That something is not identical to its mirror image is a property known as chirality. Hands (etymologically the word chirality is derived from the Greek word for hand), spiral galaxies, and the DNA helix are all examples of chiral objects. In particle physics chirality is a more abstract notion defined by transformations of a particle with respect to a left right of left handed representation in the Poincaré group. In chemistry chirality is well described by analogy to your hands wherein left and right hands cannot be superposed on each other even though the fingers are the same and match up.
This article is an exploration of chirality in biochemistry. I want to ask what makes life chiral, why is life chiral, and how did life become chiral. In order to supplement my limited knowledge of the subject I interviewed a world expert and author of over twenty papers on the subject, Robert Compton, who I must give a deep thanks to for being willing to answer my silly questions.
spiral tree trunks, the human form, and sea shells (which generally only coil in one particular direction according to species). The remarkable thing is that this macro asymmetry can be traced back to a micro asymmetry in the chemistry of life. The arrangement of atoms in a molecule defines the function of that molecule, but even molecules with the same chemical configuration can behave differently as in the case of chiral molecules which are like mirror images of the same molecule that come in 'left' and 'right' handed forms. The great asymmetry of life is that all living organisms on Earth almost exclusively utilize the left handed (or levorotatory) configuration for amino acids and the right handed (or dextrorotatory) configuration for sugars belonging to DNA or RNA.
Perhaps it is trivial or obvious that life is chiral when looking at the nautilus, but this obvious chirality is a macroscopic feature which belies the fine arrangement of atoms which defines the chirality of biomolecules.
Different structural forms of compounds with the same molecular formula are known as isomers to chemists. A stereoisomer is an isomeric molecule which has the identical constitution and sequence of bonded atoms, but has a different three-dimensional geometry in space. An enantiomer is one specific steroisomer of the two possible mirror images that are non-superposable. The dominance of the left handed chiral enantiomer in biology is a massive blow to the idea that nature is perfectly symmetric and is an unsolved mystery as to why nature is this way.
Many molecules are chiral, however because molecules are constantly vibrating the instantaneous structure of a molecule may lack the exact structure or symmetry seen in an ideal model. Regardless, enantiomers have identical chemical properties except when they react with other molecules which are also enantiomeric in which case chiral forces yield a difference in behavior. Further, and perhaps more important for biology, particularly astrobiology, is that enantiomers have identical physical properties except with respect to the way they interact with plane-polarized or circularly polarized light or other chiral compounds. A pure enantiomer compound will rotate the plane of a monochromatic plane-polarized light by a certain angle in one direction, say clockwise, while the other enantiomer form of the compound will rotate the light by an equal amount in the opposite direction. Things that rotate light are said to be optically active. Measurements with a polarimeter allow chemists to determine if a compound is chiral or not. Polarization of light by organic compounds was discovered in 1815 by the French physicist and chemist Jean-Baptiste Biot. He found that organically produced chemical solutions consistently rotated plane polarized light, but laboratory synthesized chemicals did not reproduce the rotation. Beyond conjectures he had no explanation for the phenomena.Years later Louis Pasteur preformed a similar experiment with tartaric acid produced from grapes and tartaric acid synthesized in the lab. Pasteur went further and somehow used tweezers and a microscope (I do not conceive to understand how) to separate the tartaric acid crystals which he produced in the laboratory into piles of left and right handed molecules. He found that polarized light was rotated by the left handed molecules that he had selected in the same way the polarized light was rotated by the organic tartaric acid. He concluded that chiral molecules are responsible for the rotation of polarized light.
So chiral molecules rotate light, but actually so does an individual achiral molecule! In an ensemble of achiral molecules each individual molecule may rotate the plane of the polarization, but the net rotation averaged over the ensemble will result in zero rotation. A mixture of two enantiomers in a 1:1 ratio (which is what you get when you create chemicals in the lab) is optically inactive because the rotation results in a zero net polarization rotation. When a reagent or catalyst is optically active the chiral product will also be optically active, or in the presence of chiral forces such as circularly polarized light this may also induce optical activity via enantimoeric excess in the products as well. Generally you can get optically active compounds in two ways 1) The reagent in already optically active. 2) The reaction of achiral but optically inactive precursors in a chiral optically active environment occurs. It takes an optically active molecule or chiral force to produce a product that is optically active.
Most chemical reactions are not enantiomerically selective so that the initial reason for a completely homochiral biology on Earth remains a mystery just as when Biot and Pasteur discovered chirality through optical activity. Of course chirality is simply geometric in nature and thus this geometric asymmetry is what makes life chiral. Any molecule that contains a tetrahedral carbon or other central atom bonded to four different atoms or constituents will exist in enantiomeric forms; given that all biological molecules are at least this complicated, then (almost?) all biological molecules exist in enantiomeric forms. It may be that the chirality of biomolcules is simply a consequence of the emergent complexity of basic physics. The conditions necessary for a solution initially containing near equal number of chiral forms to evolve towards pure chirality has been explored (see Frederick Frank 1953) and is plausible. A tiny initial imbalance has spiraled out of control and now each successive generation of biomolecules on Earth is produced by the previous generation of chiral reagents, thus this is the why life is chiral. None of this explains how life is chiral, but a common answer is that because life can be chiral it is chiral.
From this persepctive this topic is not so interesting, honestly. I have come to the conclusion that chriality is as it must be given that each generation of life is spawned from the previous generation under conditions which do have enantiomeric selection forces present. The question is why was left handed chirality chosen for life on our Earth?
Now actually, the chirality of biomolecules is not just a philosophical diversion; it a serious issue of biochemistry in balance. There are over 530 synthetic chiral drugs worldwide today. It is technically and economically prohibitive to make enantiomerically pure drugs in all cases. This results in drugs that may have strange, null, or fatal interactions with human subjects. In some cases the difference between two enantiomeric forms is simple, as in the case of the olfactory exciting chemical carvone; in one configuration it smells like spearmint and the other configuration like caraway seeds. So, "Perhaps," as Alice said to her cat in Lewis Carroll's Though the Looking Glass, "looking-glass milk isn't good to drink".
I digress. Before we hear what Dr. Compton has to say on this matter lets look at what fundamental physics would bias the chirality of life on Earth and how homochirality was selected on Earth. There are three distinct mechanisms that seem plausible.
- The weak force. Of the fundamental forces, nuclear, electroweak and gravitational, only the weak force can distinguish between left and right parity particles. The weak force it turns out does not conserve parity (although it does conserve CPT symmetry) during some interactions such as the radioactive beta decay corresponding to the emission of an electron with intrinsic spin 1/2 hbar. Also, the weak force induces a parity-violating energy difference, PVED, between molecules or the interactions of left-handed electrons emitted during beta decay with molecules. So the weak force could preselect a handedness in nature through either beta decay or PVED. The idea is that if one chiral configuration is a lower energy state then nature will prefer that configuration (in fact the exact scaling from thermodynamics is that the reaction rate for the oppositely chiral molecules is proportional to the canonical partition function in physics going as e-PVED/kT where e is the Euler's number, k is Boltzmann's constant, T is the temperature in Kelvin). The difference in energy from the PVED can be theoretically calculated from a Hamiltonian operator that is scaled by the Fermi electroweak coupling constant. The energy difference between chiral configurations is predicted to be small, around 10-14 Joules per mol which means that not only is this energy difference believed to be minimally important to early life's synthesis, but it is also out of reach of current experimental techniques. However, it turns out that the PVED predicts that the left handed chiral states would be of lower energy, just as they are found dominantly in life on Earth. The weakforce influences chemical reactions because during beta decay, spin polarized electrons produce a an abundance of left-circularly polarized gamma-rays which, if present during the synthesis of biomolecules would tend to create an enantiomeric excess of left handed molecules. However, laboratory experiments have not shown conclusively that this effect is strong enough to matter either. There is some debate as to the exact nature of the PVED which will depend on further experimental measurements. The weakforce seems to preselect a hand in nature, but it is a feeble force.
- Polarization. Optically active organic molecules being synthesized in the presence of polarized light will be chiral. Sunlight is slightly polarized just before sunrise and after sunset. This averages out to zero, but chemical activity that dominates in the morning/evening or occurs in the presence of shadows could feel a net polarization effect. Also plausible are astronomical sources of polarized light outside the solar system. Supernovae have been known to emit circularly polarized light as have star forming or nebulae regions. These sources while weak would create conditions necessary for the synthesis of homochiral biomolecules.
- Vorticity. A chemical solution being stirred or agitated results in the synthesis of homochiral biomolecules, however, the handedness of the chirality is random. Certaintly there were chaotic turbulent conditions on the early Earth.
There is no conclusion to be drawn as to how life became homochiral. I found more questions than answers. So I asked Robert Compton for some answers.
1) Enantiomers are allotropes then?
Interesting question but allotropes are "two or more existing forms of an element" such as graphite, diamond and fullerenes. There are a number of fullerenes which are chiral, e.g. C84. The C84 molecule has an R and and S enantiomer as well as the meso- form (the meso is a combination of the R- and S- form). This is illustrated below. One goes from the left to the right by rotating two of the carbons twice and reconnecting the dots going through the achiral meso-form.
2) Life on earth is overwhelmingly 'left-handed' homochiral. Is this merely a coincidence of history? Do you think a fundamental effect like PVED or an environmental effect like polarized light from a local supernova or nebula was responsible for primordial chirality on Earth?
This is the 64 million dollar question. Did life end up as “left-handed” (or better L-amino acids) due to some fundamental bias or was it pure chance and there may be a planet out there that is made up of D-amino acids. I have discussed the possibilites in my review article on “Chirality of Biomolecules” but to summarize it may be due to 1) the influence of circularly-polarized light on biomolecules giving rise to life over long periods of time, or 2)the chiral weak force making one enantiomer be lower in energy than the other ( this is certainty true but the effect is really, really small) or 3) chiral beta – rays interacting with matter to produce handed biomolecules. At the present time I favor the interaction of circularly polarized light on molecules in interstellar space (maybe chiral microwaves). Meteors then bring these molecules to Earth to begin life forms.
I really feel that PVED is too small. My bet is on circularly polarized light from a local supernova or nebula.3) Subsequent proteins formed in the presence of a particular enantiomer protein will be preferentially homochial to that primordial protein. Assuming 'the spark of life' only occurred once with a particular enantiomer protein present, would you agree that it isn't surprising that life is homochiral? Is this assumption realistic?
Homochirality is understandable. Everyone agrees that life molecules are preordained to be homochiral. But there is no reason that they will be of only one handedness, of a specific chirality. That’s the rub. It has always been a big surprise to me that ALL life is made of L-amino acids ( and chiral sugars in the backbone of the DNA).4) Imagine we went to planet X and found the same flora and fauna as Earth, but planet X's organisms were dominated by the opposite chiral form as that found on Earth. Could we mate with or consume nutrients from creatures on this planet?
No. I don’t think I would like to try this.5) It is obvious in biology that chirality is a function of emergent complexity in the system. In physics chirality is often considered to be a function of the deepest laws of nature. Could chirality in physics also be a function of emergent complexity from unseen laws of nature?
I believe the current view is that the Universe began as both matter and anti-matter but bifurcated into matter due to some fundamental reason. Thus the Weak force gives rise to left spinning beta particles. There are a lot of scientists trying to understand how the Universe ended up as matter. This may be the “emergent complexity from unseen laws of nature” that you asked about.6) In biology and chemistry symmetry breaking on macro scales is understood through chiral selection processes which act on micro scales. That symmetry breaking on micro scales may be traced back to an asymmetry in fundamental physics. Is the universe fundamentally asymmetric?
I think this is ture. You may have seen the book “The Left Hand of Creation” by Barrow and Silk or “The New Ambidextrous Univesre” by Martin Gardner. They touch on this subject. Gardner is fun reading and can be (should be) read by high school students.
Robert N. Compton, Richard M. Pagni, & Volume 48, 2002, Pages 219-261 (2002). The chirality of biomolecules Advances In Atomic, Molecular, and Optical Physics, 48, 219-261