Serine octamer cluster

The Serine octamer cluster in physical chemistry is an unusually stable cluster consisting of eight serine molecules (Ser) implicated in the origin of homochirality.^[1][2] This cluster was first discovered in mass spectroscopy experiments. Electrospray ionization of an aerosol of serine in methanol results in a MS spectrum with a prominent ion peak of 841 corresponding to the Ser₈+H⁺ cation. The smaller and larger clusters are virtually absent in the spectrum and therefore the number 8 is called a magic number. The same octamer ions are also produced by rapid evaporation of a serine solution on a hot (200-250°C) metal surface or by sublimation of solid serine. After production, detection again is by mass-spectroscopic means. For the discussion of homochirality it is important to note that these laboratory production methods resemble prebiotic conditions.

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The cluster is not only unusually stable but also unusual because the clusters have a strong homochiral preference. A racemic serine solution produces a minimum amount of cluster and with solutions of both enantiomers a maximum amount is formed of both homochiral D-Ser₈ and L-Ser₈. In another experiment cluster formation of a racemic mixture with deuterium enriched L-serine results in a product distribution with hardly any 50/50 D/L clusters but a preference for either D or L enantioenriched clusters.

A model for chiral amplification is proposed whereby enantioenriched clusters are formed from a non-racemic mixture already enriched by L-serine as a result of a mirror-symmetry breaking process. Cluster formation is followed by isolation and on subsequent dissociation of the cluster a serene solution forms with a higher concentration of L-serine than in the original mixture. A cycle can be maintained in which each turn results in an incremental enrichment in L-serine. Many such cycles eventually result in enantiopure L-serine. This model has been experimentally verified.

Chiral transmission is assumed to take place through so-called substitution reactions of serine clusters. In these reactions, a serine monomer in a cluster can be replaced by another small biologically relevant molecule. For instance Ser₈ reacts with glucose (Glc) to the Ser₆ + Glc₃ + Na⁺ cluster. Moreover the cluster of synthetic L-glucose with Ser₈ is less abundant than that with the biological D-glucose.

See also

• Other magic numbers in chemistry: methane clathrate, Magic angle spinning
• Other stable clusters in: aluminium superatoms

References