Life on Earth: UV and HCN

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  • Published: Nov 1, 2017
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Life on Earth: UV and HCN

Pro-biotic chemistry

UV-Vis spectroscopy has revealed that the experimental conditions for the polymerization of aqueous HCN (hydrogen cyanide) affect the kinetics of the reaction. Given the purported role of HCN in the prebiotic organic chemistry of our planet, such insights might ultimately have implications for our understanding of how life on earth began.

UV-Vis spectroscopy has revealed that the experimental conditions for the polymerization of aqueous HCN (hydrogen cyanide) affect the kinetics of the reaction. Given the purported role of HCN in the prebiotic organic chemistry of our planet, such insights might ultimately have implications for our understanding of how life on earth began.

Margarita Marín-Yaseli, Miguel Moreno, Carlos Briones, and Marta Ruiz-Bermejo of the Centro de Astrobiología (INTA-CSIC), in Madrid, Spain, and colleague José de la Fuente of the Instituto Nacional de Técnica Aeroespacial "Esteban Terradas" (INTA), also in Madrid, explain that the polymerization of HCN is one of the most important chemical transformation in prebiotic chemistry. "HCN polymers are," the team explains, "plausible prebiotic precursors of biomonomers such as amino acids, keto- and carboxylic acids, purines, pyrimidines and pteridines." HCN polymers may also have an important and growing role to play in modern materials science where they are being investigated as advanced coatings for biomedical applications, for instance. And yet, the kinetics of this reaction are not well known.

The team further explains that our current models of atmospheric chemistry suggest that the concentration of HCN reached just tens of parts per million by volume in the lower atmosphere of the early Earth while the oceans of the time contained just 2 × 10-6 molar HCN solution at pH 8 and 0 degrees Celsius. "These calculated concentrations are very low for water polymerization of HCN," the team suggests. Therefore, the team adds, if HCN oligomerization or polymerization were relevant to the emergence of primordial biomolecules, there must have been other routes by which it could be synthesized under such atmospheric or oceanic conditions.

Cyanide and life

Writing in the journal Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, the team describes a study, which they say is a first approach to the kinetics of two sets of aqueous HCN polymerizations from ammonium cyanide, NH4CN, and sodium cyanide, NaCN. The study they hope will help take us a step closer to understanding the role of prebiotic HCN polymerization, they explain that HCN cannot pre-concentrate to the levels necessary for polymerization but the salts of the compound might do so through precipitation. They have thus looked at reactions of the salts of HCN occurring at mid-temperature, between 4 and 38 degrees Celsius, the team points out. For each reaction series, the team investigated whether or not the presence of air and salts in the reaction medium would have an impact on the reaction kinetics. Indeed, they have explored a range of reaction conditions systematically, monitoring the sol fractions using UV-Vis spectroscopy and the kinetics of the gel fractions with gravimetric analyses.

The team explains that UV-Vis spectroscopy is commonly used to study initiation or catalysis and the consumption of monomers in a polymerization. However, in the present research UV-Vis was used because the reactions were carried out in solution. The technique provides an easy, rapid, and straightforward way to monitor the processes directly, the team says.

UV test

The UV-Vis spectra and the gravimetric data led the team to similar conclusions about the kinetics of each series of reactions. The kinetics of what is a complex system shows a marked difference in the linear dependence of the polymerization reaction with temperature for ammonium cyanide (the ammonium also has a catalytic effect) when compared to the sodium cyanide series. The data also reveal the influence of air, oxygen, and the saline medium in the HCN polymerization reaction.

"To the best of our knowledge, this is the first systematic kinetics approach to the study of HCN polymer formation in aqueous solutions at middle temperatures based on UV-Vis spectra from the sol fractions, and from the conversion data from the gel fractions," the team concludes.

Related Links

Spectrochim Acta A 2018, 191, 389-397: "Experimental conditions affecting the kinetics of aqueous HCN polymerization as revealed by UV–vis spectroscopy"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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