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P R E B I O T I C . S Y N T H E S I S . O F . A M I N O . A C I D S
ORIGIN OF LIFE
Andrew Gyles
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- Biased conversion of glycine to L-alanine in lightning clouds (hypothetical)
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Biased conversion of glycine to L-alanine in lightning clouds (hypothetical)
A question for organic chemists
In the non-catalytic synthesis of an organic enantiomeric molecule by the discharge of an electric current through an appropriate mixture of gases, would the presence of a strong magnetic field of about 1 tesla bias the synthesis toward one of the enantiomers?
Zeeman effect
As I see it the Zeeman effect caused by the strong magnetic field would split the energy levels of the excited electrons, including those involved in the synthesis, in an asymmetric way, favouring the formation of one enantiomer more than the other.
Lightning strokes in clouds in prebiotic atmosphere
My interest is in the synthesis of enantiomeric organic molecules in or near the channel of a lightning stroke in the prebiotic atmosphere of Earth (or in lightning in the accreting cloud of dust and gas that formed the Earth).
In 1953 Stanley Miller demonstrated in a laboratory simulation on a small scale that an electric discharge through a reducing mixture of gases produced plenty of amino acids (though not all of those that occur in living organisms). I calculate that powerful lightning strokes must build up magnetic fields of 1 or 2 teslas.
I asked the above question on the internet discussion group sci.chem.organic.synthesis and received a reply from a member of an institute for organic chemistry in Germany, who said essentially:
'As far as I know, there are no confirmed observations of any chiral induction by magnetic fields'.
A second question for organic chemists
Crossed electric and magnetic fields
I therefore asked the further question:
Would the situation be different in strong crossed electric and magnetic fields? The electric field in a lightning cloud is up to 3 million volts per metre and is not dissipated in the first strokes of a multi-stroke flash. My interest is in the synthesis of amino acids in the prebiotic atmosphere.
Oriented molecules
For example, if glycine were to be converted to alanine in solution in a cloud droplet, the glycine molecules would be in the ionised and polarised state, and would therefore be oriented by the electric field. A strong magnetic field (1 or 2 teslas) would build up during the stroke, imposing the Zeeman effect on glycine molecules all oriented alike by the electric field.
Might the result of conversion to alanine then be the synthesis of more of one enantiomer than the other?
My attempt to answer the questions
I received no anwer to the second question from members of the discussion group. I give my attempt to answer it below:
Converting glycine to alanine
If we look at the conversion of glycine (which does not exist in two forms, each the mirror image of the other) to alanine (which does exist in two such forms, called L-alanine and D-alanine) we see that one of the two hydrogen atoms on the alpha carbon atom of the glycine molecule must be replaced by a methyl group, -CH3.
In 'normal' conditions each hydrogen atom has an equal probability of being replaced, and so the conversion yields a 1:1 mixture of L-alanine molecules and D-alanine molecules.
Glycine molecules oriented by electric field and acted on asymmetrically by Zeeman effect
It seems to me that if all of the glycine molecules in a cloud droplet were oriented alike by the strong electric field, then the crossing strong magnetic field would split the energy levels of their orbiting electrons asymmetrically (through the Zeeman effect) so that one of the two hydrogen atoms on the alpha carbon atom of each glycine molecule would be more likely to be replaced than the other, and the one more likely to be replaced would always be on the same side of every glycine molecule.
More of one enantiomer than the other
Thus, more would be produced of one enantiomer of alanine than of the other enantiomer. I suggest that there would be an excess of L-alanine molecules over D-alanine molecules, but this would have to be tested by experiment.
Glycine molecules oriented and freeze-dried on droplet nucleus
Organic chemists might tell us that the prebiotic conversion of glycine to alanine in an aqueous solution is unlikely.
In that case I would suggest that glycine molecules in a cloud droplet might be all oriented alike by the strong electric field of a lightning cloud, then freeze-dried in their oriented state onto the solid particle that formed the 'nucleus' on which the droplet had condensed.
Such a layer of oriented glycine molecules might then be converted to alanine by the energy provided by a lightning stroke (assuming, of course, that the prebiotic atmosphere provided, directly, or indirectly by intermediate synthesis, a source of the methyl group, or of an energised donor of the methyl group).
Again, the Zeeman effect would cause more of one enantiomer of alanine to be produced than of the other enantiomer of alanine.
Same biased conversion to other amino acids
The same mechanism might have caused biased synthesis of other enantiomeric amino acids in prebiotic lightning clouds.
Published on this site 17 August 2001. © Andrew Gyles
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Stanley Miller's famous simulation of lightning in a flask containing a reducing mixture of gases produced several amino acids.
But this compact laboratory setup did not simulate all of the conditions in a lightning cloud. If it had done so the result of the experiment might have been even more surprising than it was.
The setup did not create a strong electric field (up to 3 million volts per metre) over a broad area and depth.
It did not create momentarily a very strong magnetic field.
It did not provide the heating and radiation caused by a great current. (The main current in a stroke from a typical lightning cloud is about 20,000 amperes, but may be as big as 200,000 amperes. One author implies currents as big as 300,000 amperes. Much of the research literature describes mid-latitude lightning; but storms with energies orders of magnitude higher develop in the tropics.)
It did not provide the freezing conditions ( -20 degrees Celsius) that exist in the negatively charged 'pancake' of a lightning cloud. It did not provide supercooled water droplets, small ice crystals, bigger ice crystals (Graupel particles) and water droplets condensed on nuclei consisting of mineral particles. In this negative 'pancake' all three phases of water (ice, liquid and vapour) can coexist. Small droplets can evaporate, or grow into bigger ones. Small ice crystals can sublimate, or grow into bigger ones.
Yet Miller's achievement in 1953 was so remarkable that everyone who followed him assumed that the next problem was to work out what happened when, in the real prebiotic world, his amino acids and adenine fell to the earth in rain and accumulated in puddles and the primeval sea. The joke about the 'prebiotic soup' was born: it was about a third as concentrated as chicken bouillon. Everyone smiled and took their eye off the lightning cloud.
Thor's bang
The channel containing the current of a lightning stroke is a few centimetres wide. About 100,000 joules per metre are deposited in it. This energy dissociates, ionises, excites and heats the air molecules (mainly nitrogen, oxygen and water in the case of the present atmosphere) in the channel. They are split into single atoms and, on average, each atom loses one electron; this happens in a few microseconds. The temperature rises to about 30,000 degreees C and the pressure to about 10 atmospheres. The channel then expands in from 10 to 20 microseconds, this taking about 98 per cent of the total energy. The supersonic expansion of the channel slows to a sound wave, which is heard as thunder. About 1 per cent of the input energy is stored in the particles in the channel, and about 1 per cent is radiated as light and infrared radiation in the band of wavelengths from 4,000 to 11,000 angstroms.
A two-tesla magnetic field?
A long unbranched section of the lightning channel can be regarded as a long straight conductor. To take as an illustrative example a large discharge: before the channel has expanded, when it has a diameter of 4 centimetres and carries a current of 200,000 amperes, the magnetic field at a radius of 2 centimetres from the longitudinal axis of the channel would be 20,000 gauss or 2 tesla.
As it happens, this is the field strength that electrical engineers have worked with for more than a century; the strongest they have ever been able to economically achieve. It is very strong. The two-tesla field pulls seven-kilometre freight trains and twists the screws of the largest ships. (Stronger fields can be created by extraordinary means in the laboratory, and cosmologists know of fields much stronger than these.)
For comparison, the magnetic field of the earth is about 0.5 gauss. A small, inexpensive type of permanent magnet has a field of about 100 gauss.
Accretion clouds
It seems reasonable to choose the largest known values of the various aspects of lightning clouds and strokes if they seem to be necessary to originate life. Life only had to originate once (though it might have originated often) and it had plenty of time to do it in. It would be helpful to know the values in the lightning clouds of dust and gas that must have formed as our planet accreted. Heavier dust particles would have fallen through the clouds of small dust particles, and the heavy and small particles must have acquired electric charges of opposite sign. When the electric field had increased enough to break down the insulating atmosphere a stroke must have passed between the cloud and the accreting planet, or between different parts of the cloud.
Volcanic lightning
A similar process (though perhaps not the only one causing electrification) must happen in volcanic lightning clouds (one above the Sakurajima volcano, Japan, is shown on the cover of 'Science', volume 275, 28 February 1997). These clouds contain great quantities of dust at certain phases of an eruption. During the eruption of the Krakatoa volcano in 1883 a ship 120 kilometres from it saw a black billowing cloud rise to about 25 kilometres. A ship 64 kilometres away described the cloud as like 'an immense pine tree, with the stem and branches formed with volcanic lightning'.
Accretion clouds and volcanic clouds might have contained mineral grains that oriented and selected amino acids once formed, or catalysed their polymerisation; iron sulphide, for example. (Water clouds might contain similar mineral grains as the nuclei of droplets.)
Perhaps life originated at various times in all three types of cloud: the accreting cloud, the volcanic cloud and the water-cloud. Indeed, it might still be originating today above any volcano emitting a suitable reducing mixture of gases, if such a volcano exists.
An enantiomeric excess without photolysis
Magnetic circular dichroism
I propose that L-amino acids (or at least an L-enantiomer core from which L-amino acids could be derived by further chemical reactions) were preferentially formed from molecules or atoms of the prebiotic atmosphere that were in or near the channel of a lightning stroke.
In molecules, atoms or ions that have been excited by the energy of a lightning stroke the orbits of their electrons would be affected by the strong electric field in the cloud, which persists during a stroke because only a part of the energy stored in the electric field goes into each stroke. (A lightning 'flash' commonly consists of three or four strokes; one flash to the ground had 26 strokes and lasted two seconds. Other observers have counted 40 strokes in a single flash).
These excited molecules, atoms or ions would also be affected by the strong magnetic field built up by the electric current of a stroke in the channel, because the motions of orbiting electrons perpendicular to a magnetic field are subject to Lorenz forces. The degeneracy of the eigen vibrations of the electrons is removed by the magnetic field so that their energy levels are split (this is the Zeeman effect, discovered in 1897). The oscillating electrons emit radiation in all directions. That emitted in the direction of the field is left circularly polarised. That emitted in the opposite direction is right circularly polarised. (These definitions of direction conform to the chemical convention, as opposed to that used in the physical literature.) At various wavelengths the left circularly polarised radiation might be more intense than the right, or the right circularly polarised radiation might be more intense than the left; these differences in intensity depending on wavelength are referred to as magnetic circular dichroism. Some researchers believe that at certain ultraviolet wavelengths magnetic circular dichroism would cause ultraviolet radiation to destroy (photolyse) more of one enantiomer than the other of an organic molecule.
Biased synthesis of L-amino acids
The effect of a strong magnetic field in removing the degeneracy of the eigen vibrations and splitting the energy levels of the electrons of excited molecules, atoms or radicals in or near the lightning channel would bias those molecules, radicals or atoms toward the formation of one enantiomer as they bonded with each other. I propose that in the formation of amino acids in or near the channel an L-enantiomer core would be formed in preference to a D-enantiomer core and, directly or indirectly, L-amino acids would be formed in preference to D-amino acids. In this case there would be no need to invoke biased photolysis of the D-enantiomer to explain an excess of L-amino acids in a meterorite; the bias would have been in the synthesis of the amino acids, not in their destruction.
A universal bias for L-amino acids
In this proposed biased synthesis of enantiomers the bias would be the same anywhere in the universe for the same synthesis under the same conditions. Thus this hypothesis differs from one of preferential photolysis, which depends on the filtering out of certain wavelengths of energetic circularly polarised radiation and the non-filtering of other wavelengths having the opposite rotation. Because the particular wavelengths filtered out (by dust, for example) in different places would depend more or less on chance, preferential photolysis might result in an excess of one enantiomer in one part of the universe and the other enantiomer in another part of the universe.
Nature's rules about electric and magnetic fields are asymmetrical
It seems to be assumed by many writers on the origin of life that the physical conditions in an accretion cloud or on our planet are on average symmetrical, and that therefore an asymmetry in biological molecules - the existence of only L-amino acids and D-nucleic acids, for example - must have evolved by chance. But nature has only one rule relating the direction of an electric field, the direction of the electric current that it drives and the direction of the magnetic field that the current creates. And it has only one rule relating the direction of a magnetic field and its effects on the oscillations of electrons in orbit about a nucleus or a molecule. There is asymmetry in these rules.
Other possible effects of a strong magnetic field
Enantiomers facing different ways
If amino acids were diamagnetic or paramagnetic they would become oriented antiparallel or parallel with a strong magnetic field. Proteins are sometimes made paramagnetic in laboratory investigations by replacing some of their hydrogen atoms with fluorine atoms. It seems unlikely that this would happen to amino acids in nature, except perhaps above a volcano emitting fluorine gas.
Nonetheless it is illuminating to note that if fluorinated amino acids were dissolved in water droplets in a volcanic lightning cloud they would firstly be oriented by the persistent strong electric field of the cloud; then when a stroke passed close to them they would also be oriented by the strong magnetic field built up momentarily by the stroke. If both enantiomers were present in the droplet, and if their electric axis and paramagnetic axis were not coincident, one enantiomer would have its 'front' facing the radiation and the shock wave from the stroke; the other enantiomer would have its 'back' facing the radiation and the shock wave.
The effects of the energy received by the two enantiomers from these sources might be different because of the different orientation of the molecules with respect to the stroke. This is another example (though an unlikely one) of how nature could place enantiomers in an asymmetric situation in a lightning cloud. It raises the question of whether the strong electric field in a cloud could slightly polarise the electron orbits of amino acids and make them paramagnetic enough to be oriented by a strong magnetic field.
A chance of preferential photolysis
Any non-spherical diamagnetic, paramagnetic or ferromagnetic mineral particles suspended by updrafts in the lightning cloud would be aligned in a direction antiparallel (in the case of a diamagnetic particle) or parallel with the direction of the magnetic field surrounding the lightning channel. These aligned particles might by reflection circularly polarise electromagnetic radiation travelling radially outward from the plasma in the channel. (Radiation that began its journey parallel or antiparallel with the magnetic field would already be circularly polarised.)
The particles might also filter out certain wavelengths.
This radiation would extend to ultraviolet frequencies in an accretion cloud because of the presence of ions of high atomic number.
However, even if ultraviolet radiation were not present, a beam of circularly polarised visible electromagnetic radiation close to the channel would be very energetic and would carry more angular momentum than an ultraviolet beam of the same energy, because the angular momentum of circularly polarised radiation varies inversely with the frequency of the radiation. Thus a very energetic beam of circularly polarised visible light might have asymmetric effects on enantiomers of organic molecules in its path.
Polymerisation of L-amino acids in repeatably ordered sequences
Alignment by the electric field
Having produced L-amino acids the lightning cloud might have aligned them 'head to tail', that is, carboxyl group to amino group, and might have arranged them in order in the line. The strong electric field built up in the cloud would act on amino acids dissolved in water droplets. Because the carboxyl group is acidic and the amino group is basic a proton is transferred from the former to the latter in water, leaving the carboxyl group with a negative electric charge and giving the amino group a positive charge. In a strong electric field in a cloud the carboxyl group must be attracted toward the effective positive part of the cloud or earth and the amino group must be attracted toward the negative part of the cloud.
(I say 'effective' because typical lightning clouds have a main positive charge at the top, a negative part in the middle and a small positive part at the bottom. A lightning stroke to ground connects the negative middle part of the cloud with the earth, which is relatively positive. Electric fields within a cloud are complex. However, whatever the orientation of the field may be at a particular place, it will affect all amino acids dissolved in water in a consistent way.)
Because water molecules are polar they would be aligned by the electric field of the cloud; this might strengthen the alignment of the amino acids dissolved in the water.
Ordering of sequence by the electric field
Some amino acids have extra charged groups in water, so that they have a net positive charge or a net negative charge (depending on the nature of their R group). Those with a net positive charge would be pulled bodily through the water droplet (though remaining inside it) toward the negative part of the cloud, and those with a net negative charge would be pulled bodily through the droplet toward the positive part of the cloud or earth. Amino acids with no net charge would be jostled toward the middle as the charged ones found their respective places.
Then the water of a small droplet might partly or completely evaporate; in freezing dry air this could happen quickly. The aligned and ordered amino acids might be 'dried' and remain as lines of head-to-tail molecules on the surface of the mineral particle on which the water droplet had condensed. This mineral particle might be one that catalysed the elimination of water from the juxtaposed amino group of one molecule and the carboxyl group of another molecule in a line when a nearby lightning stroke supplied the necessary energy.
Energy for polymerisation from inductive effect
The quickly building magnetic field might induce forces on the positive amino group and the negative carboxyl group that helped to eliminate a water molecule and form a chemical bond. As the circular magnetic lines of force of the building field travelled outward they would induce the electric charges that they 'cut' to move in the direction perpendicular to the line of magnetic force and perpendicular to its direction of motion. A positively charged amino group would be induced to move in one direction and a juxtaposed negatively charged carboxyl group would be induced to move in the opposite direction.
A short time later, as the magnetic field collapsed, they would be induced to move in the 'switched' directions. Either during the building-up or the collapsing of the magnetic field the amino group and the carboxyl group might be forced together by induction with enough energy to enable them to eliminate a molecule of water and form a chemical bond.
The same forces would be induced on all of the charged amino and carboxyl groups in an aligned, ordered sequence of amino acids. Thus a complete line of amino acids might be polymerised into a short protein.
A variety of ordered sequences
These short proteins, produced by the trillion in the cloud, would not all be alike. Some might be longer than others. Their arrangements would depend on which amino acids had been present in their 'mother' water drop, how many of them there were, how big the drop was before it evaporated and what mineral 'nucleus' was present. (The drop might alternatively have been frozen and then sublimated; in this case the aligned and ordered amino acids would have been 'freeze-dried'.)
Positive, negative and neutral domains
Some proteins (I shall call them proteins to avoid using more cumbersome names) might have charged amino acid residues at each end (positive at one end, negative at the other) and uncharged amino acid residues in the middle, following the order I described above. When redissolved in water these would fold consistently into specific shapes, depending on the charges of their parts and other effects, including hydrogen bonding.
Membrane-forming proteins?
Other proteins might have charged amino acid residues at only one end, and uncharged residues in the middle and at the other end. In some of these proteins the uncharged residues might be hydrophobic, 'water-hating'. If a lot of the latter kind of protein gathered together after being redissolved in a water droplet they might have formed a two-layered membrane: an essential part of a living cell.
Thus tiny sacs formed by two-layered protein membranes and containing water and other proteins folded in various configurations might have been formed in lightning clouds and fallen in raindrops onto the surface of the prebiotic earth.
A laboratory built to withstand a blast
The conditions I have described would not be difficult to simulate in a well-funded laboratory. Stanley Miller and his supervisor could hardly have been expected to simulate them. But 48 years have passed since Miller synthesised amino acids in a 'prebiotic' atmosphere (and, in a remarkable coincidence, Watson and Crick described the structure of DNA). No biased synthesis of L-amino acids, and no polymerisation of L-amino acids into various short proteins each having a repeatable sequence of amino acid residues, has been achieved in that time. (By 'repeatable sequence' I mean each of many specific sequences that will be produced each time the experiment is done.)
What kind of experimental chamber and atmosphere would be required to simulate these conditions?
An electric field between plates
A strong electric field could easily be set up between big metal plates at the top and bottom of the chamber.
Reducing mixtures of 'prebiotic' gases
Various reducing mixtures of gases could readily be sent into the chamber (there seems to be no consensus on the composition of the prebiotic atmosphere, so various plausible mixtures should be tried, including sulphurous mixtures that might have been emitted from primeval volcanoes).
Water as liquid, solid and vapour
Conditions in which water coexisted in its three states as liquid droplets, ice particles and vapour could be provided by freeze-drying equipment. This dries the atmosphere in one part of a chamber, but it humidifies the atmosphere in another. It is not a simple refrigerator. A small fan might be needed to provide enough updraft to prevent the droplets falling.
A puff of mineral powders
Mineral particles to act as nuclei for water vapour to condense on could be puffed into the experimental chamber. In some experiments these particles should include minerals likely to exist in an accretion cloud, such as nickel and iron.
Lightning stroke and strong magnetic field
A sufficiently strong cylindrical magnetic field (not necessarily as strong as 2 teslas) could be momentarily built up by the sudden discharge of a large bank of capacitors through the prebiotic atmosphere in the chamber. The insulating property of the atmosphere would first have to be broken down by a high-voltage, small-current electric discharge between pointed electrodes at the top and bottom of the chamber; this would form a 'channel' of deposited charge and ionised atoms. This discharge would be immediately followed by a lower-voltage, great-current discharge through the same electrodes.
(Experimental work on confinement of plasmas for nuclear fusion has accumulated much experience on the use of banks of capacitors to provide currents of several hundred thousand amperes in pulses lasting a few milliseconds.)
The passage of so much electric current through the channel would have the same effects on the gas molecules as a lightning stroke, dissociating, ionising, exciting and heating them. The temperature in the channel would rise to about 30,000 degrees C and the pressure to about 10 atmospheres. Electromagnetic radiation would travel outward from the channel. Then in from 10 to 20 microseconds the channel would expand supersonically, this taking about 98 per cent of the energy of the discharge. The chamber would have to be built strongly enough to withstand the blast. It should be wide enough to allow the supersonic shock wave to decay to a sound wave.
The finale: gentle rain
After the experimenters had sent a sufficient number of simulated lightning strokes through the prebiotic atmosphere in the chamber the fan would be switched off and the conditions of relative humidity and temperature would be adjusted so that small droplets of water began to condense, coalesce and fall like rain, or trickle down the walls of the chamber.
This water would carry dissolved in it whatever organic molecules had been formed. It would be drawn off from the floor of the chamber.
Then the task of identifying these molecules could begin.
(I sent this article to the internet discussion group 'bionet.molbio.evolution' on 04 July 2001, where it was subsequently published.)
Published on this site 05 July 2001. © Andrew Gyles
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