Chiroselective self-directed octamerization of serine: Implications for homochirogenesis

Serine undergoes chiroselective self-directed oligomerization to form a sin gly protonated octamer under positive ion electrospray conditions, as ident ified by ion trap tandem mass spectrometry. The experiments also show a ser ies of higher-order clusters (metaclusters) corresponding to [(Ser(8)H)(n)] (n+), n = 1, 2, 3. There is a magic number effect favoring formation of the protonated octamer over its homologues and also a strong preference for oc tamer formation from homochiral serine molecules. Collision-induced dissoci ation suggests that the protonated octamer is composed of four hydrogen-bon ded dimers, stabilized by further extensive hydrogen bonding. Density funct ional calculations support this model and show that the protonated homochir al octamer is energetically stabilized relative to its possible fragments ( dimer plus protonated hexamer, etc). The calculations also show that hetero chiral octamers are less stable than homochiral octamers (e.g., the protona ted 7:1 cluster is 2.1 kcal/mol less stable than the 8:0 analogue). The imp lications of these results for the origin of homochirality are discussed.