An important aspect of amino acid analyses of Martian samples is distinguishing those produced abiotically from those synthesized by either extinct or extant life. Amino acid homochirality provides an unambiguous way of distinguishing between abiotic vs. biotic origins (see ref. 22 and references therein). Proteins made up of a mixture of D- and L-amino acids would not be efficient enzymes because they could not fold into bioactive configurations such as the -helix. However, enzymes made up of all D-amino acids function just as well as those made up of only L-amino acids, but the two enzymes use the opposite stereoisomeric substrates. There are thus no apparent biochemical reasons L-amino acids would be favored over D-amino acids. On Earth, the use of only L-amino acids in proteins by life is probably simply a matter of chance. We assume that if proteins and enzymes were a component of extinct or extant life on Mars, then amino acid homochirality would have been a requirement. However, the possibility that Martian life was (or is) based on D-amino acids would be equal to that based on L-amino acids. The detection of a nonracemic mixture of amino acids in a Martian sample would be strong evidence for the presence of an extinct or extant biota on Mars. The finding of an excess of D-amino acids would provide irrefutable evidence of unique Martian life that could not have been derived from seeding the planet with terrestrial life (or the seeding of the Earth with Martian life). In contrast, the presence of racemic amino acids, along with nonprotein amino acids such as -aminoisobutyric acid and racemic isovaline, would be indicative of an abiotic origin, although we have to consider the possibility that racemic amino acids were generated from the racemization of biotically produced amino acids (23).
A potential impediment to the search for life on Mars is the forward contamination of the planet by a spacecraft with either terrestrial organisms, or more likely terrestrial biomolecules. This problem would be of great importance in assessments of whether there are any amino acids indigenous to Mars. Because of the distinctive mixture and the L-enantiomeric signature of amino acids associated with terrestrial life, chiral amino acid analyses can be used to monitor the level of forward contamination of Mars that occurs during the course of planetary exploration. A long-range monitoring program would provide a critical baseline data set for assessing forward contamination during the eventual human exploration of Mars.
A relatively new technology that shows promise for spacecraft-based amino acid enantiomeric analysis is microchip-based capillary electrophoresis (µCE). With µCE, both the identity and enantiomeric composition of amino acids can be determined at subpart-per-billion levels. The µCE-based analyses are about an order of magnitude faster than analytical methods such as conventional CE and HPLC. In addition, µCE has a detection limit more than 3 orders of magnitude better than conventional HPLC. Thus, proportionally smaller samples (100 picoliter or 1010 liter) can be analyzed.
A µCE chip system has been used to explore the feasibility of using such devices to analyze for amino acid enantiomers in extraterrestrial samples (24). The test system consisted of a folded electrophoresis channel (19.0 cm long × 150 mm wide × 20 mm deep) that was photolithographically fabricated in a 10-cm-diameter glass wafer sandwich, coupled to a laser-excited confocal fluorescence detection apparatus providing subattomole (18 mole) sensitivity. The µCE analysis system consists of a stack of wafer scale components, which individually provide the liquid flow channels, the capillary separation zones, the electrophoretic controls, the fluid reservoirs, and the detection system. This µCE system is more than an order of magnitude smaller in size than conventional laboratory bench-top amino acid analytical instruments. Analysis times with µCE are on the order of a few minutes compared with almost an hour for HPLC-based analysis.
A critical aspect is that enantiomeric ratios can be rapidly and accurately determined by using the microfabricated CE chip instrument. Using a SDS/g-cyclodextrin, pH 10.0 carbonate electrophoresis buffer and a separation voltage of 550 V/cm at 10°C, baseline resolution is observed for the enantiomers of valine, alanine, glutamic acid, and aspartic acid in only 4 min (see Fig. 2). Enantiomeric ratios of amino acids extracted from sediments and the Murchison meteorite using this µCE chip system closely matched values determined by HPLC-based methods (24).
For spacecraft-based µCE chip analyses, a microfluidics-based sample processing system is required to deliver an amino acid extract suitable for analysis. In a design scheme presently being tested, amino acids are first extracted from a sample by heating in hot water for about an hour, a procedure similar to that used to extract amino acids from meteorites in the laboratory (13). The aqueous extract obtained by this procedure is frozen and then sublimed at Mars ambient pressure onto a cold finger. The sublimed ice/amino acid mixture is thawed and collected in a reservoir interfaced with a µCE chip instrument. With this design, no desalting is required, thus eliminating a procedure that requires reagents and ion-exchange chromatography.