A feasibility study of Raman optical activity for the detection and discrimination of chiral biological compounds for planetary science instrumentation

Amino acids and carbohydrates, as individual compounds, can be formed abiotically and have been identified in a variety of extraterrestrial materials. Therefore, in these cases, it is not merely the presence, but rather the chirality of these molecules that is indicative of biological origin. Biological materials on Earth are composed exclusively of L enantiomer amino acids and D enantiomer carbohydrates. To date, no spacecraft-based Raman spectrometer has the capability to distinguish between enantiomers of these compounds. While there is only one commercial-off-the-shelf (cots) Raman optical activity (ROA) spectrometer available, TRL 3, surprisingly to date however, no research using this technology as an innovative analytical technique for biosignature detection in astrobiology and planetary science has been undertaken. Therefore, for the first time, this work would carry out a feasibility study to establish if ROA would afford the discrimination of the chirality of amino acids and carbohydrates in astrobiologically relevant simulant samples. This feasibility study is not about building a ROA instrument, but rather, undertaking experiments to report and observe basic principles of ROA of chiral biological compounds in astrobiologically relevant simulant samples, entry TRL 1, and demonstrating the feasibility of this proof-of-concept, exit TRL 3. This in turn, would provide invaluable information and baseline data that would feed forward into instrument design and implementation of a miniaturized ROA system.

A substance is optically active if it shows a difference with the interaction of left circularly polarized and right circularly polarized light. ROA is based on the phenomenon of inelastic scattering of circularly polarized light by chiral molecules. It is measured as the difference in intensity of the right and left circularly scattered Raman light. Funding is requested to purchase the cots ROA spectrometer to conduct the proposed work. Firstly, the proposal would investigate if ROA can distinguish between the D and L enantiomers of a collection of amino acids and carbohydrates most commonly found in biology and meteorites. Common amino acids that most frequently occur in abiotic and biotic samples include glycine, alanine, glutamic acids, aspartic acid, leucine, serine, and valine, and likewise, common carbohydrates include glucose and fructose. These compounds would be purchased in their D and L enantiomers from Sigma-Aldrich and Alfa Aesar. ROA spectra would be collected and chirality between D and L enantiomers would be elucidated. Subsequently, a database of amino acid and carbohydrate chirality relevant for astrobiology would be constructed. Then, the limit of detection would be determined by serial dilution of the D and L enantiomers of the above compounds in water, then likewise for saline solutions relevant for Europa, such as, NaCl, KCl, MgCl, MgSO4, and Na2Mg(SO4)2. To further test the feasibility of this technique, mixtures of Raman active substances, amino acids, and carbohydrates would be analyzed to determine the strength of chiral signatures. To facilitate this, statistical methods such as chemometrics would be employed to discriminate characteristic chirality signals and further determine enantiomeric ratios.

The proposed work is highly relevant to the PICASSO goals of supporting planetary and astrobiology science instrument feasibility studies, concept formation, proof-of-concept instruments, and advanced component technology development. With the discovery of water-ice and a subsurface liquid ocean on Europa and Enceladeus, measurement of chirality is becoming a highly desirable capability for search for life centric missions. To this end, NASA is currently investigating a potential Europa lander mission, and instrument and technologies relevant to such a mission are also especially of interest. Goal 1 of this mission, which ranks the highest, is to search for evidence of extant life. More »