Physics, Earth & Space
Homochirality: The Truth Is Out Where?
When it comes to origin of life scenarios, the only way to explain the emergence of life from purely naturalistic premises is that life, and all of its necessary components, arose by chance, or by natural laws or by some combination of both.
Whether life arose in a “warm little pond” or in heat vents or in ice crystals, origin of life scenarios must rely only on chance and natural laws (chemical and physical properties) to explain seemingly impossible phenomenon, such as how DNA first arose.
Right-handed amino acids are found in nature, but for whatever reason proteins only employ left-handed amino acids. Laboratory synthesis of amino acids produces a 50/50 mixture, as is expected with chiral molecules, and both are chemically equivalent. Obviously this cannot be attributed to chance any more so than someone flipping a coin 100 times and getting all heads can. So from naturalistic premises, the only way to solve this is to find some reason why the chemistry favored a particular orientation of biomolecules.
One piece of the origin of life puzzle is that biological molecules have a particular chemical orientation. Biological molecules are composed of carbon, and carbon can form four bonds. When there are different things coming off of those four bonds, it makes a difference where those bonds are with respect to each other. For example, when you look at your hands, your thumb could come off of the right side of the hand or the left side of the hand. The orientation of your thumb and fingers relative to each other makes a difference. Your hands are not superimposable (the same); they are mirror images. Carbon bonds have a similar characteristic. When a carbon-containing molecule has a bond structure around a carbon such that if the bonds were arranged differently the molecules would not be superimposible, it is said to be a chiral carbon or a chiral molecule (from the Greek for “hand”). An interesting thing about chirality is that it does not change the chemical nature of the carbon bonds. So if you have a reaction that ends up making a chiral carbon, you will probably get a 50/50 mixture of left- and right-handed molecules.
All of the amino acids, except glycine, have a chiral carbon. Here is the rub: Amino acids that are from naturally occurring proteins are all left-handed.
The two pervading naturalistic theories on this are: 1) Evolution selected left-handed biomolecules or 2) they were formed in outer space. The exact mechanism for these two theories is the fuel for research and publications. Recently, the Astrophysical Journal published a paper that studies ultra-violet circularly polarized light because this is one of the few ways that can produce a particular handedness (e.g. more right-handed molecules than left-handed molecules) in organic molecules (The Astrophysical Journal Letters, 727:L27, February 2011). Since the mirror images of chiral molecules are called enantiomers, a mixture that has more than 50% of one enantiomer over is another is said to have an enantiomeric excess (e.e.).
Some primitive meteorites contain a small enantiomeric excess of certain chiral amino acids, leading many people to believe that perhaps the origin of homochirality is found in outer space. The authors of the Astrophysical Journal paper sought to investigate what kind of cosmic phenomenon would cause this. One possibility is UV-circularly polarized light (UV-CPL) acting on cosmic “ices” or chunks of cold rock that eventually become meteors. “…we tested such a scenario in a laboratory simulation using UV-CPL to drive the photochemistry of cosmic ice analogs under plausible astrophysical conditions and search for the generation of chiral species with significant e.e.’s…”
They decided to compare their experiments to the Murchison Meteorite, which landed in Australia on September 28, 1969 (Garrett & Grisham Biochemistry 2nd Ed, Harcourt, Inc, 1999). This meteorite showed an enantiomeric excess of several amino acids of 2-9%. The authors looked at alanine, which had an enantiomeric excess of 1.2% on the Murchison Meteorite.
For the experiment, the authors selected a particular photon energy that they knew was close to the maximum needed for alpha hydrogenated amino acids, and directed those photons to a synthesized ice mixture that is theoretically similar to cosmic ice mixtures. To simulate interstellar conditions, they used ice mixtures of the composition H2O:CH3OH:NH3 (2:1:1).
This experiment resulted in a 1.34% e.e. of left-handed alanine, close to the 1.2% reported for the Murchison Meteorite. Other amino acids of interest were too dilute to take meaningful measurements. Usually with these syntheses the smallest amino acids are formed in greater amounts. Alanine is the smallest chiral amino acid. The authors’ assumptions are first, amino acids formed in outer space under similar conditions to what they have reconstructed in the lab, and secondly, that these molecules were exposed to UV-CPL under the appropriate conditions to cause an enantiomeric excess of left-handed amino acids.
A couple of experimental procedural notes:
First, to get the laboratory experiment to work, the UV-CPL radiation temperature was increased to higher levels than what is thought to be in outer space, but the authors explain that this is still reasonable since the meteor will likely go through various temperatures: “The use of a higher temperature (80 K), rather than observed temperatures for interstellar ices (10-20 K), was decided to enhance diffusivity and recombination of photoproducts within the ices…However, since the complete cycle of inter and circumstellar grain evolution comprises cycles through warmer regions (hot cores) in which grain temperatures rise significantly (200 K), astronomical organic residues should be produced via pathways similar to those in our laboratory simulations.” The authors justify this by referencing experimental studies that showed irradiation temperature (10 or 80 K) does not greatly influence product composition.
Second, they chose to irradiate for 10 hr with the molecules at room temperature (this would be about 20 C or 293 K) “to potentially favor enantioselective photoreactions based on a photon-molecule asymmetry transfer…” In other words, they shot photons at the compounds for a particular amount of time at a particular temperature that they knew would favor the reactions they were pursuing. The authors still contend that this remains astrophysically relevant particularly in “hot molecular core environments, where grains are heated and complex gas-phase molecules, thermally desorbed from the ices, are observed…”
While the authors may have a point that there are hot cores in outer space, they are adding an additional factor by assuming that the meteor must apparently pass through a hot core for a period of time and under certain conditions. At this point, there has been quite a bit of experimenter intervention for what they consider a “natural” process.
A 1.34% enantiomeric excess is significant enough to measure, but does this experiment solve the mystery behind homochirality? Let’s consider what is most reasonable and probable. Obviously scientists are looking for an explanation for homochirality because the chances of nature accidentally selecting left-handed amino acids in a pool of a 50/50 mix are impossible. So, instead, the possibility presented here supposes that a meteor somehow went through a heat core and was exposed, apparently within the heat core, to UV-CPL with particular helical properties ended up with some amino acids with a slight e.e. on it. It hit the earth, and happened to hit in such a way that the amino acids were not destroyed upon entry, which is possible given the Murchison Meteorite, but then landed in a place conducive for reactions to take place. Furthermore, the authors mention that other amino acids on the Murchison Meteorite show a greater e.e for molecules such as isovalene “that cannot be explained solely by the asymmetric effect of UV-CPL,” meaning that the meteor had to have gone through yet another process to produce the enantiomeric excess of other amino acids. Then those reactions apparently beginning with anywhere from 1.2 to 9% e.e. of left-handed amino acids ended up producing 100% e.e. of the chiral amino acids, which is still inexplicable. How is this any more probable than nature just happening to find the one place where a 50/50 racemic mixture was not made and chemistry did not behave as it normally would?
This paper is interesting because it may account for the 1.2% e.e. of L-alanine in the Murchison Meteorite, although even this is questionable since their procedure does not account for the other amino acids on the meteorite. However, it does not offer much by way of explaining this very difficult origin of life puzzle. The final section of the paper speculates on where UV-CPL sources of the particular helical value necessary for hydrolyzed amino acids might be found. They suggest this type of UV-CPL was “most probably produced by dichroic scattering [ref removed] on aligned grains by a magnetic field in reflection nebulae close to regions containing massive stars.” They speculate that this might be similar to where the Sun was formed. By continuing to add specific conditions upon specific conditions, the likelihood that a meteor brought left-handed amino acids to earth which lead to the subsequent beginning of life is quickly diminishing.