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The Origin of Homochirality

A Major Problem for Origin of Life Theories

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All living organisms are based upon certain "mirror" forms of amino acids and sugars. Although normal chemical reactions produce right and left mirrors in equal amounts ("racemic" mixtures), life uses specialized molecular machinery to produce only right handed forms of sugars and left handed forms of amino acids (called "enantiomers"). Because these chemicals exist in only one form, they are referred to as being "homochiral." Origin of life theories must explain how chemistry could produce the proper mirrored building blocks to support the generation of the first self-replicating life form.

Why homochirality is important

Origin of life theories often ignore the homochirality problem, even though the question is critical to the origin of life. Both DNA and RNA are incapable of complementary pair bonding in the absence of being homochiral. Practically, this means that racemic DNA or RNA cannot replicate.

Because of this problem, organic chemist William Bonner, professor emeritus at Stanford University, has dismissed the point of view that homochirality in nucleic acids and/or amino acids did not precede the origin of life. Because of the importance of the question, Bonner spent considerable effort looking for a solution, but admitted, "I spent 25 years looking for terrestrial mechanisms for homochirality and trying to experimentally investigate them and didn't find any supporting evidence." He added that "Terrestrial explanations are impotent or nonviable."

Amino Acids

Researcher Stanley Miller demonstrated in 1953 that mixtures of reducing gases, thought to be present in the primordial earth, when subjected to electrical discharges, produced many organic compounds, including several amino acids.

Years later, in 1969, a meteor that landed in Murchison, Australia, was shown to contain the same organic compounds and amino acids, in roughly the same proportion as those generated through the Miller experiments. An initial examination of the chirality of the amino acids revealed either no chiral excesses or L-amino acid excesses that were probably the result of contamination by terrestrial sources. However, in 1997, a study examined chiral proportions of non-terrestrial amino acids. The results showed a 7% and 9% L-enantiomeric excess within extracts of the meteorite. Although statistically significant, such as small excess of L-amino acidswould not solve the problem of 100% L-enantiomeric excess required by earth's life forms.

The carbonaceous meteorite GRA 95229 was discovered in 1824 in Antarctica and appears to be relatively free of terrestrial contamination. Analysis of the organic materials revealed the presence of several amino acids and aldehydes with L-enantiomer excesses up to 14%. Again, such excesses are not enough to account for the origin of homochirality. The most pristine carbonaceous chondrite examined to date is the Tagish Lake Meteorite, which was observed to fall on a frozen lake in Canada during Winter, 2000 and was collected to minimize human contamination. Amino acids in the meteorite were observed only in parts-per-billion concentrations and no L-enantiomeric excess was found.

An attempt to be explain the origin of homochirality in amino acids has been made by invoking some rather complex chemistry. Researchers have shown that transfer RNA (tRNA), the molecule responsible for binding to amino acids during protein synthesis, does not selectively bind to L-amino acids in solution. So, it has become apparent that the prebiotic soup hypothesis will not explain the origin of homochirality.

However, researchers thought that if RNA were bound to a solid substrate, the selectivity of RNA might be constrained, since the molecular configuration would be more limited. So, researchers have hypothesized that homochiral RNA bound to polar mineral surfaces would alter stereospecificity. In fact, this technique has produced up to a 35-60% enantiomeric excess of L-amino acids, but not nearly enough to produce any reasonably-sized protein.

In another strategy, researchers isolated the amino acid binding site of several tRNAs and modified them to make an "RNA minihelix" that demonstrated a four-fold enantiomeric selection. Although impressive compared to previous attempts, such a system would still produce a 20% error rate, which would prevent the formation of anything larger than a small peptide. In addition, the system assumes that the problem of RNA homochirality had already been solved. In addition, the author assumed that some primitive life form would have come up with the minihelix design, instead of stealing it from current life forms, which is what the scientists did.

Another means by which L-amino acids could be selected is through a specific molecule that prevents or removes binding of D-amino acids to tRNAs. In most living systems, specific molecules are used to discriminate between structurally similar amino acids. In general, the tRNA itself cannot discriminate and allows the smaller amino acid to bind.

However, another molecule will remove the smaller intruder amino acid, preserving the accuracy of the genetic code. It turns out that in some species of Archaea (ancient, bacteria-like, single celled organisms), the molecule that corrects for improper amino acid charging can also distinguish between certain L- and D-amino acids. It is possible that such a system could select for homochiral amino acids. However, the requirement for 20 specifically-designed accessory molecules (one for each amino acids), would add a level of design that would not be expected in primitive early life forms.

A recent study has found differential separation of racemic mixtures of the amino acid proline when dissolved at high concentrations (25-100 mM) in pure DMSO. Aldol condensation reactions produce amino acids under these conditions with an enantiomeric excess from 46-99%. Although this seems impressive, there would have been no prebiotic source of high concentrations of purified amino acids nor DMSO solvent.

Peptides

Attempts to produce amino acids have been plagued with problems involving unreactive byproducts. In particular, the formation of peptides under primordial conditions have resulted in the formation of large amounts of unreactive diketopiperazines. This problem can be circumvented by incubating purified L-amino acids (obviously not available on the primordial earth) in a slurry of (Ni, Fe)S at boiling temperatures under alkaline conditions (similar to those observed in undersea hydrothermal vents).

The problems with such a system was that, although the system produced very short peptides, the process itself resulted in racemization of the peptides. In addition, under these conditions, these peptides hydrolyzed rapidly (reversal of the process). Ultimately, because these problems, the process produced low amounts of usable peptides.

Ribose

The sugar ribose forms the backbone of RNA (and its related sugar, deoxyribose, makes up the backbone of DNA).Researchers have shown that enantiomeric excess of sugars can be produced by using chiral amino acid catalysts. However, even a 100% homochiral amino acid increased the enantiomeric excess by only 10%. Subtle chiral excesses of amino acids (like those found in meteorites) produced negligible enantiomeric excess in synthesized sugars.

Homochirality in extraterrestrial sources

In 1997, scientists hypothesized that the enantiomeric excess of L-amino acids in extraterrestrial sources could be due to circular polarization of synchrotron radiation in neutron stars, which selectively destroys the opposite handed enantiomer.12 According to this theory, a neutron star was originally present near the interstellar molecular cloud from which the Solar System formed, resulting in excess L-amino acids and/or their precursors.

However, over time, such radiation would destroy all amino acids, even the enantiomer that is destroyed at a lower rate. In addition, over the entire spectrum of circularly polarized radiation, the susceptibility of specific enantiomers to differential photochemical degradation sum to zero. So, any preferential degradation of an amino acid would require some means of providing monochromatic circularly polarized radiation, which is extremely unlikely.

There are even more problems with circularly polarized radiation theories. Although, such sources of radiation are theoretically possible, the reality is that none have yet been found. For example, the Crab Nebula has been proposed as a possible source of synchrotron radiation, but detection has shown a maximum amount of 0.03%. What has been detected is a level of 0.05% at a wavelength of 1415 MHz. However, this wavelength is one million times longer than the wavelengths that actually produce an effect. So, to date, there is no evidence that natural sources of circularly polarized radiation of the proper wavelength actually exist.

Despite these problems, researchers have found that phthalic acid crystals differentially scatter circularly polarized light, making it possible that such crystals might be involved somehow in the synthesis of homochiral amino acids. Now, where did the universe put those phthalic acid crystals?

Conclusion

The origin of homochirality is extremely important in origin of life research, since non-optically pure mixtures of amino acids or sugars cannot be used to make RNA, DNA, and proteins, the building blocks of all living organisms. There is no terrestrial or extraterrestrial explanation that describes how homochirality could have arisen through completely naturalistic processes.

Processes that can enhance the enantiomeric excess of appropriate amino acid or nucleic acid building blocks produce only modest increases in the percentage of those products, while requiring unrealistic, laboratory conditions, which could not have been present on the primordial earth. Coupled with the inability of unaided chemistry to even produce some of the required molecular building blocks of life, a completely naturalistic origin of life seems extremely unlikely.

Such insurmountable problems led me, as an undergraduate biology major at USC, to the conclusion that, at minimum, there must be a Creator God who designed the first life form, prompting me to go from atheism to deism.

Homochirality - The primary molecules of life exist in one specific form even though ordinary chemical reactions produce equal amount of both mirror forms. Living organisms contain complex enzymes that specifically produce molecules of only one form. This article discusses the problems associated the generation of specific enantiomers of amino acids or sugars within naturalistic origin of life theories. Since the jargon is necessarily specific to the topic, you may want to review the concepts and terminology of introductory molecular biology in order to understand the concepts.

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