Prebiotic chemistry: In the beginning, there was sugar

Date:
October 19, 2020
Source:
Ludwig-Maximilians-Universita"t Mu"nchen
Summary:
Organic molecules formed the basis for the evolution of life. But
how could inorganic precursors have given rise to them? Chemists
now reports a reaction pathway in which minerals catalyze the
formation of sugars in the absence of water.



FULL STORY ========================================================================== Organic molecules formed the basis for the evolution of
life. But how could inorganic precursors have given rise to
them? Ludwig-Maximilians-Universitaet (LMU) in Munich chemist Oliver
Trapp now reports a reaction pathway in which minerals catalyze the
formation of sugars in the absence of water.


==========================================================================
More than 4 billion years ago, the Earth was very far from being the Blue Planet it would later become. At that point it had just begun to cool and,
in the course of that process, the concentric structural zones that lie
ever deeper beneath our feet were formed. The early Earth was dominated
by volcanism, and the atmosphere was made up of carbon dioxide, nitrogen, methane, ammonia, hydrogen sulfide and water vapor. In this decidedly inhospitable environment the building blocks of life were formed. How
then might this have come about? Researchers have puzzled over the
question for decades. The first breakthrough was made in 1953 by two
chemists, named Stanley Miller and Harold C. Urey, at the University
of Chicago. In their experiments, they simulated the atmosphere of the primordial Earth in a closed reaction system that contained the gases
mentioned above. A miniature 'ocean' was heated to provide water vapor,
and electrical discharges were passed through the system to mimic the
effects of lightning. When they analyzed the chemicals produced under
these conditions, Miller and Urey detected amino acids -- the basic constituents of proteins - - as well as a number of other organic acids.

It is now known that the conditions employed in these experiments did
not reflect those that prevailed on the early Earth. Nevertheless,
the Miller-Urey experiment initiated the field of prebiotic chemical
evolution. However, it not throw much light on how other classes of
molecules found in all biological cells -- such as sugars, fats and
nucleic acids -- might have been generated.

These compounds are however indispensable ingredients of the process that
led to the first bacteria and subsequently to photosynthetic cyanobacteria
that produced oxygen. This is why Oliver Trapp, Professor of Organic
Chemistry at LMU, decided to focus his research on the prebiotic synthesis
of these substances.

From formaldehyde to sugar The story of synthetic routes from smaller precursors to sugars goes back almost a century prior to the Miller-Urey experiment. In 1861, the Russian chemist Alexander Butlerov showed
that formaldehyde could give rise to various sugars via what became
known as the formose reaction. Miller und Urey in fact found formic
acid in their experiments, and it can be readily reduced to yield
formaldehyde. Butlerov also discovered that the formose reaction is
promoted by a number of metal oxides and hydroxides, including those
of calcium, barium, thallium and lead. Notably calcium is abundantly
available on and below the Earth's surface.

However, the hypothesis that sugars could have been produced via the
formose reaction runs into two difficulties. The 'classical' formose
reaction produces a diverse mixture of compounds, and it takes place
only in aqueous media. These requirements are at odds with the fact that
sugars have been detected in meteorites.

Together with colleagues at LMU and the Max Planck Institute for
Astronomy in Heidelberg, Trapp therefore decided to explore whether formaldehyde could give rise to sugars in a solid-phase system. With a
view to simulating the kinds of mechanical forces to which solid minerals
would have been subjected, all the reaction components were combined in a
ball mill -- in the absence of solvents, but adding enough formaldehyde
to saturate the powdered solids And indeed, the formose reaction was
observed and several different minerals were found to catalyze it. The formaldehyde was adsorbed onto the solid particles, and the interaction resulted in the formation of the formaldehyde dimer (glycolaldehyde)
-- and ribose, the 5-carbon sugar that is an essential constituent of ribonucleic acid (RNA). RNA is thought to have merged prior to DNA,
and it serves as the repository of genetic information in many viruses,
as well as providing the templates for protein synthesis in all cellular organisms. More complex sugars were also obtained in the experiments,
together with a few byproducts, such as lactic acid and methanol.

"Our results provide a plausible explanation for the formation of sugars
in the solid phase, even under extraterrestrial settings in the absence
of water," says Trapp. They also prompt new questions that may point to
new and unexpected prebiotic routes to the basic components of life as
we know it, as Trapp affirms. "We are convinced that these new insights
will open up entirely new perspectives for research on prebiotic,
chemical evolution," he says.


========================================================================== Story Source: Materials provided by
Ludwig-Maximilians-Universita"t_Mu"nchen. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Maren Haas, Saskia Lamour, Sarah Babette Christ, Oliver
Trapp. Mineral-
mediated carbohydrate synthesis by mechanical forces in a primordial
geochemical setting. Communications Chemistry, 2020; 3 (1) DOI:
10.1038/ s42004-020-00387-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201019125356.htm

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