All about crystal Eve

Scientists have reported the discovery of what may be the "ancestral Eve" crystal that billions of years ago gave life on Earth its curious and exclusive preference for so-called left-handed amino acids. Fourier transform IR spectroscopy and powder X-ray diffraction provide the evidence. Molecules of aspartic acid of a sinister, or left-handed, orientation, could be the ancestral Eve of all amino acids, the building blocks of proteins, terrestrial life.

At the molecular level nature is not symmetrical. Louis Pasteur was among the first to spot this in teasing apart tiny crystals of left- and right-handed tartaric acid salt crystals. Moreover, natural proteins almost invariably use only the left-handed form of amino acids. There have been countless theories to explain life's chiral bias, some based on electron spin and quantum physics, others alluding to polarised synchrotron radiation from a supernova, yet others talk of sunrises and stability. None has yet explains the bias, which lies at the very roots of life itself.

On one hand, you might think that nature could simply choose either chiral form of any molecule with which to work. Countless molecules exist in two non-superimposable mirror image forms. Without this handedness biomolecules would not function as they do and it has been suggested that the primordial soup would never have given rise to bio in the first place without it being intrinsic to pre-biotic chemistry. Simply reacting methane, ammonia, water, and hydrogen at temperatures ranging from 0 to 70 Celsius under the reducing atmospheric conditions of the primitive earth somehow led to a bias even though in laboratory experiments equal quantities of both left- and right-handed amino acids form.

Now, Tu Lee and Yu Kun Lin of the Department of Chemical and Materials Engineering, at the National Central University, in Jhong-Li City, Taiwan, R.O.C. believe their work may help resolve one of the most perplexing mysteries about the origin of life.

Lee and Lin point out, as many before them have, that conditions on the primordial Earth are likely to have held an equal chance of forming the same amounts of left-handed and right-handed amino acids, if these molecules formed spontaneously from simpler abiotic organic molecules. Presumably from the outset, however, the earliest simplest self-replicating molecules and molecular systems that gave rise to the first life forms more than 3 billion years ago, used only left-handed amino acids. That bias exists in all modern plants, animals, and fungi.

The researchers experimented with various temperatures and reactions conditions to see if they could induce a spontaneous bias in mixtures of both left- and right-handed form of the simple amino acid aspartic acid. They found that under conditions that could have existed on primitive Earth, left-handed aspartic acid molecules and right-handed aspartic acid molecules could stay separated in water instead of well-mixed (a conglomerate solution) even though aspartic acid is believed to be a racemic forming system, then it became a simple matter of a well-known process called preferential crystallization for the left-handed version to prevail on a relatively large scale.

"The aspartic acid crystal would then truly become a single mother crystal: an ancestral Eve for the whole left-handed population," the team says.

"We found that it took more than 36 h at 25 Celsius and 5 h at 45 Celsius just to complete the solution phase transformation of a conglomerate solution of aspartic acid to a racemic compound solution of aspartic acid," the researchers explain. They tracked the changes in their solutions and resulting crystals using Fourier transformed infrared spectroscopy and powder X-ray diffraction.

We used FTIR and PXRD to distinguish a racemic compound (DL) from a conglomerate (D+L physical mixture), Lee explains, "Obviously, FTIR and PXRD cannot distinguish the bias (i.e., D or L from a D+L physical mixture); only polarimetry can do that."

However, if there were an equimolar of succinic acid present in the solution phase, then the transformation from conglomerate to racemic delayed the process by at least 8 h even at 60 Celsius. This time delay allows the thermodynamically stable racemic aspartic acid to convert into a temporarily stable, or metastable, conglomerate in water either through a rapid acid-base reaction or if a solvent is added when the temperature falls, this then delays the subsequent back conversion to a racemic compound thermodynamically thus locking in one handed formed of the aspartic acid in crystals. As a result, the left- and right-handed forms of aspartic acid could preferentially crystallize on a large scale and may well have done so commonly on the primitive earth.

Previous research that has focused on chiral fields, parity non-conservation, chiral symmetry breaking, asymmetric adsorption, chiroselective synthesis, co-crystal engineering, enantiomeric enrichment, ultraviolet photolysis, asymmetric autocatalysis, diastereomeric resolution, and racemization has simply ignored the liquid structure of the solution phase and its evolution over time. Lee and Lin suggest that they have remedied this situation in their experimental explanation of chiral bias in aspartic acid. Once bias was induced in one amino acid, then it would be triggered in others in the primordial chemistry of life.

It is commonly thought that aspartic acid is a racemic compound forming system (i.e., of the 20 natural common amino acids, only two: threonine and arginine, are conglomerate forming systems. When D and L molecules are mixed in water, they will do so homogeneously giving a racemic compound upon drying.

The researchers have demonstrated something new with aspartic acid. "The D and L molecules do not mix in water instantly," Lee told SpectroscopyNOW. "They tend to stay separarted in water for at least 36 hours at 25 Celsius. If we remove water during this period of time, we will get a conglomerate. Once we saw that a conglomerate system exists for aspartic acid for at least 36 hours, we got 'goose bumps' because the separation (or enrichment) of D and L will be very easy and routine by preferential crystallization or simply by a bumpy landscape," Lee adds.