Exploring a world with chiral biochemistry: chirality detection

Question

Space explorers have landed on an Earth like planet and, to their surprise, the place is luscious and covered in life, to the point of resembling some earthly tropical atoll.

What they don't know yet is that the entire biochemistry of the planet they are exploring has opposite chirality with respect to theirs.

Once you have a polarimeter, measuring the chirality of certain molecules is a rather easy task.

Is there any way for them to detect or at least suspect that something is odd with the local chirality, before they measure samples with a polarimeter?

To be clear, I prefer something empirical. For example to observe if a substance is fatty, one can simply spread it on a surface and observe how water interacts with it, without needing any more complex instrument.

Among the exploring crew there is a biologist and a chemist.

Answer 1

This is tricky because to some degree it requires the crew to expect the local biochemistry to present one way and know to be confused when it doesn't.

An alien landing in an orange grove wouldn't know what to expect oranges to smell like, for example, let alone wonder why they don't smell like lemons. The molecule responsible for the smell of oranges and lemons is chiral; limonene is R-enantiomer in oranges and S-enantiomer in lemons.

Likewise there are pharmaceuticals (90% of pharmaceuticals are chiral) in which one arrangement is beneficial and the other toxic:

Citalopram, an antidepressant, is produced as a racemic mixture, but only the (S)-(+) enantiomer is responsible for its beneficial effects. The drug d-penicillamine can treat rheumatoid arthritis, but l-penicillamine is toxic.

We couldn't expect either reaction when taking an unlabelled pill, let alone suspect chirality is behind the results. Similarly your explorers wouldn't know what to expect of the local organic biochemistry.

There are ways to observe chirality at the macroscopic level without a polarimeter:

The chiral signals of the d- or l-Trp residues are seeded into poly(acrylamide) (pAAm)-based hydrogels (d- or l-Trp(x)-gels), in which the chirality is successfully transferred and amplified to the macroscopic scale. The interactions between the pAAm-gel bearing βCD moieties (βCD-gel) and the d-Trp(x)-gel and L-Trp(x)-gel are investigated in aqueous solutions with different components. Under appropriate conditions, such as in aqueous NaCl, the βCD-gel can successfully discriminate the d-Trp(x)-gel from l-Trp(x)-gel on a macroscopic scale by amplifying enantioselective host–guest recognition through interfacial multisite interactions.

But again your explorers would already have to suspect chirality to perform this test or one like it.

Answer 2

Introduction

With my answer I wanted to go into a directly observable clue if you know the physics behind circular polarised light and bragg reflectors. However while you can make this to work for expecting chirality, I haven't found a way to directly suspect different chirality without using an external light source. So I have added two answer, one regarding alien ants and a second extensive one giving my thought on how you might be able to get a visual distinctive clue about strange chirality.

Answer 1 (Alien Ants Behaving Strangely)

So your explorers are enjoying lunch and a cup of coffee during their lunch break whilst exploring. But even on alien worlds you can't have a picnic without ants (or whatever passes for ants on this world) showing up and trying to grab some food. However these ants only seem interested in the artifical sugar (Tagtose) used by one of your explorers instead of the normal crystaline sugar. It would seem quite weird after a while a small stream of insects carrying away your artificial sugar just past your sugar cubes.

The reason is that (as pointed out in the comments) chirality can matter a lot in nature and affect smell, taste, toxicity and other things. The insects pick up on this nature and since cyrstaline sugar is left oriented (most often combination of D-glucose and D-fructose I think) while the artifical sweetner is right oriented. Disclamair, I am physicist not a chemist or food scientist so I can have it completely wrong but I just gave two examples but there are many more to choose from. Some lactose are right oriented and most sugar have a right oriented enantiomer. I think fruit sugar (levilose) is one, however here I am not certain if the molecular chirality and the optical rotation properties match. Depending on your story and how strict quarantine rules are you can of course setup a single experiment to see how the local wildlife interacts with your food and come to the same conclusion without taking off your hazmat suit in an alien environment.

Answer 2 (Circular Polarized Structural Color)

The big polarimeter

So instead of the molecular chirality you want to measure the optical rotational polarity and get out your big polarimeter. It might turn out not to be so big, Ultrathin, flat lens resolves chirality and color. So with this lens you can easily image the chirality of the exoskeleton of a beetle as shown in the paper.

Fig 1: Chirality of the exoskeleton of a beetle. Image source: M. Khorasaninejad et al.: Multispectral Chiral Imaging with a Metalens, Nano Lett., online 07 June 2016; DOI: 10.1021/acs.nanolett.6b01897

Detecting "Opposite" Chirality

With a little bit of tuning your world you might be able to check for chirality and even suspect that it is opposite from what you would expect. I will assume here that the rotational polarization direction and chirality are one and the same.

So as seen with the beetle a bunch of insects have chirality based on structural optics. One of the most beautiful and easily observed examples is with butterfly wings shown in Fig 2a, where the the right wing was saturated with alcohol. Fig 2b and 2c show the TEM cross-section of the wings.

Fig 2. Structural color in a butterfly. Image Source: M. Sc. Christoph Fenzl, et al.:Photonic Crystals for Chemical Sensing and Biosensing; DOI:10.1002/anie.201307828

The color change in this case is based on an expansion of the structure of the wing which influence the reflection of light due to a change in layer thickness of a Bragg reflector. A Bragg reflector, reflects or transmits light based on the interference pattern created by the controlled layer thickness, see:Dielectric mirror and Fiber Bragg grating.

Similar in nature polarization of light is used, again an example of this in butterflies is swhon in Fig 3a and 3b, where the butterfly in 3a does not exhibit polarization and the butterfly in 3b does. The blue color denotes no specific polarization direction while green and red denote a specific polarization.

Fig 3: Two butterfly species. Image Source: Jonathan M. Douglas, et al.; Light habitats and the role of polarized iridescence in the sensory ecology of neotropical nymphalid butterflies (Lepidoptera: Nymphalidae), DOI:10.1242/jeb.02713

So combining circular polarization with a bragg reflector would allow for species to change the color of their wings if they can expand the lattice strucutre (or if somebody scientist pours alcohol on it, (ohh the ethics of those scientist)). That light can further more be circular polarized and in this case due to the chirality of the creatures make up, it would have right-handed circular polarization as opposed to the expected left-handed circular polarization one would expect.

How to detect the different circular polarization direction

So it is easy to see structural color (it looks colorfull) and to determine it is indeed structural by pouring alcohol over it. It is easy to detect linear polarization, just look through your polaroid sunglasses, turn them 90 degrees and look again. If the polarization axis are aligned with your glasses the butterfly wings should turn from looking normal to completely black. If you look at circular polarized light through linear polaroid sunglasses the intensity would simple half irrelevant of direction, making it distinguishable from linear polarized light but not from un-polarized light (which would also half). This is where some story telling should come in.

Either for some reason you have circular polarized sunglasses of which you know the polarization direction. Knowing or deducing the polarization direction should not be hard if you know the material of which it is made. Or you have a device which emits circular polarized light. This second option might not be as far fetched as you think. You will have devices with displays emitting light, if you have use circular polarized LCD screens instead of the old fashion mostly currently used linear polarized LCD screens (example of lineary polarized screen through polaroid sunglasses) you have a circular light emitter. A butterfly landing on the screen will suddenly become black if the polarization direction is different, if polarization is in the structural color part of the wings it might be arranged such that it will suddenly change color. The scientist trying to explain the darkening and color shift of this beautiful butterfly that has landed on their laptop might let them think about the polarization direction and thus the chirality. Presumably the direction of the color shift and knowing the circular polarization direction of the screen might lead you to suspect a different polarization orrientation than you would have expected.

Additional stuff

Sorry for the long winded answer but I wanted to explain an at least feasible optical solution. Here some extra material if you really want a rigorous physics calculation:

• With the Law of Malus you can calculate the intensity through a single polarizer.
• You can calculate stacked polarized transmission with Jones Calculus to see how circular polarized butterfly wings would behave with certain polarizers.
• With gratings you can calculate color divergence.