circular polarisation in beetle shells

Is nature even handed?

Many coffee mugs have an aspect of handedness to them: they have one handle and we tend to pick them up with our dominant hand. During zoom meetings, this point has recently been emphasised to me because of the design of some Ritzenhoff mugs. Picking up the mug with one hand, the image shown on the screen is quite different to picking up the mug with the other. Overall, about 90% of us are right handed with 10% being left handed (though it becomes a little bit more complicated than this). And while we can also have mugs with no handles and which don’t have any difference when picked up with the left or the right hand, this reflection on handedness in mugs could prompt a question, what about the coffee inside it, does it have directionality or “handedness” to it?

These Ritzenhoff mugs each have a different character, but they also each show a handedness. The face would appear to the drinker if picked up with the right hand or to the viewer if picked up with the left.

This is in fact not an unreasonable question as it is about how light interacts with the coffee. But to see how the coffee could show a handedness, it is worth a brief diversion into the nature of light. Light consists of an oscillating electric (and magnetic) field which oscillates perpendicularly to the direction that the light is travelling in. Apart from the fact that the oscillations are perpendicular to the direction of travel, these oscillations can be in random directions, a situation in which we would say the light is ‘unpolarised’. If however the electric field oscillates in one direction only, the light is said to be polarised. We can find out how the light is polarised by using a pair of polarised sunglasses or a piece of polaroid and rotating it to see how the intensity of the light changes as the polaroid is (or sunglasses are) rotated.

We encounter polarised light all of the time, although we may not necessarily realise it. The reflections of light off the surface of a cup of coffee are partially polarised, and if viewed above a certain angle, known as the Brewster angle, the polarisation is completely in the plane of the reflection. The same is true of reflections generally, while the light scattering caused by the effect that makes the sky appear blue also polarises the (otherwise unpolarised) sunlight. Perhaps for reasons such as these, Sir Lawrence Bragg in one of his lectures to the Royal Institution said “I’ve always found it useful to carry round a piece of polaroid with me”. A life lesson that I fully intend to take on board.

When light is reflected from a surface, including from the surface of a cup of coffee, the reflected light is partially polarised.

This so called linear polarisation is only one type of polarisation however. If you imagine viewing the electric field of the light head-on coming towards you, it could also rotate rather like a corkscrew. And just like a corkscrew, it could either rotate clockwise or anticlockwise; this is circularly polarised light. When light interacts with, or reflects from some chemicals, it can turn from being unpolarised to left or right circularly polarised. We’d say that it has chirality or ‘handedness’, and it is this effect that we are asking about in coffee. One fantastic example of a surface that reflects (mostly left) circularly polarised light is the shells of certain beetles in the Lomaptera and Hybosoridae families. Here, the brilliantly shimmering colouring of these green and occasionally other coloured beetles is entirely structural, meaning that there is no pigmentation on the shell, the colour is caused by how the light interacts with the (colourless) layers of the shell. In the case of the beetles it is because the shell is made from layers of strongly linearly orientated chitin molecules. Because the beetle shell is composed of many layers each twisted slightly from the one beneath it, the light ends up interacting with a corkscrew type reflecting surface that gives the reflection a left circular polarisation.

While this is a cool effect in beetle shells, the consequences of this handedness in nature can be catastrophic. Some molecules have an intrinsic ‘handedness’ to them, so although two molecules have the same chemical composition, they are the mirror reflection of each other and so not identical. It is like the cartoon molecule in the image below. Both ‘molecules’ contain the same number of coloured circles but their positioning means the molecule on the right is not the same as the one on the left. In some cases, these molecules will interact with light differently, one will polarise the light with a left circular polarisation and the other a right circular polarisation. As the molecules are chemically identical but do not map onto each other (they have ‘handedness’) they are called enantiomers. Years ago I had a summer job at Pfizer in Sandwich, Kent, UK, analysing various candidate drugs to check both that they were chemically pure and that they were what they were thought to be. One of the tests that I had to do repeatedly was polarimetry which measures the optical activity of the molecules in a sample. In short, this measures whether the chemical in the sample shows a handedness and if so, how much. It may at first sight seem not to make too much of a difference, after all the molecules are chemically the same. However it makes a large difference, not just to the way that light interacts with the molecules, but to the way that our bodies do too.

If you imagine each of these circles as representing different atoms, these two molecules are not quite the same. Though they are the same compositionally, one is the mirror image of the other, they are enantiomers.

In the late 1950s and the early 1960s, the drug thalidomide was prescribed for, among other things morning sickness. Thalidomide is an example of a drug in which there are two enantiomers which, ordinarily exist in equal amounts. The problem was that one of these enantiomers (the s-enantiomer) was teratogenic which means that it caused birth defects in forming embryos. It was suggested in the 1970s that if the r-enantiomer of thalidomide had been isolated from the mix and given without any s-enantiomer present, the birth defects could have been avoided. While this conclusion has since been questioned, nonetheless, now all drugs are tested to ensure that this problem can never happen again, and part of that test involves looking at the optical handedness of the drug sample with a polarimeter.

What does this mean for coffee? Does coffee contain any handedness? The chemistry of coffee is complex, with up to 900 volatile aromatic compounds and then further chemicals dissolved within the brew. We can get an answer to the question though by just looking at some of the main compounds in coffee: caffeine, the various thiols that create the aroma and substances such as caffeic acid that contribute to the flavour. Caffeine itself has no chiral centre, meaning it is even handed however the same is not true of the thiols nor necessarily the acids, both of which can contain some degree of chirality or handedness. For the case of the aromatic thiols, this may even be important as we do not seem to sense the two types of molecule in the same way. Handedness matters. Some researchers have even looked at how roasting affects the amount of different enantiomers in robusta and arabica coffee. All of which shows that, just as our own coffee mugs reflect our handedness in zoom calls, so too the coffee has a handedness when it interacts with light.

Now who thought that coffee was balanced?