Alkali Metal-Cationized Serine Clusters Studied by Sonic Spray Ionization Tandem Mass Spectrometry

Serine solutions containing salts of alkali metals yield magic number clusters of the type (Ser4+C)+, (Ser8+C)+, (Ser12+C)+, and (Ser17+2C)+2 (where C = Li+, Na+, K+, Rb+, or Cs+), in relative abundances which are strongly dependent on the cation size. Strong selectivity for homochirality is involved in the formation of serine tetramers cationized by K+, Rb+, and Cs+. This is also the case for the octamers cationized by the smaller alkalis but there is a strong preference for heterochirality in the octamers cationized by the larger alkali cations. Tandem mass spectrometry shows that the octamers and dodecamers cationized by K+, Rb+, and Cs+ dissociate mainly by the loss of Ser4 units, suggesting that the neutral tetramers are the stable building blocks of the observed larger aggregates, (Ser8+C)+ and (Ser12+C)+. Remarkably, although the Ser4 units are formed with a strong preference for homochirality, they aggregate further regardless of their handedness and, therefore, with a preference for the nominally racemic 4D:4L structure and an overall strong heterochiral preference. The octamers cationized by K+, Rb+, or Cs+ therefore represent a new type of cluster ion that is homochiral in its internal subunits, which then assemble in a random fashion to form octamers. We tentatively interpret the homochirality of these tetramers as a consequence of assembly of the serine molecules around a central metal ion. The data provide additional evidence that the neutral serine octamer is homochiral and is readily cationized by smaller ions.