Absolute Configuration of Aliphatic Hydrocarbon Enantiomers Identified by Gas Chromatography: Theorized Application for Isoprenoid Alkanes and the Search of Molecular Biosignatures on Mars

Guillaume Leseigneur,Jean-Jacques Filippi,Uwe Meierhenrich,Nicolas Baldovini

doi:10.3390/sym14020326

Guillaume Leseigneur, Jean-Jacques Filippi + Show 2 more

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https://doi.org/10.3390/sym14020326

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Abstract

Acyclic saturated hydrocarbon enantiomers were resolved by gas chromatography using a β-cyclodextrin-based chiral stationary phase. The stereospecific synthesis of single enantiomers of 4-methyloctane allowed to assign the absolute stereochemical configuration to the resolved enantiomers. Data show that the (S)-4-methyloctane shows higher chromatographic retention as compared to the (R)-4-methyloctane due to stronger van der Waals interactions with the β-cyclodextrin chiral selector. This introductive research presents future prospects for the separation of stereoisomers of larger branched hydrocarbons. We discuss the importance of chiral hydrocarbons, more precisely the stereochemistry of the isoprenoid alkanes pristane and phytane, as potential biosignatures stable on geological timescales. The origins of pristane and phytane in Earth sediments are presented, and we detail the implications for the search of extinct or extant life on Mars. The data presented here will help to systematically investigate the chirality of hydrocarbon enantiomers in biological and nonbiological samples and in samples to be analyzed by the ESA’s ExoMars rover to trace the chiral precursors of life in 2023.

Highlights

Introduction and Eve BergerThe stereochemistry of chiral alkanes is of significant importance in various scientific fields [1,2]
Isoprenoid hydrocarbons are abundant in biology, often in the form of the hydrophobic side chains of an active molecule with one or several functional groups: this includes the main core membrane lipids of methanogenic and halophilic archaea [4,5,6], most chlorophylls including those used by algae and cyanobacteria [7] and tocopherols that are relatively abundant in most photosynthetic organisms [8]
The results presented here prepare for the chirality experiment onboard the ExoMars mission that is capable to search for the chirality of amino acids and sugar molecules, but as described here hydrocarbon enantiomers to distinguish their biological versus nonbiological origin

Summary

Introduction

Introduction and Eve BergerThe stereochemistry of chiral alkanes is of significant importance in various scientific fields [1,2]. Chirality and absolute configuration of branched hydrocarbons are, often unknown and neglected, because hydrocarbon enantiomers lack functional groups, cannot be derivatized into diastereoisomers and are thereby difficult to resolve, to quantify, and to identify by enantioselective chromatographic and NMR techniques. They bear no classic chromophores that often are the basis for attributing their absolute configuration. Its polymerization can form cyclic compounds and repeating patterns of branched aliphatic carbon chains every four or five carbons (head-to-tail or tail-to-tail linkage, respectively) This often gives rise to multiple stereocenters in the resulting isoprenoid. Isoprenoid hydrocarbons are abundant in biology, often in the form of the hydrophobic side chains of an active molecule with one or several functional groups: this includes the main core membrane lipids of methanogenic and halophilic archaea (archaeol) [4,5,6], most chlorophylls including those used by algae and cyanobacteria [7] and tocopherols that are relatively abundant in most photosynthetic organisms (see Figure 1a–c) [8]

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