Homochirality: A Big Challenge for the Naturalistic Origin of Life
“On the other hand . . . ”
Whenever we hear that phrase, we know exactly what’s coming next. We are about to hear a second perspective. Just like every debate has two sides so, also, do some molecules. This two-sidedness is especially the case with the building block molecules that comprise the proteins, DNA, and RNA that are the critical molecular components of all living organisms. In particular, any molecule with four different chemical groups bonded to a central carbon atom manifests two distinct three-dimensional configurations akin to our left and right hands (see figure below). Chemists call these mirror image molecular configurations as left-handed and right-handed.
Image: Two Chiral Configurations for a Generic Amino Acid.
Image credit: NASA
Molecules possessing such mirror image configurations are referred to as chiral. An aggregate of chiral molecules where all the molecules display only left-handed configuration or only right-handed configurations are said to be homochiral.
Life Requires Homochirality
Homochirality is a distinguishing characteristic of living organisms. Living organisms possess homochiral amino acids (the building blocks of proteins) and ribose sugars (molecules essential for linking together the nucleobases that comprise DNA and RNA), while inanimate systems do not. All of Earth’s organisms possess only left-handed chiral amino acids and only right-handed ribose sugars.
Functional proteins—which are long, folded chains of amino acids—cannot be assembled unless all the chiral amino acids (19 out of the 20 bioactive amino acids are chiral) either are 100 percent left-handed or 100 percent right-handed. One wrong-handed amino acid incorporated into a protein is enough to disrupt the folding configuration of the protein and, thus, block its capacity to function.
Similarly, DNA and RNA molecules cannot be assembled unless all the ribose sugars are 100 percent left-handed or 100 percent right-handed. For example, two complementary strands of DNA cannot bind together into the life-critical double helix unless all the ribose sugars bonding the nucleobases together possess the same handedness.
These requirements demand that the origin of homochiral amino acids and ribose sugars must precede the origin of proteins, DNA, and RNA. That is, without preexisting large reservoirs of exclusively left-handed amino acids for each of the 19 bioactive amino acids and preexisting large reservoirs of exclusively right-handed ribose sugars, any naturalistic assembly of proteins, DNA, and RNA is ruled out. Without such reservoirs, naturalistic origin-of-life models are prohibited.
Searching for Naturalistic Homochirality Causes
Understandably, nontheistic origin-of-life researchers have devoted decades of research in their efforts to find naturalistic solutions to the homochirality problem. Since no reservoirs of homochiral amino acids or ribose sugars are known to exist outside of living organisms, these researchers first sought to find out if there was any physical or chemical pathway that at least theoretically could produce such reservoirs.
Theoretically, possible physical and chemical pathways do exist. These “symmetry breaking” pathways either induce or take advantage of subtle imbalances between the left-handed and right-handed configured molecules.
Search for Homochirality Causes: Parity Violation
Three of the four fundamental forces of physics—gravity, electromagnetism, and the strong nuclear force—obey strict parity conservation or parity symmetry. The weak nuclear force does not. Only the left-handed components of particles and the right-handed components of antiparticles participate in weak nuclear force interactions. This parity violation in the weak nuclear force can affect chiral molecules in two ways:
- It produces spin-polarized electrons that preferentially ionize one-handedness over the other, resulting, for example, in more right-handed amino acids being destroyed than left-handed amino acids.
- It induces differences in the energies of the two configurations that make, for example, the right-handed ribose sugars more stable than the left-handed ribose sugars.
Calculations showed that the first pathway would generate an enantiomeric excess, or ee, where ee = (R – L)/(R + L) and R and L are the amounts of right- and left-handed molecules, respectively, on the early Earth of at most 10-17 (a quadrillionth of a percent).1
For the second pathway, theoretical calculations showed that the induced energy difference was only 10-30 (one part in a million trillion trillion). For this pathway to have any possible significant effect, Earth, at the time of life’s origin, would need to have had at least 1034 amino acids reacting at the same time!2
Given the extremely tiny impacts that parity violations in the weak nuclear force could possibly have on chiral molecules, it is inconceivable, as several independent researchers have pointed out, that the weak nuclear force could have played any significant role in making the reservoirs of homochiral molecules that life requires.3
Search for Homochirality Causes: Chiral Fields
The other physical pathways for producing reservoirs of homochiral amino acids and ribose sugars are chiral fields. The two such fields that can generate symmetry breaking of chiral molecules are circularly polarized light and unpolarized light in oriented magnetic fields. Natural sources of circularly polarized light are rare in the universe, while unpolarized light in oriented magnetic fields is relatively more common.
Laboratory experiments demonstrate that for unpolarized light, a very strong oriented magnetic field is needed to produce any measurable ee in chiral amino acids or ribose sugars. For a magnetic field strength of 10,000 gauss, an ee = 0.0000001 is the best that can be achieved.4
Today, the only remaining possible candidate for a natural cause of homochirality for the chiral molecules essential for life is circularly polarized light at ultraviolet wavelengths. When chiral molecules are exposed to circularly polarized ultraviolet light (CPUL), unequal absorption of right- and left-CPUL occurs. As the enantiomer that suffers more photoabsorption is destroyed at a higher rate, the concentration of the opposite enantiomer rises.
Obtaining the greatest possible ee requires that the CPUL be monochromatic. The reason why is because of the Kuhn-Condon rule. This rule notes that while one wavelength of CPUL preferentially destroys the left-handed chiral molecules, the adjacent wavelength preferentially destroys the right-handed chiral molecules. Therefore, any broadband source of CPUL will destroy approximately the same number of left-handed and right-handed chiral molecules.
The Kuhn-Condon rule explains, in part, why such small ee’s are observed in laboratory experiments on amino acids. Even with reasonably monochromatic CPUL, physical chemists were only able to induce a maximum of 2.6% ee for the amino acids leucine5 and 4% for the amino acid alanine.6Ang
Even smaller ee’s are detected when physical chemists run laboratory experiments designed to simulate natural sources of CPUL. The only proven natural sources of CPUL are black holes and neutron stars. Other possible sources are supergiant stars and supernovae during their eruption phase. Several laboratory experiments designed to simulate possible CPUL incidents upon cometary ices and interstellar molecular clouds (the richest sources of complex carbonaceous molecules) failed to produce any ee’s distinct from racemic mixtures (mixtures with only small random departures from equal amounts of left- and right-handed chiral molecules).7 One other laboratory experiment where alanine was individually exposed to left- and right-handed CPUL from a synchrotron radiation source produced an ee = 0.65%.8
To get ee’s as high as 0.65–4%, the laboratory experimenters had to expose the amino acids to CPUL wavelengths in the range of 165–182 nanometers (1650–1820 angstroms). They also had to make the CPUL intense.
The intensity and the wavelength of the CPUL poses a problem for any long-term survival of the amino acids. It also stymies the synthesis of many other life-essential building block molecules.
The necessary fine-tuning of the intensity and wavelength of the CPUL that the experimenters needed to obtain their temporary results is unlikely to have an astrophysical analog. Astrophysical sources of CPUL are notoriously unstable. Furthermore, all it takes is one strong burst of ultraviolet and/or X-ray radiation to destroy any conceivable reservoir of amino acids and ribose sugars.
Whether natural reservoirs of significant concentrations of amino acids and ribose sugars even exist is debatable. Astronomers have yet to detect ribose or any of the bioactive amino acids in the only possible astronomical synthesizers of these molecules, namely interstellar molecular clouds.9
Enhancement through Laboratory Autocatalysis and Trapping
Chemists have successfully taken amino acid samples initially exposed to CPUL to generate an ee = 0.1–1.0% and amplified that ee through subjecting the samples to autocatalytic reaction mechanisms in the laboratory. The most productive of these mechanisms are the Frank and Soai asymmetric autocatalytic replication mechanisms. Ee’s as high as 85% have been reported for chiral molecules that are close analogs to bioactive amino acids.10 However, these high ee’s are not stable over time.11 Moreover, to achieve these high ee’s the experimenters inevitably end up destroying almost the entirety of the original sample.
Other laboratory researchers have taken chiral molecule samples (not amino acids) initially in a tiny ee state and amplified that ee through a sophisticated experiment that preferentially trapped one-handedness of the chiral molecules in a solid state while leaving much of the opposing handedness in a liquid solution.12 However, as with the autocatalytic replication experiments, highly specified, highly concentrated pure solutions of the chiral molecules under highly controlled chemical and physical conditions established by elaborate laboratory equipment are needed to get the high ee yields. Furthermore, both sets of experiments required thousands of iterations. Thus, these experiments are far from analogs of anything conceivable in the natural realm. Rather than supporting a naturalistic origin-of-life scenario, they demonstrate that Someone more knowledgeable, more intelligent, and better equipped than the chemists that achieved these results likely is the Originator of life.
The higher the ee’s achieved in the laboratory experiments, the lower the percentage of the original chiral molecule sample that remains. Moreover, the smaller the initial ee, the greater the number of both left-handed and right-handed molecules that are destroyed before any significant ee enhancement can occur. For an example of the first situation, to achieve an ee = 2% for the amino acid leucine, the lab experimenters had to destroy 59–75% of the original sample of leucine.13 All the numbers in the laboratory experiments designed to generate or enhance ee’s are consistent with the conclusion that anything close to 100% ee would leave behind 0% of the original sample. They also are consistent with what is theoretically possible for ee. None of the theories unequivocally predict the 100% chiral purity observed in all living organisms.
Chiral ee in Nature
Chemists have found a few of the bioactive chiral molecules in comets and meteorites, but only at a few parts per million concentration or less and with ee’s for the bioactive amino acids of, at best, a few percent.14 The highest concentrations of bioactive amino acids found are in meteorites where samples have been recovered soon after impact. This outcome is consistent with theoretical work demonstrating that the plasma torch generated by super-high-velocity impacts of meteorites will synthesize a few amino acids.15 As noted already, neither ribose nor any of the bioactive amino acids have yet been found in interstellar molecular clouds.16
Thirteen years ago, I described in chapter 9 of the book I cowrote with Fazale Rana, Origins of Life,17 how the lack of a reasonable natural hypothesis, or even a scenario, to explain an abiotic origin of reservoirs of 100% left-handed bioactive amino acids and 100% right-handed ribose posed an intractable problem for naturalistic/materialistic origin-of-life models. Thirteen years later, the homochirality problem for researchers intent on a naturalistic/materialistic origin of life has become even more intractable. Is it not now well past time to consider a supernatural, super-intelligent Originator of Earth’s life?
Featured image: The giant molecular cloud Sagittarius B2 near the center of our galaxy is where astronomers have found the richest treasure of diverse carbonaceous molecules, more than 135 different species to date.
Featured image credit: European Southern Observatory.
- G. L. J. A. Rikken and E. Raupach, “Enantioselective Magnetochiral Photochemistry,” Nature 405 (June 22, 2000): 934, doi:10.1038/35016043.
- V. A. Avesitov, V. V. Kuz’min, and S. A. Anikin, “Sensitivity of Chemical Chiral Systems to Weak Asymmetric Factors,” Chemical Physics 112 (April 1, 1987): 179–87, doi:10.1016/0301-0104(87)80160-X.
- William A. Bonner, “Parity Violation and the Evolution of Biomolecular Homochirality,” Chirality 12 (February 25, 2000): 114–26, doi:10.1002/(SICI)1520-636X(2000)12:3<114::aid-chir3>3.0.CO;2-N; Pedro Cintas, “Elementary Asymmetry and Biochirality: No Longer Twinned,” ChemPhysChem 2 (July 16, 2001): 409–10, doi:10.1002/1439-7641(20010716)2:7<409::aid-cphc409>3.0.CO;2-B; Ralf Wesendrup et al., “Biomolecular Homochirality and Electroweak Interactions. I. The Yamagata Hypothesis,” Journal of Physical Chemistry A 107 (July 31, 2003): 6668–73, doi:10.1021/jp022568v; S. K. Tokunaga et al., “Probing Weak Force-Induced Parity Violation by High-Resolution Mid-Infrared Molecular Spectroscopy,” Molecular Physics 111 (September 2013): 2363–73, doi:10.1080/00268976.2013.821186.
- Rikken and Raupach, “Enantioselective Magnetochiral Photochemistry,” 932–5.
- Uwe J. Meierhenrich et al., “Asymmetric Vacuum UV Photolysis of the Amino Acid Leucine in the Solid State,” Angewandte Chemie 44 (September 5, 2005): 5630–5634, doi:10.1002/anie.200501311.
- Cornelia Meinert et al., “Anistropy-Guided Enantiomeric Enhancement in Alanine Using Far-UV Circularly Polarized Light,” Origins of Life and Evolution of Biospheres 45 (June 2015): 149–61, doi:10.1007/s11084-015-9413-x.
- Michel Nuevo et al., “Urea, Glycolic Acid, and Glycerol in an Organic Residue Produced by Ultraviolet Irradiation of Interstellar/Pre-Cometary Ice Analogs,” Astrobiology 10 (April 2010): 245–56, doi:10.1089/ast.2009.0358; Michel Nuevo et al., “Nucleobases and Prebiotic Molecules in Organic Residues Produced from the Ultraviolet Photo-Irradiation of Pyrimidine in NH3 and H20+NH3 Ices,” Astrobiology 12 (April 2012): 295–314, doi:10.1089/ast.2011.0726; Pierre de Marcellus et al., “Aldehydes and Sugars from Evolved Precometary Ice Analogs: Importance of Ices in Astrochemical and Prebiotic Evolution,” Proceedings of the National Academy of Sciences USA 112 (January 27, 2015): 965–70, doi:10.1073/pnas.1418602112; Cornelia Meinert et al., “N-(2-Aminoethyl)Glycine and Amino Acids from Interstellar Ice Analogues,” ChemPhysChem 77 (March 2012): 186–91, doi:10.1002/cplu.201100048; G. M. Muñoz Caro et al., “Amino Acids from Ultraviolet Irradiation of Interstellar Ice Analogues,” Nature 416 (March 28, 2002): 403–6, doi:10.1038/416403a.
- Yoshinori Takano et al., “Asymmetric Synthesis of Amino Acid Precursors in Interstellar Complex Organics by Circularly Polarized Light,” Earth and Planetary Science Letters 254 (February 15, 2007): 106–14, doi:10.1016/j.epsl.2006.11.030.
- L. E. Snyder et al, “A Rigorous Attempt to Verify Interstellar Glycine,” Astrophysical Journal 619 (February 1, 2005): 914–30, doi:10.1086/426677.
- Takanori Shibata et al., “Amplification of a Slight Enantiomeric Imbalance in Molecules Based on Asymmetric Autocatalysis: The First Correlation between High Enantiomeric Enrichment in a Chiral Molecule and Circularly Polarized Light,” Journal of the American Chemical Society 120 (November 1, 1998): 12157–58, doi:10.1020/ja980815w.
- Frederic C. Frank, “On Spontaneous Asymmetric Synthesis,” Biochimica et Biophysica Acta 11 (March 12, 1953): 459–63, doi:10.1016/0006-3002(53)90082-1.
- Donna G. Blackmond, “The Origin of Biological Homochirality,” Cold Spring Harbor Perspectives on Biology 2 (May 2010): id. a002147, doi:10.1101/cshperspect.a002147.
- Jose J. Flores, William A. Bonner, and Gail A. Massey, “Asymmetric Photolysis of (RS)-Leucine with Circularly Polarized Ultraviolet Light,” Journal of the American Chemical Society 99 (May 1977): 3622–25, doi:10.1021/ja00453a018.
- Sandra Pizzarello, “The Chemistry of Life’s Origin: A Carbonaceous Meteorite Perspective,” Accounts of Chemical Research 39 (March 1, 2006): 231–37, doi:10.1021/ar050049f.
- Vadim A. Davankov, “Homochirality of Organic Matter—Objective Law or Curious Incident?” Israel Journal of Chemistry 56 (November 2016): 1036–41, doi:10.1002/ijch.201600042.
- Snyder et al., “A Rigorous Attempt.”
- Fazale Rana and Hugh Ross, Origins of Life: Biblical and Evolutionary Models Face Off (Colorado Springs: NavPress, 2004): 123–133. The reprinted edition (RTB Press, 2014) includes a web link to article and podcast updates.