The prebiotic origins of sugars and their polymers pose a challenge to our current understanding of the chemical origins of life. Historically, formaldehyde has been considered as the only starting material and formose reaction as the only pathway for the prebiotic synthesis of various sugars – in spite of the enormous complexities and, difficulty in taming its inefficiency and varied product distribution. Work within the Center for Chemical Evolution (CCE), based on the glyoxlate scenario, has focused on, an alternative source molecule (glyoxylic acid) and has demonstrated a different and viable pathway for the production of sugars.  This “glyoxylose” pathway is initiated by the reaction of glycolaldehyde with glyoxylate and proceeds, via aldol reactions, carbonyl migrations, and decarboxylations, to produce only linear sugars and sugar acids with no tars. A prebiotically plausible synthesis of glyoxylate from the reaction of glycine with formaldehyde and its interception with dihydroxyacetone to produced sugar acids has also been demonstrated. As the next step towards a plausible prebiotic origin of polysaccharides, condensation of sugars was investigated. Using glucose as a model, CCE researchers demonstrated that the acid-catalyzed condensation of aqueous solutions of glucose employing a wet-dry (night-day) protocol with 0.01 M HCl at 50 <sup>o</sup>C leads to the formation of oligosaccharides containing up to eight monomeric units identified by high resolution mass spectrometry. Employing NMR spectroscopy, ten out of eleven possible acetal linkages, including α- and β- anomers, were identified with the α- and β- 1,6-acetals being the dominating the product distribution. That this product distribution was a consequence of thermodynamic control was supported by the study of acid-catalyzed oligomerization of several glucose disaccharides which all showed a similar partitioning of products and linkages. Work is continuing within the CCE based on the hypothesis that oligosaccharides that are produced via such wet-dry cycles could have been readily available to facilitate prebiotically-relevant chemical transformations or impact the properties of other classes of molecules, by (a) direct participation in chemical conversions and becoming part of structural scaffolds and/or (b) altering the physical properties of the environment (e.g. lowering water activity) by acting as gelling agents or viscogens.<br/>

Life is sweet: Prebiotic relevance of studying the abiotic evolution of sugars and oligosaccharides

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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