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The Alchemist, 28.3.2002
Mysterious amino
It's been known for more than a century that amino acids are nothing special. They might be the building blocks of life on earth, but there is no real mystery as to how they can be synthesised. They are simply a blend of carbon, nitrogen, hydrogen, oxygen and the occasional sulfur atom. Nothing mystical. Nothing preternatural, writes David Bradley.
But saying that, there is still a great deal of intrigue about the monomeric units (to coin a tautology) of the natural proteins from which we are made. For instance, if there is nothing mysterious about them, how come we do not know exactly how life began to use them? The elemental constituents of amino acids are found throughout the universe, or so we believe, there are countless ways that amino acids may be produced, from irradiation or lightning flash of the primordial soup here on earth, the blast of a supernova or the tail-end chemistry of a comet.
Indeed, laboratories have spent decades finding ways to replicate the dawn of life by discharging electricity across a blend of precursor compounds, shining ultraviolet radiation on potentially vital cocktails, sending shock waves, radiation and heat through everything from hydrogen cyanide, formaldehyde, and methane to carbon monoxide, oxalic acid, and carbides with the anticipation of creating the stuff of life. The amino acid.
According to Everett
Shock, writing in this week's Nature, "It appears that nearly every experimental
scenario produces organic compounds of some form, which could hardly be
more frustrating when trying to unravel what actually produced the organic
inventory of the Solar System." He suggests that rather than simply trying
to recreate life's most primitive conditions in this way that researchers
should be "quantifying reaction rates, the relative stabilities of synthesized
compounds and the effects of variables such as temperature and pressure."
Indeed, two papers to
which his commentary relates, in this week's Nature, reveal that conditions
onboard ice-coated grains in the interstellar medium may be suitable for the
synthesis of amino acids rather than the more conventionally suggested solar
system based environments of meteors and earth itself.
Interstellar ice photochemistry
The delivery of extraterrestrial organic molecules to earth is certainly a plausible alternative or complement to the explanation that amino acids were generated on the prebiotic Earth. Amino acids have been found on several meteorites, and the usual explanation for their presence is that they formed in liquid water reactions on parent bodies such as comets or asteroids. Now, Max Bernstein of the SETI Institute, Mountain View, California, and NASA Ames Research Center, Moffett Field, California, and colleagues, and Uwe Meierhenrich of Bremen University, Germany, and the Centre de Biophysique Moléculaire, Orléans, France, and co-workers have independently discovered evidence that suggests ice photochemistry in the interstellar medium is a possible source of meteorite-borne amino acids. The teams found that irradiating ice mixtures containing known 'interstellar' molecules (among them water, methanol, carbon monoxide, hydrogen cyanide, ammonia) with ultraviolet in a vacuum at the kind of temperatures found in interstellar space — below 15 K — produced amino acids.
The Bernstein team synthesised three amino acids (glycine, serine and alanine) in a mixture of water, methanol, ammonia and hydrogen cyanide (ratio 20:2:1:1). Meierhenrich's group, on the other hand, obtained 16 amino acids and other organic compounds from water, methanol, ammonia, carbon monoxide and carbon dioxide (ratio 2:1:1:1:1). According to Shock, the differences seen between the two experiments, which were carried out under comparable conditions, demonstrate that the composition can have a 'profound effect' on the results of the chemical syntheses. He points out that the Bernstein's water-rich method resulted in less than a handful of amino acids while Meierhenrich and colleagues generated a wide range of amino acids and some of these were carrying two amino groups. However, Meierhenrich's team has carried out additional experiments with different ice mixtures that show that the exact composition of the starting ice mixture has only a negligible effect on the quantities of amino acids produced. "The relatively (to the NASA Ames group) low percentage of water in our starting ice mixture is definitely not the reason for the identification of a higher complexity of amino acids in our case," he explains. "However, we are convinced that differences in the applied analytical procedures are the obvious reason."
Seeing the light
At first glance, it might seem that the teams have done little more than confirm that in the frozen conditions of space, onboard a meteor, for instance, carbon, hydrogen, nitrogen and oxygen can and do combine to form amino acids. But, while the reactant ratios Meierhenrich used do exist on meteors, it those diamino acids that are the critical difference. The Murchison meteorite has not yielded such compounds. "Yet," says Meierhenrich, "the fact that in the Murchison no such compounds had been identified does not necessarily mean that they were not there; that could mean that researchers have not looked for them carefully enough." He cites the joke about the drunken man searching for something within the cone of light below a street-lamp. A helpful passerby comes along asking what the man is looking for. "I am looking for my keys". The passerby enquires, "are you sure you lost it within the cone of light, here?" and the drunk replied, "no, of course not, but it's dark over there."
Bernstein points out that while more than 70 indigenous amino acids have been found in the Murchison meteorite alone, not only is the water in the Murchison meteorite depleted in deuterium relative to indigenous organic acids, the meteoritical evidence for an excess of 'left-handed', laevo-rotatory, amino acids cannot be fully explained as having taken place in the liquid-water reactions on a meteorite parent body. "These are the first papers to show that amino acids can form in ice, and this is very important because of the peculiarities of the amino acids in meteorites," explains Bernstein. "The amino acids in meteorites have two molecular signatures that indicate they have an interstellar ice origin: their deuterium enrichment and their chirality." The present results point to a putative interstellar origin, certainly.
Bernstein also feels it a bit unfair that Shock should suggest in his commentary that researchers should shift away from (he quotes) "seemingly endless series of phenomenological experiments and to concentrate instead on quantifying reaction rates." "After all," Bernstein says, "it is difficult to quantify something if you have not shown that it can happen in the first place, which is what we have just done!" He and his colleagues have also recently published research in which they do indeed measure rates of decomposition of amino acids and discuss their stability.
So, what does any of this tell us about the origins of life? It seems that one of the most archetypal of life molecules, the amino acids, are more likely to be found to have an abiological origin and to be found in the far-flung reaches of the universe rather than here on earth. Now, if that thought is not enough mystery for you, I don't know what is.
Shock muses that one can study rocks but knowing geology tells us nothing about the Teotihuacán, the Taj Mahal or Tony's Tavern. On the other hand, an understanding of geological processes, materials science, architecture and culture provide much more of an insight. Similarly, if amino acids are a common feature of the universe and truly, as has been suspected for many years, nothing particularly special, then the search for an understanding of origins of life perhaps should be couched in terms of life processes and not simply the raw world of organic chemistry. After all, once Wöhler had synthesised urea it was patently obvious that life would never look the same again.
References:
Everett L. Shock. Astrobiology: seeds of life? Nature 2002, 416(6879):380–381.
Max P. Bernstein, Jason P. Dworkin, Scott A. Sandford, George W. Cooper & Louis J. Allamandola. Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues.Nature 2002, 416(6879):401–403.
G. M. Muñoz Caro, U. J. Meierhenrich, W. A. Schutte, B. Barbier, A. Arcones Segovia, H. Rosenbauer, W. H.-P. Thiemann, A. Brack & J. M. Greenberg. Amino acids from ultraviolet irradiation of interstellar ice analogues. Nature 2002, 416(6879):403–406.
Last updated May 2002