August 11, 2003
Volume 81, Number 32
CENEAR 81 32 p. 5
ISSN 0009-2347


Amino acid's enantioselectivity may answer homochirality-of-life question


Serine may have played a central role in the prebiotic chemistry that led to living organisms, according to researchers at Purdue University. The simple amino acid has been proposed as a key player in primordial chemical processes because of its unique tendency to form stable homochiral clusters.

For years, scientists have tried to explain the puzzling observation that certain biochemical building blocks found in living organisms come almost exclusively in just one enantiomeric form:
L for amino acids and D for sugars. Questions regarding the origin of the "homochirality of life," as the topic is known, have led to investigations by many research groups.

Two years ago, Purdue chemistry professor R. Graham Cooks and coworkers reported that serine--unlike other -amino acids--forms unusually stable octamers in which all members of the cluster are of a single enantiomeric form (C&EN, Aug. 13, 2001, page 13). The group also reported that these homochiral clusters undergo enantioselective substitution reactions. They found that other amino acids could replace serine in the cluster, provided that all amino acids share the same chirality.

Now, Cooks, postdoctoral research associate Zoltan Takats, and graduate student Sergio C. Nanita have discovered additional facets of serine chemistry that uniquely tie the amino acid to other species of fundamental significance in biochemistry, including glyceraldehyde, glucose, phosphoric acid, and transition-metal ions [Angew. Chem. Int. Ed., 42, 3521 (2003)]. Their observations strengthen the case that serine may provide a key part of the answer to the homochirality-of-life question.

Cooks proposes that homochirality in life is a result of three processes: chiral selection (also known as symmetry breaking), which selects the dominant enantiomer; chiral accumulation; and chiral transmission (to other molecules). "Our hypothesis is that chiral selection occurred with the amino acids--specifically with serine," Cooks says. The exact nature of the symmetry-breaking event is still unclear, he notes. But after this unidentified event took place, serine's unique chemical properties took over and provided the chiral accumulation and transmission steps, Cooks says. Chiral accumulation came about through serine's ability to form homochiral octamers, as reported two years ago. The present study addresses chiral transmission to other biomolecules via enantioselective substitutions and other reactions involving serine clusters.

In the current work, which is based on a gentle ionization process coupled with tandem mass spectrometry methods, the Purdue group finds that serine forms adducts enantioselectively with glyceraldehyde, the simplest aldose. The adducts, which are composed of L-serine and the D-sugar, form stable clusters, they report. The group also observes that glyceraldehyde in serine clusters can dimerize to form C6 sugars--a reaction that may have taken place in the primordial soup. In addition, they find that serine clusters bind phosphoric acid and transition-metal ions, such as copper and iron, which may have led to prebiotic phosphorylation reactions and oxidations, they say.

"Now we need to focus our attention on the symmetry breaking step," Cooks remarks. "To show that serine really could have been a key molecule in prebiotic chemistry, we need to take a mixture that has just a slight enantiomeric excess and figure out how to tip the balance."

HANDY CHEMISTRY The tendency of L-serine (left) and its homochiral octamers (right) to undergo enantiospecific reactions with sugars and other molecules leads Purdue scientists to propose that the amino acid played a key role in homochirality in living organisms.


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