Astronomers have known for a very long time that simple compounds such as ammonia, methanol, carbon monoxide, carbon dioxide and water are present in icy grain mantles in interstellar gas clouds. Simple amino acids were first detected in meteorites in the 1960s, but it has only been in the most recent decade, by using laboratory simulation experiments and sophisticated analytical instrumentation, that scientists have been able to uncover the astounding array of organic molecules strewn throughout the solar system and galaxy and determine that a slight excess of the L-enantiomer occurs for some chiral amino acids. Investigations today focus on how that excess could have been created, and whether these compounds contributed to the development of life on Earth.
To date, multiple amino acids, hundreds of different isomers of 10 carbons or less, diamino acids and nucleobases, nucleobase analogues, carboxylc acids, ketones, aldehydes, nitriles, xanthenes, aromatics and polycyclic aromatic compounds have been detected in meteorites and comets by direct analysis of isolated material, and in interstellar gas and dust clouds by spectroscopic analysis. More than 160 compounds have been detected in the gas or solid phase of star-forming regions using IR and radio spectral analyses.
Initially scientists discounted the discovery of amino acids in meteorite samples as contamination by Earth-based substances. But the following factors suggest that this belief is not the case:
- there is a much higher 13C content found in interstellar amino acids than is found in organic compounds on Earth;
- some of the non-protein-forming amino acids and nucleobase analogues found in interstellar space are not found on Earth;
- both left-handed and right-handed protein-forming amino acids are found in roughly equal abundances in space, but are only present in living organisms on Earth as the left-handed isomer;
- analysis of soil/ice samples from the area where the meteorites are collected shows different concentrations of organic compounds; and
- a similar range of compounds, including non-biological derivatives, has been produced in laboratory experiments that simulate the conditions in space.
Advances in analytical techniques have made some of these analyses possible. In fact, re-analysis of older samples with new instruments has led to the identification of many more compounds than previously detected with less sophisticated equipment. A few grams of a meteorite may contain thousands of organic compounds at incredibly low concentrations. Certain laboratory experiments also generate minute quantities of organic molecules.
Today, extremely high resolution two-dimensional gas chromatography coupled with time-of-flight mass spectrometry and ultra-high resolution Fourier transform ion cyclotron resonance mass spectrometry can detect individual compounds at incredibly low concentrations. Formation of diastereomers of amino acid enantiomers with fluorescent tags enables detection of compounds at the femto (10-15) and even the attomole (10-18) level, according to Daniel Glavin of NASA’s Goddard Space Flight Center. ‘We still have difficulty identifying the different structures found by mass spec analysis, though, as there often are no standards available for comparison. Many of the compounds detected in meteorites, and especially the non-biologic substances, are not commercially available,’ he notes.
The presence of amino acids and nucleobases has attracted the most attention because of their importance in biochemistry. Different amino acids are present on different types of meteorites, depending on the conditions to which the material has been exposed. α-Aminocarboxylic and α-aminodicarboxylic acids could have formed at temperatures well below 0°C via the Strecker cyanohydrin synthesis involving ammonia, aldehydes/ketones and water. β-Amino acids are believed to be generated via Michael addition of ammonia to α,β-unsaturated nitriles followed by reduction/hydrolysis.
Straight chain amino acids, however, must have been formed by another mechanism; higher temperature (ca 600°C) Fisher-Tropsch synthesis from CO, H2 and N2 or NH3 is one possible route. Hydrolysis of polymeric materials, modified radical–radical mechanisms with nitriles and ion–molecule reactions in the gas-phase have also been proposed as routes for amino acid synthesis, according to Uwe J. Meierhenrich at the Université de Nice-Sophia Antipolis, France.
Furthermore, the exposure of meteorites to water and heat affects the concentration and variety of amino acids present. Water, in particular, is critical. Fragments of the Tagish Lake meteorite from British Columbia, Canada were found to have different levels of water by evaluation of the mineral content. Where there was little water, the abundance of amino acids was high but their distribution was limited.More water led to an increase in abundance but a decrease in L-isovaline excess. Higher water levels caused destruction of the amino acids. ‘These results clearly indicate that percolation of water through the rock caused both the formation and destruction of organic molecules,’ comments Christophe Herd, associate professor at the University of Alberta, Edmonton, Canada.
Amino acids that are either rare or non-existent on Earth have even been found on a fragment of an asteroid that is believed to have experienced temperatures exceeding 1100°C. Glavin believes that once the asteroid cooled to below 500°C, CO2, H2 and NH3 could have reacted with grains of iron or nickel in the meteorite, a process that has been observed in laboratory simulations. ‘Clearly, organic chemistry is taking place in environments where previously not thought possible, and we should be careful about making assumptions regarding potential reactivities in different interstellar conditions,’ he says.
Several researchers have attempted to simulate the conditions in interstellar clouds and explore the reactivity of the different basic compounds present. For example, Maria Colín-García of the Instituto de Ciencias Nucleares, UNAM, Mexico, found that γ-irradiation of frozen and liquid HCN solutions generates several organic products, including carboxylic acids, free amino acids, amines, oligomers and urea, even at low temperature (77K) and low radiation doses (3–419kGy (1.87x1019–2.61x1021erg-1)). HCN is detected throughout the galaxy in interstellar clouds, in the interstellar medium and in comets. Meanwhile, astrochemists from Heriot-Watt University, Edinburgh, UK, found that acetonitrile molecules isolated on a silica/water ice surface react with water to form more complex organic compounds when bombarded with low-energy electrons.
While reactions clearly take place under the harsh conditions in space with temperatures ranging from 10–8000K and the presence of many types of intense radiation, there is still question as to how more complex organic material is produced. More than 70% of organic carbon occurs in the form of insoluble, high molecular weight polymeric material, and it has been detected throughout the universe. Scientists reason that such material is likely to have been formed by a very different pathway in high temperature environments – perhaps via gas phase radical reactions – and may have formed before amino acids were synthesised.
Stability and chirality
The stability of amino acids in interstellar space has been investigated by Marylene Bertrand, at the Centre de Biophysique Moléculaire-CNRS at the University of Orléans, France. Her group exposed different amino acids and dipeptides to space conditions aboard the International Space Station and in simulation chambers using vacuum ultraviolet (VUV) radiation. After irradiation, the molecules were extracted, derivatised and analysed by liquid chromatography, electrophoresis, or GC–MS. ‘We found that resistance to irradiation is a function of the chemical nature of the exposed molecules and of the wavelengths of the UV light, and that some of the exposed compounds could be sufficiently stable in space conditions to survive transport in interstellar space, especially if they are embedded in appropriate mineral matter and if they are in a suitable crystalline form,’ she says.
Stability may also play a role in transferring chirality to more complex compounds. To date, enantiomeric excesses of amino acids have all been found to be the L-isomer. Water exposure is believed to cause an increase in the enantiomeric excess of certain amino acids such as L-isovaline.
‘Because α-methyl α-amino acids are not prone to rapid racemisation under aqueous or radiogenic conditions, it is thought that they might have acted as chiral catalysts, transferring their chiral information to the α-hydrogen in proteinogenic amino acids common to life and other prebiotic compounds such as sugars,’ comments Glavin. He points out that other researchers including Ronald Breslow at Columbia University, US, and Donna Blackmond at Imperial College, London, UK, have reported that, in the process of crystallising a slightly enantio-enriched solution, the enantiomeric excesses for some amino acids in solution can be increased from 1% to 99%.
But the question remains: what caused the initial enrichment of enantiomeric excess in amino acids? Laurent Nahon at Synchrotron SOLEIL, near Paris, France, says that in regions of space greater in size than the solar system, asymmetric photochemistry or enantioselective photosynthesis (photochirogenesis) with interstellar ultraviolet (UV) circularly polarised light (CPL) may have induced an enantiomeric excess in chiral organic compounds.
Laboratory simulations involving the irradiation of interstellar ice analogues, composed of H2O, CO, CO2, NH3 and MeOH, with circularly polarised vacuum ultraviolet light (VUV) from a synchrotron beam on a transparent substrate (MgF2) at 80K and high vacuum have been followed using FTIR and two-dimensional GC–MS and do indeed result in slight (1–2%) enantiomeric excesses in alanine, which is reversible upon light helicity switching. ‘The ultra high sensitivity of these new analytical techniques is critical to the success of these experiments, as the number of molecules being produced is extremely small,’ observes Nahon. He adds that a key advantage of simulations is the ability to vary the conditions, including the composition of the starting molecules, temperature, and energy of the light.
In a separate experiment, racemic leucine was condensed into a metastable film at low temperature under high vacuum and then bombarded with circularly polarised VUV energy at 130–190nm. One enantiomer reacts a little faster and is thus destroyed more rapidly, according to Meierhenrich. After one or two days, therefore, an excess of one enantiomer is created. ‘Unfortunately,’ he notes, ‘this approach is not very elegant. To achieve an excess of 10–20%, 99.5% of the amino acids must be destroyed.’
If circularly polarised light does cause enrichment of stereoisomers, then probably there should be regions in space where R-amino acids are predominant. At this point, however, there is no spectroscopic technique sophisticated enough to detect individual enantiomers at such great distances.
An alternative theory has been proposed by Richard Boyd at Lawrence Livermore National Laboratory, US, and his collaborators, Toshitaka Kajino and Takashi Onaka of the University of Tokyo, Japan. It involves coupling the spin of the 14N in amino acids or their precursors to their chiral molecule and selective destruction of one orientation of 14N by neutrinos. The neutrinos are produced by the collapse to neutron stars in core-collapse supernovas, as is the magnetic field required to orient the 14N spins. The neutrinos could process the molecules in dust and ice particles in interstellar dust clouds near the supernova.
Since neutrinos would pass through even large meteoroids, they would potentially process all the 14N nuclei within their volume. The selected chiral molecules would then catalyse reactions that produce other identical molecules, thus imposing their chirality throughout the galaxy.
Another intriguing question relates to why the specific 20 amino acids found in all life on Earth were chosen out of the numerous possibilities. Did it happen randomly or was there something special about them? Stephen J. Freeland of the NASA Astrobiology Institute at the University of Hawaii, US, found that, compared with other groups of 20 amino acids made up of compounds found on meteorites – both biologic and non-biologic – the biologic set has the best, and widest, range of sizes, charges and hydrophobicities. ‘The data strongly support the notion that through evolution, a unique set of amino acids was selected as the building blocks for life,’ Freeland remarks.
Understanding the attributes of the standard set of amino acids could have implications for the likelihood of other life forms existing on other planets. Freeland is using computers to generate the 23,000-plus chemically feasible amino acids to find out more about how life maps onto the ‘chemistry space’.
Beyond astrobiology, this information could be useful to synthetic biologists who are exploring the effects that different amino acids might have on biological traits. Already, over 70 human engineered amino acids have been inserted into the genetic codes of bacteria and fungi to control enzymatic reactions, and some have even been inserted into mammals.
One of the challenges for all of these researchers is the need to work with materials that have fallen from space or use spectroscopy to analyse compounds at great distances. NASA’s Stardust mission, which collected samples from the tail of a comet and sent the material back to Earth for analysis, was the first attempt to collect samples in space. Two additional missions are eagerly anticipated by astrobiologists and astrochemists.
The European Space Agency’s Rosetta spacecraft, launched in 2004, will send a lander to the surface of comet 67P/Churyumov–Gerasimenko in November 2014. The Cometary Sampling and Composition Experiment (COSAC) will involve in situ analysis of comet samples – after converting to the gas phase – by GC using three different capillary columns, two of which contain chiral stationary phases, coupled with TOF mass spectrometry. Two years later, NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) will land on the asteroid ‘1999 RQ36’ and return a sample to Earth in 2023.