Natural Source of Life’s Homochiral Molecules?

When we hear someone say “on the other hand,” we know we are about to hear an alternative perspective. Every debate has two sides, and so, too, do the building block molecules of life. Any molecule with four different chemical groups attached to a central carbon atom manifests two distinct three-dimensional configurations. Chemists refer to these mirror image configurations as left-handed and right-handed (see figure 1). They identify such molecules as chiral molecules.

Figure 1: Two Chiral Configurations for a Generic Amino Acid.
Image credit: NASA

The essential building blocks of life are proteins, DNA, and RNA. Proteins are made up of long chains of amino acids. DNA and RNA are made up of long chains of nucleobases that are linked together by pentose sugars. Proteins cannot assemble unless all the chiral amino acids (21 out of the 22 proteinogenic amino acids are chiral) are either 100 percent left-handed or 100 percent right-handed. Likewise, DNA and RNA molecules cannot assemble unless all the pentose sugars are 100 percent left-handed or right-handed. All organisms on Earth manifest only left-handed chiral amino acids and right-handed pentose sugars, what chemists call homochirality.

Homochirality Challenge
Homochirality presents a challenge to anyone attempting to explain life’s origin by purely natural mechanisms. Explaining how homochirality arises from a racemic (random) mixture of left-handed and right-handed chiral molecules is the simplest problem for naturalistic models of life’s origin—but it is a major problem. Laboratory demonstrations of many of life’s essential chemical pathways¹ are irrelevant to solving the origin-of-life problem unless a naturalistic means for generating homochiral amino acids and pentose sugars can be proven. As I documented in our book, Origins of Life, no possible terrestrial pathways exist for producing the homochiral molecules required for life’s emergence.² Organic chemist William Bonner agrees:

“I spent 25 years looking for terrestrial mechanisms for homochirality and trying to investigate them and didn’t find any supporting evidence. Terrestrial explanations are impotent or nonviable.”³

Because of this dead end, for the last 20 years the search for a naturalistic origin of homochiral molecules has focused on possible astronomical sources.

Looking to the Stars
The best developed, and until just a few years ago, the only viable astronomical model for suggesting how a racemic mixture of amino acids could be pushed toward one exhibiting at least a slight preference for left-handed amino acids, has been the Circularly Polarized Light (CPL) model. In this model, ultraviolet CPL produced by scattering light from an extremely hot star to polarize it falls on an aggregate of amino acids and preferentially destroys right-handed amino acids more efficiently than it destroys left-handed amino acids.

A Japanese research team used a laboratory cyclotron to simulate the kind of ultraviolet CPL arising from the most intense emitters from known neutron stars and black holes. They reported generating up to a 0.65% enantiomeric excess ([NL-NR]/[NL+NR] where NL and NR are the number of left-handed and right-handed amino acids in an ensemble) for the bioactive amino acid alanine.⁴ A team of French researchers reported achieving up to a 1.34% enantiomeric excess for alanine using right ultraviolet CPL.⁵

The reported 0.65% and 1.34% enantiomeric excesses match what geophysicists have found in the few meteorites that do possess amino acids (once probable contamination from terrestrial life is accounted for). These meteorites, however, never show a preference for right-handed amino acids. In the few instances where a slight preference is indicated it is always for left-handed amino acids.

This consistent preference for left-handed amino acids runs counter to the Kuhn-Condon rule. Werner Kuhn observed in 1930 and Edward Condon proved in 1937 from quantum mechanical principles that while one wavelength of ultraviolet CPL preferentially destroys left-handed chiral molecules, a different wavelength of ultraviolet CPL preferentially destroys right-handed chiral molecules. Any broad band ultraviolet CPL, therefore, would destroy equal amounts of left-handed and right-handed molecules. Likewise, monochromatic (single wavelength) ultraviolet CPL sources where the wavelengths differ among the individual sources would destroy similar amounts of left-handed and right-handed molecules. As it is, no astrophysical sources of monochromatic ultraviolet CPL are known to exist.

Anti-Neutrino Processing
The Kuhn-Condon rule and the lack of astrophysical sources of monochromatic ultraviolet CPL appear to rule out ultraviolet CPL as a candidate for explaining the slight preference for left-handed amino acids seen in some meteorites. Consequently, a team led by Richard Boyd, formerly of the Lawrence Livermore National Laboratory and now at Ohio State University, has suggested that a dense neutrino flux in the presence of a strong magnetic field such as one would encounter near a supernova or neutron star, explains the slight preference for left-handed amino acids.⁶ Since the nitrogen-14 nucleus has a spin of 1, electron anti-neutrinos preferentially interact with the nitrogen-14 atoms in the right-handed amino acids (all amino acid molecules contain nitrogen-14) and convert the nitrogen-14 into carbon-14. The conversion of nitrogen-14 into carbon-14 in an amino acid destroys the amino acid. Thus, the electron anti-neutrinos destroy more right-handed amino acids than they do the left-handed amino acids.

In a paper published in the March 20 issue of the Astrophysical Journal, Boyd’s team assesses whether meteoroids could have their amino acids processed by astrophysical sources of electron anti-neutrinos to deliver an approximately 1 percent excess of left-handed amino acids relative to right-handed amino acids.⁷ They investigated four possible sources:

1. a single massive star that becomes a supernova
2. a neutron star that is recoiling right after it has been produced by a supernova
3. a Wolf-Rayet star (see featured image) that becomes a supernova
4. a neutron star in a close orbit about a massive star (see figure 2)

Figure 2: Neutron Star Accreting Matter from a Companion Massive Star. Image credit: NASA

Boyd’s team judged the third and fourth possibilities as the most productive. In the fourth possibility the neutron star siphons off the massive star’s outer layers. The massive star eventually becomes a supernova that provides a robust electron anti-neutrino flux to debris material near the neutron star.

Boyd’s team close their paper by addressing two questions. The first, “Could this model populate the entire galaxy with enantiomeric amino acids?”⁸ They point out that Wolf-Rayet stars and neutron stars in close orbits about massive stars, though not extremely rare, are, nonetheless, rare. Thus, only a few isolated sites in our Milky Way Galaxy would be expected to be populated with meteoroids possessing amino acids with a slight preference for the left-handed configuration.

The second question Boyd’s team addressed was could their model “produce meteorites that can make it to Earth’s surface?”⁹ They explain that, given their penetrative power, the electron anti-neutrinos would have processed the entire meteoroid, no matter how large it was. Thus, whatever portion of the meteoroid remained after experiencing collisions and fractures on its way to Earth and after passing through Earth’s atmosphere would still carry the enantiomeric excesses it had originally gained.

Lastly, Boyd and his team concede that their model at this point is completely theoretical. They point out that experiments to test their predictions are feasible but have yet to be performed.

Do Anti-Neutrinos Solve the Homochirality Challenge?
In the abstract of their paper Boyd and his colleagues state that their results “have obvious implications for the origin of life on Earth.”¹⁰ They hint that perhaps some autocatalytic mechanism operating on Earth 3.8 billion years ago amplified the approximately 1 percent enantiomeric excess up to the needed 100 percent. However, no such autocatalytic mechanism has ever been observed to operate in nature.

Chemists, using sophisticated laboratory experiments, have achieved some success in amplifying enantiomeric excesses in amino acids. I discussed these laboratory results in a previous blog.¹¹ However, the higher the enantiomeric excesses achieved, the lower the amount of the original sample of amino acids that remains. All the laboratory experiments are consistent with the conclusion that before a 100 percent enantiomeric excess is achieved all of the original sample would be destroyed.

The “obvious implications for the origin of life on Earth” are that Someone with a lot more intellect, knowledge, laboratory control, and funding than our best chemists must have been responsible for producing the required samples of 100 percent left-handed amino acids and 100 percent right-handed pentose sugars. There is only one possible candidate for the Someone who could have done the deed on Earth 3.8 billion years ago.

Featured image: The Wolf-Rayet Star WR 124 and Its Nebula. Image credit: NASA/ESA/Hubble