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Origin of Life: The Left-Handed Molecule Problem

Many important molecules required for life exist in two forms. These two forms are non-superimposable mirror images of each other, i.e.: they are related like our left and right hands. Hence this property is called chirality, from the Greek word for hand. The two forms are called enantiomers (from the Greek word for opposite) or optical isomers, because they rotate the plane of plane-polarized light.

Nearly all biological molecules must be homochiral (all molecules having the same handedness. Another term used is optically pure or 100% optically active) to function. All amino acids in proteins are left-handed, while all sugars in DNA and RNA, and in the metabolic pathways, are right-handed.

A 50/50 mixture of left and right-handed forms is called a racemate or racemic mixture. Racemic polypeptides could not form the specific shapes required for enzymes, rather, they would have the side chains sticking out all over the place. Also, a wrong-handed amino acid disrupts the stabilizing a-helix in proteins. DNA could not be stabilized in a helix if even a small proportion of the wrong-handed form was present, so it could not form long chains. This means it could not store much information, so it could not support life. 1

However, as a well-regarded organic chemistry textbook states a universal chemical rule in bold type:

Synthesis of chiral compounds from achiral reagents always yields the racemic modification. or Optically inactive reagents yield optically inactive products.2

To resolve a racemate (i.e. separate the two enantiomers), another homochiral substance must be introduced. The procedure is explained in any organic chemistry text-book. The idea is that right-handed and left-handed substances have identical properties, except when interacting with other chiral phenomena. The analogy is that our left and right hands grip an achiral (non-chiral) object like a baseball bat equally, but they fit differently into a chiral object like a left-handed glove. Usually, an organic chemist will use a ready-made substance from a living organism.

However, this does not solve the mystery of where the optical activity in living organisms came from in the first place. A recent world conference on The Origin of Homochirality and Life made it clear that the origin of this handedness is a complete mystery to evolutionists. 3

The probability of forming one homochiral polymer of N monomers by chance = 2-N For a small protein of 100 amino acids, this probability = 2-100 = 10-30. Note, this is the probability of any homochiral polypeptide. The probability of forming a functional homochiral polymer is much lower, since a precise amino acid sequence is required in many places. Of course, many homochiral polymers are required for life, so the probabilities must be multiplied. Chance is thus not an option.

A further problem is that homochiral biological substances racemise in time. This is the basis of the amino acid racemization dating method. It is not very reliable, since the racemization rate is strongly dependent on temperature and pH, and depends on the particular amino acid. But it shows that the tendency of undirected chemistry is towards death, not life.

Origin-of-life researchers have tried to think of other means of producing optical activity. There have been unsuccessful attempts to resolve racemates by other means:

Circularly Polarized UV Light

This means light where the electric field direction rotates along the beam. This can destroy one form more readily than another. However, because the light also destroys the right form, this method would not produce the necessary 100% homochirality required for life. The best result has been 20% optically pure camphor, but this occurred after 99% had been destroyed. 35.5% optical purity would have resulted after 99.99% destruction. 4

A practically optically pure compound (99.99%) is obtained at an asymptotic point where absolutely no material remains.5

Circularly polarized light has recently been revived as a solution in a paper by the Australian astronomer Jeremy Bailey in a recent issue of Science, and widely reported in the media. His team has discovered circularly polarized infrared radiation in a nebula. He admits that he hasn't discovered the required circularly polarized ultraviolet light nor any evidence that amino acids are produced in nebulae. But even if we grant this wishful thinking, the chemistry just doesn't work, as shown above.

Another proposed source of circularly polarized light is synchrotron radiation from a neutron star, 6 but this is speculative and doesn't solve the chemical problems.

b-decay

This is one form of radioactive decay, and it is governed by one of the four fundamental forces of nature, the weak force. This force has a slight handedness, so some theorists thought this could account for the chirality in living organisms. However, the weak force is aptly named, and one specialist in the chirality problem, organic chemist William Bonner, said that none of this work has yielded convincing conclusions. 7

Optically active quartz powders

Quartz is a very common mineral on Earth, and its crystals come in left and right-handed forms. Some investigators tried to use optically active quartz powders to adsorb one form more than the other. But they had no success. Besides, there are equal amounts of left and right-handed quartz crystals on Earth. 8

Clay minerals

Some investigators have reported a very small chiral selection effect on clay minerals, but the effects may have been an artifact of the technique used. Selective adsorption and binding have now been rejected. 9 Even if modern clays did have a chiral bias, this could be due to previous absorption of optically active biomolecules (which are, of course produced by living things). Prebiotic clays would then have no chiral bias.

Self-selection

A small minority of chiral substances crystallize into homochiral crystals. Louis Pasteur was not only the founder of the germ theory of disease, the destroyer of spontaneous generation ideas, and a creationist, he was also the first person in history to resolve a racemate. He used tweezers to separate the left and right-handed crystals of such a substance, sodium ammonium tartrate. 10

This separation only happened because of outside interference by an intelligent investigator, who could recognize the different patterns. On the supposed primitive Earth, there was no investigator. Therefore the two forms would have re-dissolved together and re-formed a racemic solution.

Also, Pasteur was fortunate to choose one of the few substances that self-resolve. And even this substance has this property only below 23C, so its fortunate that 19th century laboratories were not well heated!

Fluke seeding

Some theorists have proposed that a fluke seeding of a supersaturated solution with a homochiral crystal would crystallize out the same enantiomer. However, the primordial soup, if it existed, 11 would be extremely dilute and grossly contaminated, as shown by many writers.12 Also, nothing could be done with the resulting homochiral crystal, because it would be immersed in a solution of the wrong enantiomer. Concentrating the solution would crystallize out the wrong enantiomer. Diluting the solution would dissolve the crystal, and the process must start from scratch.

Homochiral template

Some have proposed that a homochiral polymer arose by chance and acted as a template. However, this ran into severe problems. A template of 100% right-handed poly-C (RNA containing only cytosine monomers) was made (by intelligent chemists!). This could direct the oligomerisation (formation of small chains) of (activated) G (guanine) nucleotides. Indeed, pure right-handed G was oligomerised much more efficiently than pure left-handed G. But racemic G did not oligomerise, because:

monomers of opposite handedness to the template are incorporated as chain terminators This inhibition raises an important problem for many theories of the origin of life.13

The textbook cited above states:

We eat optically active bread and meat, live in houses, wear clothes, and read books made of optically active cellulose. The proteins that make up our muscles, the glycogen in our liver and blood, the enzymes and hormones are all optically active. Naturally occurring substances are optically active because the enzymes which bring about their formation are optically active. As to the origin of the optically active enzymes, we can only speculate.14

If we can only speculate on the origin of life, why do so many people state that evolution is a fact? Repeat a rumor often enough and people will swallow it.

Author: Jonathan Sarfati, Creation Ministries International.

References

  1. W Thiemann, ed., International Symposium on Generation and Amplification of Asymmetry in Chemical Systems, Jlich, Germany, pp 3233, 1973; cited in: A.E. Wilder-Smith, The Natural Sciences Know Nothing of Evolution, Master Books, CA, 1981. Return to text.
  2. R.T. Morrison and R.N. Boyd, Organic Chemistry, 5th ed. Allyn & Bacon Inc. p.150, 1987. Return to text.
  3. J. Cohen, Getting all turned around over the origins of life on Earth, Science, 267:12651266, 1995. Return to text.
  4. G. Belavoine, A. Moradpour, and H.B. Kagan: Preparation of Chiral Compounds with High Optical Purity by Irradiation with Circularly Polarised Light, J Amer. Chem. Soc. 96:515258, 1974. Return to text.
  5. W. Thiemann, Ref. 1, pp 222223. Return to text.
  6. W.A. Bonner, Origins of Life 21:59111, 1991; cited in C.F. Chuba, A Left-handed solar system, Nature 389:234235, 18 Sept 1997. Return to text.
  7. As stated by Cohen, Ref. 3. Return to text.
  8. A. Amariglio and H. Amariglio in Chemical Evolution and the Origin of Life, eds. R. Buvet and C. Ponnamperuma (1971), North-Holland Publishing Co., Amsterdam-London. Return to text.
  9. B. Youatt and R.D. Brown: Origins of Chirality in nature: A reassessment of the postulated role of bentonite, Science 212:11456, 1981. Return to text.
  10. L. Pasteur: Annales de Chimie Physique 24:44259, 1848. Return to text.
  11. J Brooks and G Shaw, Origins and Development of Living Systems, Academic Press, London and New York, 1973, write on p. 359:
    If there ever was a primitive soup, then we would expect to find at least somewhere on this planet either massive sediments containing enormous amounts of the various nitrogenous organic compounds, acids, purines, pyrimidines, and the like; or in much metamorphosed sediments we should find vast amounts of nitrogenous cokes. In fact no such materials have been found anywhere on Earth. (emphasis added). Return to text
  12. C.B. Thaxton, W.L. Bradley and R.L. Olsen, The Mystery of Lifes Origin, Philosophical Library Inc., New York, 1984. Return to text.
  13. G.F. Joyce, G.M. Visser, C.A.A. van Boeckel, J.H. van Boom, L.E. Orgel, and J. van Westrenen, Chiral selection in poly(C)-directed synthesis of oligo(G), Nature 310:602-4, 1984. Return to text.
  14. R.T. Morrison and R.N. Boyd, Ref. 2, p.157. Return to text.

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