It is told that a wise Rabin, was trying to resolve a quarrel between husband and wife. The husband complains to him of his wife. “You are right” answered the Rabin. His wife complained to him of her husband. “You are right” answered the Rabin. The Rabin’s wife comes out of behind a curtain, from where she had heard everything. “How can you say to both of them that they are right?”, she asked her husband. At this the wise Rabin answered: “You too are right”.
This little story, evoked by R. Panek in “L’universo oscuro” 2011, has been told by Vera Rubin, in a conference in 1996, to reassume the situation in which at this epoch cosmology found itself. But the story also illustrates in adequate way the state in which prebiotic chemistry finds itself today.
Who were the Alchemists? Chemists without rules.
Alchemy, at its origin had as its aim, the search for the philosopher’s stone and the origin of life. Such research led to the knowledge
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of the specific properties of many substances. However every alchemist had his own ideas, rules, methods, and the largest part of their work did not produce any result. When beginning at the 16th century rules were introduced, alchemy became science.
What were those rules? The fundamental laws of chemistry: the law of the conservation of mass by Lavoisier, the definition of compound by Proust, and the law of Dalton which introduced the atom.
Without laws, in whatever field of human knowledge, it is necessary to put rules or to establish conventions or reference points, otherwise we are in full chaos. Imagine how the street traffic would be without rules, that is without the highway code, without road signs and lights. In science also conventions are necessary. By convention hydrogen is indicated by the letter H, oxygen by the letter O, and water by H2O.
To measure the temperature of a body we need a thermometer. But to create a thermometer we must have reference points. These reference points are: the freezing point of water fixed at 0°C, and its boiling point at 100°C. We divide into a hundred parts this interval and we have the centigrade scale.
In astrophysics one wanted to understand what the universe would do: would it continue to expand or would it turn back? It was hence necessary to have a reference point, a source far away of light, and study its luminosity. For this purpose, it seems that the best sources of light are the supernovas of the group SNIa, called: standard candles. Studies of standard candles affirm that the universe is in continual expansion. Without reference points, without rules and conventions, one can go no way. Football also has its rules. Imagine football without rules: at the moment in which a penalty is about to be shot, the goalkeeper displaces the goal and puts it in the point of the corner. Rules whether agreeable or not must be everywhere. Only for artists, for their work, it is permitted not to have rules.
On the question of the origin of life, one starts with the ascertainment that life had its origin on our planet and that it could not have been a chance event. Prebiotic chemistry treats the formation of the fundamental substances for the synthesis of the macromolecules necessary to the origin of life in the prebiotic era. It is however difficult to establish what the physical-chemical conditions in prebiotic era were like. And so we cannot know with certainty if certain substances, the result of precise experiments, were or not present in the prebiotic era. There exists no rules, conventions, reference points; every researcher chooses his own, and so all are right. It seems absurd, but things are really like that, as confirmed by Orgel:
Leslie E. Orgel (Critical Reviews in Biochemistry and Molecular Biology, (39:99–123, 2004): «Prebiotic chemistry is concerned with molecules that are interesting to students of the origin of life which, they believe, could have been formed on the primitive Earth. Since we know very little about the availability of starting materials on the primitive Earth or about the physical conditions at the site where life began, it is often difficult to decide whether or not a synthesis is plausibly prebiotic. Not surprisingly, claims of the type, “My synthesis is more prebiotic than yours” are common. Nonetheless, there is fairly general agreement about the following restrictions on organic synthesis imposed by the requirement for prebioticity:
It must be plausible, at least to the proposers of a prebiotic synthesis, that the starting materials for a synthesis could have been present in adequate amounts at the site of synthesis.
Reactions must occur in water or in the absence of a solvent.
The yield of the product must be “significant,” at least in the view of the proposers of the synthesis.
Clearly “prebiotic” is a very elastic term, and it would not be wise to try to define it too closely».
Translated, the necessity for putting restrictions is recognized, but in fact the researcher is left free to choose his own restrictions. As every researcher is free to choose the prebiotic conditions, all the research is considered as linked with the origin of life and hence it is published. The result is a great number of publications, where every research worker obtains the substances fundamental for the origin of life in his own prebiotic ambience, often the fruit of fantasy and in contradiction with the prebiotic conditions of other scientists. Now it is evident that here there is a problem to be resolved; much the opposite of prebiotic chemistry, here we are in the presence of “prebiotic alchemy”.
Nucleotides are the constituents of the RNA. At the actual state of knowledge, the nucleotides in the prebiotic era could not exist and in fact they have never been obtained in experiments on prebiotic chemistry. Now, on the nucleotides and on the RNA, the absence of rules and of reference points, has led to the publication of some article which has no link with prebiotic chemistry. Some authoritative scientists, sustainers of the RNA world, have taken these articles as reference publishing the presence of nucleotides in prebiotic experiments.
It is sad to see authoritative scientists transmitting false information. And unfortunately, as all copy all, these information are even copied in test books for superior schools transferring false information to our students.
Now, coming from the above considerations it is evident how, to face the problem of the origin of life a new attitude is necessary. As suggested by Mario Ageno (Lezioni di Biofisica3,1984). «It is the same attitude that Urey inaugurated for research on the formation of the solar system: trying to gather, examining accurately, the actual state of things, all the indices and all the testimonies which can constitute conditions within limits for a future theory. On the basis of the indices and of the testimonies gathered, try to put together a sequence of possible events, that is which are not in contrast with our scientific knowledge,[…]».
Meanwhile the conditions within limits will impose restrictions which will have the function of “prebiotic razor”, eliminating all the elements which are in contradiction one with another. To reconstitute the sequence of the possible events, we can use: the simplest of the rules of logic,
1) The simplest of the rules of logic, that is hypothetic-deductive logic if…then, of the sort: if the petrol runs out, then the car is stationary.
2) Occam’s razor; William Ockham was a Franciscan Friar of the 14th century, to him is attributed the principle of Occam razor: one should always start from simple suppositions, obvious, and add afterwards complexity if necessary.
3) At length if this was not enough, we can follow the rule of common sense which as Descartes has written: common sense is the quality most frequently distributed among men.
In the end, as already said by Mario Ageno, condition within limits and possible events must not be in contrast with our scientific knowledge.
The primitive ambience: conditions within limits.
It is now certain that the earth has a life of about 4,6 billions of years. The most ancient sedimentary rocks which conserve testimony of forms of primitive life have an age of about 3,5 billions of years. Hence life must have appeared on earth in the interval of time of about 1 billion years in which there does not exist testimony. To try to understand how life could have had its origin from inorganic matter, we must try to find out what the ambience conditions were on the surface of the earth in the first billion of years.
Prof Mario Ageno has dedicated an entire volume to the reconstruction of time and ambiance of the appearance of life on earth: “Lezioni di biofisica 2, 1984”. The sources form which he gathered the information are more than 150 publications of various researchers which go from the beginning of the last century to almost 1980, that is the years of the most intense research on the argument. Such research extend from the formation of the earth to the epoch of the appearance of life and the primitive ambiance, in particular the origin and history of the ocean and the atmosphere. The conclusions to which Mario Ageno come and to which agree the greater part of scientists are the following:
1) Our solar system and hence our planet had their origin
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with the collapse of a gas nebula and dust 4,6 billion of years ago. It seems that the gravitational collapse lasted 1 million years. Hence in the sun was lit up a strong thermonuclear reaction which produced a solar wind, made up principally of protons and electrons, a millions times more intense than the actual one. The solar wind has blew away vapor and the gas residue of the nebula, which wrapped around the earth. Our planet hence remained without an atmosphere.
2) For at least another 100 millions of years the earth would have attracted comets and wandering bodies also of important dimensions, its surface was hence fused; catastrophical sporadic impacts are not excluded until almost 4 milliards of years ago. When this phase of accumulation has ended, the surface of the earth slowly begins to cool down and after another 100 millions of years about, it goes under the boiling point of water. The water carried by the comets or coming from the earth’s losing gas begins to condense.
3) The condensation of water has slowly given origin to a primitive ocean of about 1/10 of the actual one. With the exception of carbonates and silica the composition and the pH of the ocean were similar to the actual one. Phenomena of a local increase of the salinity due to evaporation of water, are not excluded.
With the presence of the ocean the water-cycle began, and it triggered a strong mechanism of feed-back. Such a mechanism made the average temperature of the planet, already about 4 milliards of years ago, equal to the actual one, and it maintained it more or less constant until our days. Phenomena of local overheating particularly close to volcanoes are not to be excluded.
5) It is very probable that the primitive atmosphere, restored by comets, meteorites and volcanism, was constituted by Methane (CH4), Ammonia (NH3), Water (H2O). and Hydrogen (H2). It has been evaluated that an atmosphere of such a composition maintained itself for about a half milliard of years; it is not excluded that there was the presence of other substances and compounds of sulphur. Oxygen was practically absent and hence the shield of Ozone (O3) was absent. The ultraviolet rays, in quantities much larger than today, attained, in the primitive ocean the depth of 10 m., destroying the organic substances and life which was forming. After about a half a million of years the primitive atmosphere was substituted by N2 (Nitrogen) CO (Carbon oxide) and CO2 (Carbon dioxide) of volcanic origin and H2O (Water vapor).
Further workers tried to put in crisis the conclusion of these studies, in particular for that which concerned the primitive atmosphere, in 1996 Miller defined this work as lacking in concrete data to sustain it.
6) Starting from these data, according to the majority of researchers, life had its origin on our planet about 4 milliards of years ago.
Memorandum
The medium
composition of a cell puts in evidence the presence: (Escherichia coli):
Water 70% mol. Weight medium in D. 18
Inorganic
ions
( K+, Mg++, Ca, PO43-,SO42-,…) 1% " 40
Carbohydrates 3% " 150
Amino acids
(precursors
of proteins) 0,4% " 120
Lipids 2% " 750
Nucleotides
(precursors
of nucleic acids) 0,4% " 300
Proteins 15% " 40.000
Nucleic
acids 7% " 106-109
The
proteins (like enzymes) control the metabolic reactions, they are made up of 20
different amino acids and except for glycine they are chiral, that is they
exist in two forms D (Dexter) and its mirror image L (Left).
The abiotic
synthesis of the amino acids (in laboratory, in industry, in prebiotic era)
gives origin to 50% of molecules D e 50% of molecules L.
In living organisms only the form L is present.
For nucleic acids we take into consideration the RNA.
The constituents of RNA are:
The phosphate group: (H2PO4)-.
A sugar, the Ribose
A phosphate
group , a ribose molecule and one of the
four nucleobases give origin to four different aggregations which take the name
of nucleotides. As an example we report the Adenosin-5-phosphat; the nucleotides
are the constituent of RNA.
In these geometric representations, if a vertex is
empty, the presence of an atom of carbon is understood. In the cytosine, for
example, the position 3 there is Nitrogen, but in position 4 there is nothing.
In the position 4, by convention carbon atom is omitted.
The formula
is hence in reality the following:
The nucleotide
Adenosin-5-phosphat,
The RNA
A) The substances fundamental for the origin of life had their origin on
our planet, or did they come for space.
What really
happened 13,6 milliards of years ago, we do not know, yet we know that it
happened and we called it the Big Bang. We also
know that, after 380 thousands of years from the Big Bang, when the temperature
of the universe at that time decreased by some thousands of degrees, the
electrons (-) and the protons (+) linked, giving origin to hydrogen (H), Helium
(He) and small quantities of Lithium (Li). The gravitational attraction between
the atoms of these elements gave origin to the first stars. It was inside the
massive stars, at a temperature
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of hundreds of millions of degrees through the nuclear fusion that, starting from hydrogen, other
chemical elements were formed and among these Carbon (C), Nitrogen (N), and
Oxygen (O), that is those elements that form the 99% of our body. The
gravitational collapse of these stars, at thousands of millions of grades,
completed the picture giving origin to
all the other natural elements. And so, for what concerns the elements, there
is no doubt: we are “sons” of stars.
But are we
also sons of space? That is the substance fundamental for the origin of life,
amino acids, sugars, and nucleobases come from space?
The idea,
expressed at first by Juan Orò in 1961 and taken up again in the 70’s by F.
Hoyle and C. Wickramasinghe, has a certain fascination in itself. As it has
been evident in the “conditions within
limits”, the primordial atmosphere, brushed away by the solar wind, was reintroduced on our planet, probably
by volcanism, by the impacts of meteorites of various dimensions and by the
impacts of comets. With time the impacts became less frequent but they never
ceased happening. Some of these meteorites, precipitated in the last two
centuries, had been gathered and studied. A total of more than a thousand
meteorites are conserved but only some, called “carbon condrite”, are
interesting for our ends. These meteorites dated 4,5 milliards of years ago,
had their origin during the formation of the solar system.
These
chemical analyses, done at the beginning of the 70’s, have shown the presence
of amino acids constituents of proteins. These amino acids present themselves
in the two forms D and L and hence surely are extraterrestrial and not biological.
Hydrocarbons (made up of H and C) have been found, with a molecular weight even
high.
In 1929
Haldane made the hypothesis that the primitive atmosphere, in our planet, was
constituted by NH3, CO2, H2 H2O and
traces of CH4. From these substances, with the input of energy, the
substances fundamental for the origin of life would have been formed.
In 1953 S.
Miller demonstrated that in atmosphere composed of CH4, NH3,
H2O, e H2 in presence of liquid water and simulating
lighting through electric discharges, many organic substances can be produced
and among these a large number of different amino acids, these also in the two
forms D and L. It is interesting to observe that the abundance of amino acids,
in relations with their molecular weight, in the meteorites and those obtained
in Miller’s experiments, present a certain coincidence. Some of these amino
acids are equal to those present in our proteins. Moreover also in Miller’s experiments
hydrocarbons of a composition like that of the hydrocarbons contained in
meteorites have been found. Successive studies both on meteorites and on
experiments of Miller’s type have demonstrated the presence of traces of
purines and pyrimidine. These substances
have a molecular structure fairly close to that of the nucleobases. In neither
of the two cases mentioned above was however discovered the slightest trace of
sugar and nucleobases, that is the constituents of the nucleic acid, whereas it
is now certain that amino acids are present. Hence from the depth of space,
arrives evidence of the chemical processes like those that happened on our
planet according to the “conditions within
limits” already exposed. But it still remains to be cleared if the organic
molecules of the meteorites already made part of the gas cloud and dust which
gave origin to the solar system or if these were formed during the condensation
phase of the cloud itself through inputs of energy.
Miller’s
experiment has inspired many other experiments, in which were used sources of
energy and mixtures of gas of different types. These experiments have
demonstrated that, in prebiotic conditions, one can obtain a large variety of organic
substances with a biological interest and, among these, formaldehyde (HCHO) and
hydrogen cyanide (HCN). One must not under valuate the importance of these
little molecules because they could have been the precursors of sugar, and
nucleobases and amino acids. In fact ribose, a fundamental molecule for RNA, is
a pentamer of formaldehyde (HCHO) in the sense that five molecules of
formaldehyde could give origin to a ribose molecule. And adenine, nucleobase
fundamental for nucleic acid, is a
pentamer of hydrogen cyanide (HCN), that is five molecules of hydrogen cyanide could
give origin to a adenine molecule. All this leads us to conclude that, on our
planet, the natural processes can produce the fundamental substances necessary
to the origin of life.
Around the
end of the ‘70’s, the idea of an extraterrestrial origin of the substances
fundamental for the origin of life, received a new impulse. In the interstellar clouds of gas (more or less ionized) and
dust, radio astronomers
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have discovered various simple organic substances and
among these formic aldehyde (HCHO) and hydrogen cyanide (HCN). A complete list
of all the organic substances (about 40) found in space until the beginnings of
the 80’s is contained in an article by Leo Blitz: Complessi giganti di nubi molecolari nella galassia, Le Scienze
1982.
However no
trace of molecules a bit more complex, important for the origin of life.
It was
thought that one could find them in the near future, but it was not clear in
what way these molecules could have arrived from space onto our planet. As
single molecules they would have been destroyed by the solarultraviolet, mortal not only for living organisms but also for the
molecules fundamental for the origin of life. If the molecules had been
contained inside asteroids fallen on our planet, they would have been destroyed
by the enormous heat given off by the impact, or remained prisoners inside the
fragments.
However,
around the middle of the 80’s, the majority of scientists implied in research
on prebiotic chemistry, was of the opinion that the extraterrestrial origin of
organic substances only demonstrated the ease with which these molecules can be
synthesized in the presence of carbon, hydrogen, azote and oxygen.
Such an
opinion was also well synthesized by Mario Ageno who concludes (Lezioni di
biofisica 3, 1984): «Even if a minimum fraction of it succeeded in the end in
surviving and in joining in some way the hydrosphere of the planet, it would be
difficult for the chemical evolution to go further, up to the appearance of
living organisms, without a continuous supply and that of really different
magnitude, of substances of new synthesis and local origin».
The
discovery of organic molecules in space, implied however a reflection. If the
interstellar clouds contain prebiotic organic molecules, these must have been
present also in the cloud which gave origin to the solar system. It was hence
thought that, during the formation of the solar system, in the planets close to
the sun, because of the enormous heat, these molecules were destroyed. They
were however saved in the colder zones, that is at the limits of the solar
system, where they were incorporated in the comets.
Hence was
diffused, among scientists, the conviction that comets were a residue of the
nebulous which gave origin to the solar system. The hunt after the comets begins.
Around the
middle of the 90’s in the coma of the comets
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Hyakutake and Hale-Bopp, organic
compounds are discovered and among these formic aldehyde (HCHO) and hydrogen
cyanide (HCN).
However
still no trace of molecules a bit more complex, important for the origin of
life. Yet it has been observed that, when in the interstellar space water
molecules, methanol, ammonia and hydrocarbons are deposed on silica dust,
frozen grain are formed. According to some scientists, inside these grains,
organic substances could have been accumulated, complex ones included, and the
layer of ice would have protected them from the ultraviolet. Other researchers
hold that, if the layer of ice had not been sufficiently thick, the organic
substances would have been broken by the ultraviolet rays. The residues, as
they could not disperse into space, could have reacted early or late giving
origin to more complex molecules. When they enter into the atmosphere, the
organic substances contained in the cavities of the granules, are protected
from overheating. Frozen granules have been reproduced in laboratory, and named
analogous to frozen granules. In these analogous granules have been
individuated simple organic substances like ketones,
ethers and alcohols.
However
when we reread the article of M. Bernstein, A. Sandford and J. Allamandola: Dallo spazio le molecule della vita (Le
Scienze, 1999), apart from the emphasis, understandable for one who works at
the NASA, There is no trace of molecules important for the origin of life.
In 2002,
twenty years after the publication of the article by Leo Blitz and hence after
another twenty years of research in radio astronomy, is published by P.
Ehrenfreund and others: “Astrophysical and astrochemical insight into the
origin of life, (Rep. Prog. Phys. 65 (2002)
1427-1487, a list up to date (about 70) of organic substances found by radio
astronomy in space. Among these substances Ehrenfreund includes Glycine, a very
simple amino acid that makes part of our proteins, but he adds an
interrogation.
Also in
this new list there is not yet the trace of complex molecules important for the
origin of life. In the same article Ehrenfreund publishes the list of the
substances found in the comets Hyakutake and Hale-Bopp. All these substances were already found in the
interstellar space. Hence it does not seem that inside the comets particular
reactions of synthesis take place. In the Ehrenfreund table, Glycine is still
included but this time without any question mark. Reading through his comment
on the table, he writes: «Glycine, the simplest amino acid, has not yet been
detected, […]». In 2006 was brought to earth the dust of the comet Wild 2, taken
by the Stardust mission . Analysis has
shown the presence of amine and molecules made of long chains of carbon atoms.
In 2009, after three years of the publication of the first data, NASA announces
with much emphasis, that in the dust of Wild 2, when the analysis had been
redone, also Glycine was discovered.
What can
one conclude? Finally they did it.It is opportune, to conclude, to add some points, starting with some parameters.
The massive
stars, that form inside the immense clouds of gas and dust, have an average
life of about 3 million years and the greater part of their energy they give it
out under form of ultraviolet rays. Also our sun gives out ultraviolet rays
which would destroy amino acids and nucleobases up to the threshold of the
solar system. When the sun had its origin, the intensity of radiation was a thousand
times superior to the actual situation.
Space is hence permeated by rays lethal not only for the living organisms, but
also for molecules important for the origin of life. In the interstellar clouds
matter is extremely rarefied. At the temperature of -260°C the average density
is 100 molecules of hydrogen per cm3 and small fractions of azote,
oxygen, and carbon. These gas, atomized or ionized by the ultraviolet rays,
through casual impacts have given origin to very simple substances like
methane, ammonia, water and formaldehyde, hydrogen cyanide, ammines too. A part
of these substances is destroyed by ultraviolet rays to reform later in other
places. Now the molecules fundamental for the origin of life like amino acids,
sugars, and nucleobases, even if they are molecules with a molecular weight
relatively low, they are not so simple, but have their own complexity. What is
the probability that molecules of this type could have formed in space, from
atoms and ions extremely rarefied through casual impacts and at such a low
temperature and what is the probability that they could have resisted against
the ultraviolet rays? Descartes’ good sense suggests that such a probability is
practically zero. And then, even if some
molecule useful to the origin of life should form, what could be its
usefulness? The quantity of molecules necessary to the origin of life is of
such an order of magnitude that space should be permeated, and instead space is
permeated with rays which destroy these molecules. And so for what concerns
substances fundamental for the origin of life, we are not “sons” of space.
Space with
its ultraviolet rays is the dwelling of death.
The first
living organisms on earth knew something about that, being obliged to live in
mud or in water deeper than 10 meters. Only after two milliard years, when
microorganisms had learned to produce oxygen, did they succeed in strapping
from death some kilometer of space and they could occupy the surface and the
atmosphere of our planet.
However, living
organisms exist thanks to the fact that space is the kingdom of death. Life was
possible on earth thanks to the fact that space was sterilized by the
ultraviolet rays.
The organic
substances known by us are now, on the earth, more than 1,5 million. Imagine if
in space, through casual impacts and in the absence of ultraviolet rays, the
substances fundamental for the origin of life had originated. But then by
casual impact also all the sugar, that is tens of sugars, and all the
nucleobases possible, perhaps hundreds, and then hundreds of thousands of organic substances, Having arrived on earth,
in an oceanic melting pot, from these substances only a miracle could have
given origin to life. Life needs regulating principles. Without regulating
principles there is no life. And these regulating principles do not exist in
space but they can exist only on earth or on similar planets. And then, organic
molecules which radio astronomy finds in space are nothing but ashes of an over
lasting cremation thanks to which life has been able to originate on earth.
That which
has been exposed above makes one conclude that space is not the seat of the
origin of the substances fundamental for the origin of life.
The comet
impacts on our planet have restored in the prebiotic epoch the primitive
atmosphere; they have added water to our planet and probably also some simple
organic substances like formic aldehyde, hydrocyanic acid, etc.
Probably
during the phase of contraction of the primitive cloud, from compounds present
in the cloud like methane, Ammonia, Hydrogen and water, through addition of
energy, many organic substances were produced, and among these the amino acids.
Some of these amino acids were imprisoned in the meteorites, other have been
destroyed by the solar ultraviolet and the high temperature of contraction of
the cloud.
Therefore,
the conclusion to which scientists had come to in the 80’s is still valid: natural
processes, in prebiotic conditions, on our planet, are able to produce the
substances fundamental for the origin of life, and the contribution of simple
organic substances, coming from space, if ever it existed, is to be considered
without influence.
We are “sons”
of the earth.
B)
When can a chemical reaction, realized in laboratory, be defined prebiotic?
One of the
scopes of prebiotic chemistry, is that of understanding through what processes
were formed the constituents of the proteins and
of the nucleic acids in the prebiotic era. A
corollary of this scope is the question: how does one establish if a reaction,
realized in a laboratory equipped for such a scope, can be considered plausible
also in the prebiotic era? It results that without a minimum of rules, there is
no answer to this question. To establish these rules, it is however necessary
to start with concrete examples. These examples are the first publications
which concern the origin of the constituents of the nucleic acids. Such
examples, for the time being, are exposed in a synthetic way, but in the next
article they will be analyzed in detail.
In 1965 is
published by C. Ponnamperuma e R. Mack: Nucleotide synthesis under possible primitive
earth conditions (Science (1965), 148,3674): «[…]An
aqueous solution of nucleoside and phosphate was lyophilized in a Pyrex tube
and the tube then sealed off and heated to 160° for 2 hrs.». The experiment
consisted in causing a reaction between a nucleoside
(that is a ribose linked to a nucleobase) with a phosphate to obtain the nucleotides. The experiment gave origin to nucleotides and created much interest,
so much so that it was quoted by many researchers as a probable way of
synthesis of nucleotides in the prebiotic era. From one quotation to another,
it was at length inserted on a text book for students where, the way of
synthesis, from probable became certain. As ten years and more passed, doubt
was born on the prebiotic significant of the experiment. Tens of researchers
fixed themselves in experiments to render it still valid; but finally in 2004
L. Orgel, (Critical Reviews in Biochemistry and Molecular Biology, 39:99–123,
2004), with extreme fair play, concluded that the synthesis of the nucleotides
still proposed today are not convincing.
But why
Ponnamperuma’s experiment is not convincing?
Because
Ponnamperuma and all the other researchers who followed him had bought the
nucleosides in specialized laboratories, but no one knows how the nucleosides
were formed
in the prebiotic era. The different sorts of nucleosides are perhaps hundreds;
if in the prebiotic era they had spontaneously originated, there would have
been a mixture of hundreds of different nucleosides. But the experiments above
mentioned did not use a probable mixture of this type, no, the researchers
acquired the right ones.
In the
prebiotic era, the indication of the existence of the nucleosides and the
phosphate used in the experiment, are zero.
And so for
forty years discussions were made about nothing.But why, how was it possible? Because of the lack of the “rules of procedure”.
From this experience we can however extract the first procedural rule.
All the
reactions that do not adapt themselves to this first rule, can be interesting
laboratory experiments, but they have no prebiotic value.
In 1961
Juan Orò, one of the most engaged in research on prebiotic chemistry,
publishes: Synthesis of purines under possible primitive earth conditions. I.
Adenine from hydrogen cyanide. The Adenine was
obtained heating at 70°C a high concentration of cyanide acid (HCN) in the
presence of ammonia (NH3). In this experiment were obtained many
organic substances and, among these, Adenine with a concentration of the order
of 0,5%. Also this experiment was the object of a great interest and was widely
quoted. Now, in the prebiotic era, there are strong indications of the presence
of NH3 and of HCN and hence the experiment follows the first rule of
procedure. But HCN is a gas, as also NH3.
And then, how could one obtained such a high concentration of cyanide acid, in
an ammonia ambience and at the temperature of 70°C, in prebiotic era? Orò does
not explain it but, after the publication of his article he expresses the idea
that the organic substances necessary for the origin of life would have come
from space.
The
suspicion comes that he himself was not filly convinced by his experiment.
Those who
believed, on the other hand, were many other researchers who sought how it was
possible to obtain high concentrations of HCN in the prebiotic era.
In
laboratory experiments it was discovered that at the temperature of -23,4°C and
at a high concentration, 74,5 mole in % of HCN (about 800g/L), after a certain
time a polymer of HCN forms. This compound, extracted and treated with water,
gives origin to small percentages of
adenine. One must be careful though: if the temperature is some degree higher
the polymer does not form end goodbye to adenine. Moreover, a solution of this
concentration takes the name of an “eutectic solution”.
We must be careful also here, because this means that if the temperature lowers
another degree, the solution freezes entirely, immediately, the polymer can no
longer form and goodbye adenine.
How it
would have been possible for such restrictive conditions to be realized on our
planet in the prebiotic era, the researchers do not say.
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|
centrostudilaruna.it
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And all of us
imagine a planet for long periods, perhaps for tens of thousands of years, at
the temperature of -23°C, not a degree more not a degree less.
Or else to
imagine a primitive Artic where under thick layer of ice the useful
concentration and the temperature of -23°C were attained, neither a grade
higher nor a grade lower.
But in this
case, what should have been the temperature of the atmosphere, -150°C or -200°C
.
Also in the
precedent example, Ponnamperuma should have made clear what, in prebiotic era,
could have to lyophilize the solution, by what
process one could substitute the Pyrex tube hermetically
closed and how would take place the heating at 160°C.
But
Ponnamperuma does not say it.And at the end we conclude that this is not a serious way of proceeding.
From this experience we can however extract the second rule of procedure.
2) A chemical reaction, realized in
laboratory, can be defined prebiotic if it takes place inside the ambient
conditions of the primitive earth. If
the laboratory experiment takes place in ambient conditions and concentration
of the reagents improbable in the primitive earth, then the author must explain, in a convincing way, how his
experiment in the laboratory can be translated in the prebiotic era.
All the
reactions which do not adapt also to this second rule can be interesting
laboratory experiments but they are not of prebiotic value.
These
minimum rules to define prebiotic a reaction carried out in a laboratory, can
certainly be criticized and improved or changed but the rules are certainly
necessary.
Without
procedural rules, one risks to reason in emptiness for another 50 years. And
above all, as shown by the above examples, young researchers risk working for
years on research whose result is already known: zero
C)
In a chemical reaction, realized in a laboratory, what must be the
concentration of the reacting substances, and what must be its rendering so
that it be defined prebiotic
As we have
already observed at the precedent point, prebiotic chemistry has the scope of
understanding through what processes were formed the constituents of the proteins and the constituents of the nucleic acids in the prebiotic era. The problem is
that in all the laboratory experiments which simulate these processes, likely
or unlikely, the quantity of the
products obtained is very small. As we now know, Miller has demonstrated that
the amino acids form easily through an abiotic
way. Now, let us imagine that these amino acids were formed in the primitive
atmosphere. These, transported in a primordial ocean, would have given origin
to a “prebiotic mixture” very diluted. In such conditions, how would the amino
acid molecules meet to react and give origin to the first peptides? Christian
De Duve has exposed clearly the problem in “Polvere vitale” 1998: «[…] Let us
imagine a brief sequence of 3 steps from A to B, from B to C, and from C to D
each with a rendering (high for a prebiotic reaction) of 1%. In terms of A the
rendering of B will be 0,01%, that of C 0,0001% and that of D 0,000001%, that
is of one for a million. Even in the best conditions, chemists must fight
against this sort of tendency to evanescence». The fact is that we need at
least thirty steps for the proteins, and for the nucleic acids some hundred, evanescence
would already be a good result. And so, how does one resolve the problem and
what do scientists think of it?
On this
question we can divide scientists in three groups.
For the
first group the problem does not exist.
To this
group belong all those who follow the line of thought of J. B. Haldane and
Jacques Monod. According to these scientists, life would have had its origin by
chance. Some insist by chance of a probabilistic sort, others think of a chance
as almost miraculous. And if life had its origin by chance, why should we
preoccupy ourselves with the concentration of its constituents?
The second
group feigns that the problem does not exist.
To this
group belong the evolutionists fascinated by the “self-reproducing” of Dawkins
and some of the researchers who believe in the “RNAWorld”. For these researchers the primordial mixture is a fortress to be
defended. Where can exist populations of self-reproducing molecules of RNA in
competition for the availability of food, if not in the primordial mixture?
Without the primordial mixture the theory collapses. The problem hence is
avoided. Manfred Eigen in fact speaks of hydric basins, that is of a variety of
ambiences and where at least one of these would have to be adapted for its
concentration and temperature to the origin of life.
For the
third group, the problem, on the other hand, the problem exists.
But then,
if the problem exists, how shall we solve it?
To suggest
a solution, J. D. Bernal was the first, in 1951. As it is known argil are
formed of various crystalline strata superposed. Every stratus is made up of
two under strata, one of silica
tetrahedron (Si2O52-)n and the
other of hydrated alumina octahedron [Al2(OH)42+]n.
Without going in too much detail, for the presence of electric charges, the
various strata or under strata neutralize at a distance. Between one under strata and another their remain
hence empty spaces where water molecules or molecules that have electric
dipoles can lodge themselves.
It has been
calculated that in 1 cm3 of argil, the surface of these empty spaces
is almost equal to the surface of a football field. Because most molecules
fundamental for the origin of life have electric dipoles, Bernal proposed that
these molecules could have accumulated in the argil.
Other
researchers took into consideration pools of water at margins of continents and
subject to continual evaporation which could have concentrated the fundamental
molecules (Chemical evolution and the origin of life, R. E. Dickerson, Le
Scienze, 1978). Miller and Orgel proposed the cryogen methods, that is
eliminating water under the form of ice crystals. In such a way that the
solution became always more concentrated (article already quoted). However,
those who affirm that the problem exists, are convinced that some process of
accumulation of such substances must have happened. Attention though, because
some of the propositions contain a trap.
Let us take
the example of the pools of water at the margins of continents. And if the
evaporation does not reach the right point? What does one do? One waits for
another round? We shall say! In the arc of thousands or millions of years,
everything is possible. All right, but there always remains local and isolated
processes which can take place or not take place, or happen without attaining
the right point. Moreover the pools surely contained Miller’s amino acids, that
is D and L. In what way would they have been separated? The researchers do not
say. And if the pools of water contained the amino acids of Miller, these were
about 60.The amino acids which constituted the first proteins were perhaps no
more than 15. In what way proteins could have been formed excluding all the
others? The researchers don’t say.
And hence,
here the trap hides itself. Behind the appearance
of a natural process,
evaporation or crystallization, in reality resurges like the Phoenix, another time by chance. Hence, if we do not
want to fall once more in the trap of chance, the process of accumulation could
not have been sporadic and local. It must have been a process which had to realize itself at any moment, in the
sense that one could not wait for heat or cold, for the sun or the stars. This
because during the waiting the important substances could surely have participated
in secondary reactions or could have been destroyed. Moreover it must have been
a fairly diffused process on the whole surface of the planet. This because a
local event at that epoch was exposed with so many vicissitudes to render it
casual. A local and sporadic process, in fact casual, would reduce also the
possibility of life in other planets.
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| it.wikipedia.org |
Now, if a process of accumulation is all the
same necessary, then what the sense
of speaking of the rendering of a reaction? A constituent can be obtained with
a low rendering (or to use a chemical term a
trace), through some process it is accumulated and can reach also elevated
concentration.
On the
question of concentration of the substances fundamental for the origin of life,
it is impossible not to mention that which in biophysics is called: the problem
of phosphorous.
This
element is found in nature under the form of minerals of volcanic origin, the
apatite (calcium phosphate Ca5(PO4)3[F, OH,
Cl]) which, under the form of very small crystals are disseminated on all the
surface of the planet. We have no element which can indicate that the situation
in prebiotic era was different from now. The apatite however are not very
soluble and their concentration in the water is 30 µg/L (µg/L millionth of grams for litre). To give origin to
chemical reactions a concentration of this type is for chemists really
insignificant. And yet phosphorous, under the form of phosphate, enters to make
part of the structure of the nucleic acids, it is fundamental in the transfer
of energy participating in many metabolic reactions. Some researchers have
shown how the characteristics of phosphorous are unique and it does not seem
that some other element can substitute it in living organisms. One hence
concludes that it is a substance indispensable to life, even if at low
concentration but diffused on all the surface of the planet, it becomes all the
same co-opted. It hence seems that for life and for its origin the extreme
diffusion of a substance is irrelevant. To be clear, here one does not mean
that phosphorous was accumulated, but simply, as we shall see in the next
article, that perhaps a problem of phosphorous does not exist.
Hence a fundamental
substance for the origin of life can also be at a concentration very low and
undergo accumulation processes, or also as in the case of phosphorous, as it is
indispensable to life, it is anyway co-opted.
From what
is written above we can conclude with the third and last rule of procedure.
3) A chemical reaction, realized in laboratory, is
defined prebiotic even if the concentration of the reagents and the rendering
of the products are very low. That because in any way a process of accumulation
is necessary. Reaction and process of accumulation however must be realized, in
prebiotic era, at any moment and in a diffuse way on the surface of the planet.
Once the
primitive ambiance has been identified, we can recapitulate
Corollary:
1st procedural rule
A chemical reaction, realized in laboratory, can be
defined prebiotic, if for the reacting substances, there exist strong
indications of their presence in the prebiotic era.
2nd procedural rule
A chemical reaction, realized in laboratory, can be
defined prebiotic if it takes place inside the ambient conditions of the
primitive earth. If the laboratory
experiment takes place in ambient conditions and concentration of the reagents
improbable in the primitive earth, then
the author must explain, in a convincing way, how his experiment in the
laboratory can be translated in the prebiotic era.
3rd procedural rule
A chemical reaction, realized in laboratory, is
defined prebiotic even if the concentration of the reagents and the rendering
of the products are very low. That because in any way a process of accumulation
is necessary. Reaction and process of accumulation however must be realized, in
prebiotic era, at any moment and in a diffuse way on the surface of the planet.
