Abstract

With the observation of liquid water and active geology on the Jupiter’s moon Europa and Saturn’s moon Enceladus, solid organic matter on Titan, water ice in the shallow subsurface of Mars, and the expected increase in the number of observed exoplanets, planetary exploration is making fundamental steps in the search for life in the Universe [1]. However, current strategies for biosignatures detection can be affected by the underlying assumption that life elsewhere in the Universe would be based on a chemistry similar to the terrestrial one. Recent works on new strategies for the detection of universal life biosignatures suggest a paradigm shift from the traditional spectroscopic identification of known biomolecules to new approaches based on geometrical/topological complexity measures of the molecules or biologically-derived objects.

Being the building units of proteins, amino acids are high priority targets in the search for biosignatures, either in the gas phase as formed by lightening in planetary atmospheres, or in condensed assemblages in mineral matrices on planetary surfaces, icy crusts of outer Solar System’s Moons or icy mantles of small bodies. However, meteoritic samples exhibit a large set of amino acids, most of them unknown to our biosphere [2,3], suggesting that amino acids are not unambiguous indicators of life. Here we present an investigation of the underlying link between complexity, chirality and physico-chemical properties of amino acids of the isoleucine series, found with large enantiomeric excess in Antarctic meteorites. Such series features amino acids which are either involved in protein synthesis, or featured in human plasma, or external to our biosphere. For both the gas and the condensed phase, we analyze, via Density Functional Theory, wavefunction-based approaches and perturbation theory, the link between the electronic properties (the HOMO -LUMO and the band gap), a complexity measure informing on the delocalization of the electronic cloud [6], a chirality measure (as the distance for each amino acid to the closest achiral object) and the H-bonding network. The results [7] show that: a) upon condensation, L-isoleucine (involved in protein biosynthesis) gains in complexity and chirality w.r.t. to the gas phase; b) its complexity is slightly higher than the value for D-allo-isoleucine (not present in living systems), while its chirality degree is lower; c) it has a slightly weaker and less intricate H-bonding network. The findings might serve to expand the strategy for the search of biosignatures towards complementary approaches allowing for a mean to score known and unknown amino acids on a universal complexity/electronic/bonding properties scale.