Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 2003, 42 (30), 3521 - 3523
Serine – the Origin of Life?
Stable serine clusters as the starting point
for prebiotic reactions and homochiral life
It is becoming common knowledge that many biomolecules come in "right-" and "left-handed" versions -- chemists refer to these as chiral compounds -- and that each of these has a different role to play. In fact, the essential building blocks of life are found in almost exclusively one of the two forms: as L-amino acids or D-sugars (from the Latin laevus = left and dexter = right). How is it that in nature, the emergence of life resulted in such a clear preference for image or mirror image? Researchers working with R. Graham Cooks at Purdue University (USA) now have further evidence for their hypothesis that the amino acid serine played a decisive role in the development of homochiral life.
Serine distinguishes itself from the other amino acids through it special properties. For example, it forms unusually stable clusters consisting of eight serine molecules. These octamers are special because they contain exclusively either D- or L-serine. Cooks' group previously discovered that other amino acids -- in the appropriate D- or L- form -- are also incorporated into these serine clusters. Now new research by the Cooks team proves that serine also forms clusters with other key compounds, and with glyceraldehyde, in particular. A reaction between this simplest of sugars and serine produces an adduct that can be incorporated into the serine octamer. In this case, the L-octamer consists exclusively of adducts made from L-serine and D-sugar. However, combined clusters can be formed even without a prior reaction -- from six serine and six glyceraldehyde molecules. In what may be one of the first prebiotic reactions, formation of these clusters causes glyceraldheyde, a C3 sugar (a sugar containing three carbon atoms), to dimerize, forming a C6 sugar. In addition, serine clusters also bind to phosphoric acid and form adducts with transition metal ions such as copper and iron. They may thus have enabled controlled prebiotic phosphorylations and oxidations.
Serine can switch between its D- and L-forms under mild conditions. Under the influence of circular polarized light, a swirling motion, or a magnetic field, the original equal distribution between D and L could have been shifted in favor of L-serine. "In highly concentrated droplets, L-octamers and higher clusters could have formed, leading to an accumulation of other L-amino acids and D-sugars," speculates Cooks, "they could also have been the site for important prebiotic reactions."