Exobiology and Evolutionary Biology

A Laboratory Study of Low-Temperature Ice Photo-Chemistry as a Contributor to the Chemical, Chiral, and Isotopic Diversity of Meteoritic Amino Acids

PI: Max Bernstein

If the formation of amino acids and related compounds of pre-biotic importance is viable in the interstellar medium (ISM), and not restricted merely to aqueous synthesis in a planetary system, this may imply that they (and thus possible ingredients of life) are more widely spread throughout the galaxy than has often been assumed. We present here an experimental program to evaluate potential connections between interstellar (IS) ice photochemistry and amino acids and related molecules in meteorites, in an attempt to explain their relative proportions and isotopic and enantiomeric anomalies.

At the low temperatures of IS dense molecular clouds (10-50 K) deuterium fractionation is efficient and elevated D/H ratios are seen in grain mantles, as well as in several gasphase IS molecules, including amino acid precursors, such as formaldehyde and ammonia. Thus, deuterium enrichments of amino acids indicate that there is a connection between certain meteoritic molecules and these cold regions in the ISM. It has generally been assumed that this association merely derives from an interstellar precursor, such as formaldehyde, taking part in an early Solar System reaction in liquid water. However, the distribution of deuterium in glycine and glyceric acid (the smallest amino acid and corresponding hydroxy acid) in Murchison are difficult to explain by such a scheme.

Similarly, occasional but consistent meteoritic biases for left-handed (L) amino acids are difficult to rationalize by liquid water parent-body reactions. We have shown (Bernstein et al. 2002 Nature 416:401) that amino acids and related compounds are created in the lab from the UV photolysis of simple, realistic, interstellar ice mixtures under conditions representative of the dense ISM. This suggests that not just simple precursors but amino and hydroxy acids themselves were already present in the ice from which the Solar System formed.

This ice scenario has potential to simultaneously accommodate the presence and distribution of deuterium, and the bias for L amino acids in meteorites, but this possibility has not been seriously explored in the laboratory. We plan to continue to study the photosynthetic pathways that give rise to amino acids and other organic compounds in the lab and may have contributed to these molecules in meteorites. We will determine how these compounds vary in structure and abundance depending on the composition and concentrations of reactants, temperature, UV wavelength, and dose. Furthermore, we will employ isotopic labeling to uncover reaction pathways and to assess if this process will explain differences in deuterium enrichment of amino acids and related compounds in meteorites.

Formation of these compounds is only half the story, we are also interested in rates of decomposition of amino acids and related compounds on exposure to UV because even if these compounds do form in space that doesnt mean that they will be easy to observe. We must know both rates of formation and destruction to make reasonable predictions. Of what is to be expected on the surface of icy bodies in space.