title: Alkali metal-cationized serine clusters studied by sonic spray ionization tandem mass spectrometry.
abstract: Serine solutions containing salts of alkali metals yield magic number clusters of the type (Ser(4)+C)(+), (Ser(8)+C)(+), (Ser(12)+C)(+), and (Ser(17)+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 Ser(4) units, suggesting that the neutral tetramers are the stable building blocks of the observed larger aggregates, (Ser(8)+C)(+) and (Ser(12)+C)(+). Remarkably, although the Ser(4) 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.
authors.affiliation: Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA.
authors.forename: Sergio C
authors.initials: SC
authors.lastname: Nanita
authors.forename: Ewa
authors.initials: E
authors.lastname: Sokol
authors.forename: R Graham
authors.initials: RG
authors.lastname: Cooks
mesh_terms: D002412:Cations / Q000737:chemistry; D008672:Metals, Alkali / Q000737:chemistry*; D009330:Nebulizers and Vaporizers; D012694:Serine / Q000737:chemistry*; D013010:Sonication; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D013237:Stereoisomerism; D053719:Tandem Mass Spectrometry
chemical_list: D002412:Cations; D008672:Metals, Alkali; D012694:Serine