The amino acid α-carbon provides for two mirror image configurations based on the relative orientation of the side group . Terrestrial biological amino acids consist of only one configuration , however, there is no reason why proteins in extraterrestrial life would need to be based on L-amino acids as on Earth. Proteins as catalytically active as their natural biological L-amino acid counterparts have been synthesized of entirely D-amino acids , thus it is assumed that life elsewhere could be based on either L- or D-amino acids. Amino acid homochirality associated with extant terrestrial life changes over time after the bacterial community becomes deceased due to racemization. When living, the protein turnover time is sufficient to preserve the homochiral protein composition, however after death, the amino acids interconvert from the biological L-enantiomer to the abiological D-enantiomer. This interconversion is a natural process that becomes significant over geological timescales and continues until they are present in equal abundances, that is a D/L-ratio equal to 1. The D/L-enantiomer ratio along with known rates of racemization has been useful in determining the geological age of terrestrial samples up to hundreds of millions of years old . Although racemization compromises the microbial signature of terrestrial proteins over geological timescales, the determination of amino acid chirality still offers a powerful biosignature for the presence of microbial life. The detection of amino acids alone is not unequivocal evidence of life,hydroponic nft channel rather a homochiral signature is necessary to confirm a biological amino acid source.
Although sufficiently old biological samples may show racemic signatures similar to those derived from abiotic syntheses, well preserved amino acids from extinct bacterial communities at extremely cold temperatures would still show good chirality preservation for hundreds of millions of years. In future life detection experiments, the chirality of amino acids should easily discriminate between biological amino acids and those which may have formed abiotically or derived from meteorite influx.Known abiotic pathways exist for the formation of amino acids such as spark discharge experiments and laboratory hydrothermal syntheses, however they are all known to produce equal amounts of D- and L- amino acids in low concentrations. This marked difference between homochiral biological and racemic abiotic composition permits the discrimination of the source of the detected amino acids by resolving their enantiomeric abundances . Also important is that abiotic amino acid syntheses tend to form a relatively small suite of amino acids compared to those utilized in bacterial proteins. The suite of protein amino acids utilized in the bacterium E. coli is evaluated in Chapter II and compared to previous empirical studies. If detected amino acids are too old or degraded for any chiral signature to be deduced, the distribution can be used to definitively decide the source of the amino acids as microbially or abiotically derived. A variety of amino acids have been detected in meteorites as well, but these are interpreted as having formed during parent-body processes. The fact that amino acids within meteorites are all racemic , and that they show a suite of amino acids similar to those formed in abiotic syntheses, makes them easy to distinguish from biologically sourced amino acids again based on chirality or distribution. Any preferential dominance of Lamino acids detected in meteorites is assumed to be due to terrestrial contamination . Certain amino acids within the large suite of amino acids detected in meteorites are not components of terrestrial proteins, rather they are known to be indigenous because they are unique to meteorites and reflect formation during parent-body processes .
The two most abundant extra-terrestrial amino acids are isovaline and aminoisobutyric acid , however there are a variety of others that are recognized as a indicative of an extraterrestrial signature . The presence of these amino acids in geological samples is suggested to reflect deposition by during a period of high meteoritic influx.The most relevant amino acid biomarkers depend on their relative abundances in bacterial proteins and the stability of the individual residues. There are two major amino acid diagenetic pathways, degradation and racemization . The rates associated with degradation are slower than racemization by at least a factor of 100 in most cases. The most stable protein amino acids will persist through geological time and allow for the quantification of long extinct bacterial communities. Amino acids degrade primarily by decarboxylation or deamination but other processes like dehydration and aldol cleavage can also be significant .The most commonly occurring amino acids in ancient and degraded microbial communities are glycine and alanine , a finding corroborated by the analyses of natural samples of anoxic sediments . Glycine and alanine are both present in bacterial communities in very high abundances, however, only alanine shows degradation rates among the slowest of the amino acids . This implies that glycine may be better preserved in geological settings or that diagenetic pathways lead to its secondary formation from other compounds. Regardless, both alanine and glycine remain among the most important amino acids to assay for in geological samples as well as asp, glu, and ser. Valine, present in lower abundances than these other amino acids, shows slow racemization kinetics and degradation kinetics and should show good preservation in environmental samples despite composing only ~5% of total bacterial protein. The plots in Figure 1.6 show the evolution of aspartic acid concentration and D/L-ratio versus time. Aqueous rates of aspartic acid degradation and racemization were used in these models and therefore represent the fastest rates of these reactions.
Racemization is a much faster process which results in a racemization half-life of ~2,200 years whereas the half-life of aspartic acid degradation is ~10,000 years. Environmental samples always show slower degradation and racemization in colder and dryer conditions.The aspartic acid racemization rates in dry environmental conditions have been reported to be as slow as 1.20 x 10-6 yr-1 and 6.93 x 10-7 yr-1 , equivalent to half-lives of ~600,000 and 1,000,000 years, and therefore must be evaluated carefully for each geological sample for the purposes of amino acid racemization age dating . Likewise, any degradation reactions are equally dependent on the mineralogy of the environmental sample and may be accelerated by the presence of metal ion catalysts . Racemization age dating has been suggested to be applicable to amino acids from hundreds of thousands to millions of years old at low temperatures, but this range can be extended to older samples under colder conditions. Likewise, amino acids from hundreds of millions of years old up to billions of years could be well preserved under the appropriate environmental conditions . Target bio-molecules in the search for evidence of life on Mars must be stable enough to persist for geological timescales so that evidence of life on Mars does no go undetected. The fate of amino acids includes racemization, degradation, and bacterial uptake. In the absence of biological processing , racemization is faster than degradation by at least 100x. Racemization involves a planar carbocation intermediate formed by the loss of a proton on the α- carbon and subsequent attack on the top or bottom by another proton . The reactions for the destruction of amino acids include decarboxylation to amine compounds or deamination. These rates are highly matrix and temperature dependent and therefore must be evaluated for the specific environmental conditions. Although the prevailing cold and dry conditions on Mars tend to drastically increase the lifetimes of organic degradation and amino acid racemization , there are other effects that must be considered. For instance,nft growing system the surrounding mineral matrix can catalyze amino acid diagenetic reactions, especially degradation in the presence of metallic ions . Therefore, the specific preservation of organic material will be strongly a function of chemical environment.If intact amino acids are detected and show an abundance of one enantiomer over the other, this would unequivocally show that the source of these amino acids was biological. If these biomarkers from extinct life on Mars were to have been degraded over geological timescales, there are certain classes of compounds that we would expect to be diagenetic end products or intermediates. Compounds such as humic acids and kerogen are products of the diagenesis of organic matter over time, however there may be generation of other diagenetic products due to the slow degradation of amino acids over time that might indicate what might be favored on Mars in terms of diagenetic products. For instance, decarboxylation is known to be a the primary degradation reaction amino acids such as glycine, alanine, and valine and form their corresponding amine degradation products .
The study of organic inclusion in terrestrial Martian analogs allows for the characterization of similar types of environments on Earth as detected on Mars so that we can understand some of the processes that might be important on Mars. The study of organics in Mars analog minerals can offer an idea of the sequestration potential and stability of these deposits on Earth. Indeed if Mars really experienced warm and wet climate early in its history , it may have been more similar than we realize to Earth and may have a lot in common with many of the proposed Mars analog locations. The determination of the stability within terrestrial Mars analog minerals can help to approximate biochemical stability that might be expected on Mars. It turns out that low levels of amino acid degradation products that indicate diagenetic processes can often be used to determine the stabilities or diagenetic state of the included amino acids.Figure 1.9 shows the general geological history of Mars dominated by an early wet era in which clays were formed by water alteration. There was a catastrophic climate change around ~3.5 billion years ago. Water is a medium for interesting chemistry to occur, it provides a location for the origin of life, and geologists can use water abundance to explain many of the erosive features on Mars. Therefore, preservation becomes the key issue when talking about finding evidence of life on Mars. The evidence of an extinct Martian biota might be from a biological community billions of years old and must show good preservation over the history of the samples. The idea of using chirality as a bio-signature was first proposed by Halpern to search for evidence of life on Mars. This idea has resurfaced in the current strategies for life detection recognizing for over 30 years the strength of amino acid chirality as a biosignature and discrimination versus abiotic amino acid signatures . On Mars, racemization kinetics are expected to be extremely slow because of the cold, dry conditions, and any chiral signature of extinct life should be preserved for billions of years . The harsh surface conditions on Mars may limit the survival of some organics within the host regolith . Because amino acid diagenesis is so intimately linked with matrix effects, the study of amino acid preservation and diagenesis in terrestrial Mars analogs is necessary to make predictions on the best locations to search for biosignatures on Mars. Extrapolation of these diagenetic reaction rates to Mars’ surface temperatures can allow for estimates of amino acid stability and rates of diagenetic reactions on Mars.This dissertation covers my investigations of organic inclusion and sequestration within various Mars analog minerals. Throughout these studies, amino acids are investigated for their applicability as biomarkers for the detection of extinct or extant microbial communities on Mars. A variety of environments that have been suggested as analogous to Mars for mineralogical or climatic conditions are profiled and in some cases, rate data is gleaned from the coupling of amino acid degradation reactions and extrapolated to predicted rates on Mars. The stabilities of amino acids in analog minerals essentially sequesters them and offers some degree of protection from harsh surface conditions, allowing for enhanced preservation in some cases. Specifically, these studies investigate amino acid diagenetic reactions including racemization and degradation to try and predict the degree of survival of these biosignatures over geological timescales upon the surface of Mars. Chapter 2 characterizes the amino acid composition in bacteria and verifies our methods of analysis used in these studies. The amino acid distributions and concentrations are so markedly different from any type of abiotic formation process that discrimination between these processes should be possible even over very long timescales. Chapter 3 introduces a new chemical chronometer based on the detection of amino acid degradation products within ancient geological samples.