstrate that left and right or clockwise and counterclockwise are not the same.1 Moving closer to the molecular level the spiral twists of shells and plants are delightful examples of left- and right-handedness in nature. The almost complete selectivity of shells that spiral clockwise leads smoothly into discussions of the exclusivity of only right-handed sugars and left-handed amino acids in nature.2 The exciting thought that alien life forms based on carbon should also have evolved a bias to left or right brings you to the frontier of searching for extraterrestrial life by probing the left- and right-handedness of molecules from space (1). Simple tetrahedral models of carbon atoms with four different colored balls at the apexes can be used to show that carbon-based molecules can either be left-handed or righthanded, and that their mirror images are not identical. Marking the positions of three of the colored balls on a piece of paper for the right-handed molecule and then trying to place the left-handed molecule over the same marks shows that they cannot occupy the same positions and are therefore fundamentally different. Small left- and right-handed molecules come together to make larger macromolecular structures such as crystals, proteins, and DNA amplifying the left- and righthandedness of the small molecules onto the macro scale: the right-handed sugar in DNA forces it to twist into a righthanded double helix. Oranges and lemons, and spearmint and caraway are good examples of pairs of right- and left-handed molecules that our bodies respond to differently. Just as a right hand can tell the difference between another right hand or a left hand by clasping them, so taste receptors in the mouth can tell the difference between left- and right-handed limonene obtained from lemons and oranges respectively. Similarly, receptors in the nose can distinguish between the odors of caraway and spearmint, which are the left- and right-handed forms of the molecule carvone. This distinction can be appreciated by explaining that the amino acids, which make up the receptors, are themselves all left-handed and hence can distinguish between left and right. Simple shape receptors can also be constructed to further illustrate the point. 1214
Scientists use polarimeters to distinguish between leftand right-handed molecules. The effect of polarized light is simple to observe with a pair of Polaroid sunglasses and a polarized light source. In classical polarimeters there are two Nichol prisms, the polarizer and the analyzer. Right-handed molecules turn the plane of the polarized light to the right, and the left-handed molecules turn it to the left. The effect of a 50:50 mixture of left- and right-handed molecules is that the polarized light is not rotated. Our experience of explaining polarimetry to the general public is that they frequently ask how molecules rotate light, which is difficult to explain using non-technical language. Therefore we were keen to find an analogous large scale system which mimicked the polarimeter and used everyday leftand right-handed objects. In the 1960s the late Sir Robert Robinson FRS, Nobel Laureate, suggested to his then-research assistant David Morgan a possible method of demonstrating the rotation of plane polarized light by chiral molecules (personal communication, 1965). He said that in 1914, while working at the University of Sydney in Australia, he had shown that short lengths of left- or right-handed screw threads packed into a tube would rotate a stream of water flowing down the tube. He said that right-handed threads rotated the exiting water in a clockwise direction and left-handed threads counterclockwise. Despite a number of attempts David Morgan was unable to repeat this demonstration and never had the occasion to talk with Robinson about it again. We are now pleased to report that we have succeeded in repeating this experiment and confirming Robinson’s observations. The major stumbling block in the past has been the availability of a left- and right-handed threaded material with a suitably deep thread. We have now found a remarkably cheap and disposable source in our local supermarket in the form of pasta fusilli.3 Surprising as it may seem, these are available from various retailers in both left- and right-handed forms. The direction of the twist can easily be determined by examination of the pictures of the two forms of pasta (Figure 1). The light colored right-handed pasta twists in a clockwise direction as you look down the pasta and the darker colored left-handed pasta twists counterclockwise. We have also identified a miniature form of fusilli, called fusillini, and three other forms of chiral pasta: trottole, spiralli (also known as cellentani) and avvoltini all’uovo, all of which appear to be available in the right-handed form only. A simple description of the experiment is as follows. Pasta fusilli are packed into a glass tube and water is allowed to
Journal of Chemical Education • Vol. 79 No. 10 October 2002 • JChemEd.chem.wisc.edu
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Figure 1. Clockwise and counterclockwise pasta fusilli.
flow through the tube. The water leaves the tube twisting in either a clockwise direction or a counterclockwise direction, depending upon whether the fusilli have a left- or righthanded thread. The experimental apparatus is assembled as shown in Figure 2. The tube is 38 cm long, 4.5 cm i.d., and the last 3.5 cm is tapered into a cone with an exit hole of 1.7 cm i.d. Water is delivered into the center of the pasta through a joint at the top with a central 1-cm i.d. delivery tube. By experimentation it was found that the length of the cone at the bottom of the pasta tube should be approximately the length of the pasta and that the exit hole about twice the diameter of standard pasta fusilli (approximately 3-4 cm in length and 0.7 cm diameter). It is important to check that the pasta at the cone end of the tube is packed reasonably uniformly with the pasta being aligned vertically. There are normally 3 or 4 pasta ends visible just above the exit hole. If there are not, invert the tube with your hand over the bottom and then return it to the upright and check the packing again. A good flow of water (between 2 and 3 liters per minute) is required to see the effect clearly and good lighting also helps. Upon beginning the flow observers see that the water enters the tube in a straight column and exits the tube rotating in the direction of the twist in the fusilli (see Figure 3). The flow rate should be varied until the rotation can be seen more clearly. Right-handed pasta twists the water clockwise and lefthanded pasta twists the water counterclockwise. Therefore this mimics the polarimeter with the water being the polarized light and the fusilli being the left- or right-handed molecules. Thus we have named this device the Pastarimeter. This is not a perfect analogy of polarimetry. Those more versed in fluid dynamics than ourselves will have already realized that it is only the pasta in the end cone of the tube that has the effect on the rotation of the water. Indeed, if one packs the end cone with right-handed fusilli and the rest of the tube with left-handed fusilli, the water rotates a clockwise direction, that is, the same direction as the righthanded fusilli. Conclusion Of course we do not know if this is a true replication of the experiment carried out by Sir Robert Robinson. However, we feel that now, 86 years since the experiment was designed, the Pastarimeter can be used to give the general public some insight into how polarimetry works. “Left and Right Are Not the Same”, our chiral display that includes the Pastarimeter, is currently on tour in the UK.4
Figure 2. Experimental apparatus.
Figure 3. Left: Water flowing through counterclockwise fusilli; Right: Water flowing through clockwise fusilli.
Acknowledgments The authors would like to acknowledge the financial support of the Engineering and Physical Sciences Research Council (EPSRC) Partnership for the Public Awareness of Science Grant GR/N18079, Keele University and Optical Activity, Huntingdon, Cambridgeshire UK for their assistance in the purchase of a polarimeter. We thank Mel Cheney for his photography.
JChemEd.chem.wisc.edu • Vol. 79 No. 10 October 2002 • Journal of Chemical Education
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Notes 1. For many pictures of left- and right-handed objects see Rechts oder links (Brunner, H. Wiley-VCH: Weinheim, Germany, 1999). A Web search for “chirality” will yield a large number of hits. 2. For the purpose of this paper we define right-handed molecules as those that rotate polarized light to the right (⫹) and left-handed molecules as those that rotate it to the left (⫺). 3. Packets of fusilli usually contain either the left- or right-
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handed form only. However the authors have found in one supermarket that the “store brand” normal durum fusilli is right-handed and whole-wheat fusilli left-handed. 4. To learn about the “Left and Right Are Not the Same” exhibition and its availability contact
[email protected].
Literature Cited 1. Podlech, J. Cellular and Molecular Life Sciences 2001, 58, 44–60.
Journal of Chemical Education • Vol. 79 No. 10 October 2002 • JChemEd.chem.wisc.edu
