Nebulization Prior to Isolation, Ionization, and Dissociation of the Neutral Serine Octamer Allows Its Characterization

Nebulization of enantiomerically mixed serine solutions, followed by ion deflection and gentle on-line protonation, is used to isolate and characterize the strongly preferred homochiral neutral serine octamer (Ser₈). The separation of the nebulization and ionization steps and the use of the enantiomeric l-serine ion (l-d₃Ser)₈H⁺ as a soft protonating agent ensured that neutral (d-Ser)₈ was characterized without interference from serine octamer ((d-Ser)₈H⁺) ions. MS/MS experiments showed that the ionized neutral octamer and the protonated octamer formed directly by ESI have the same structures. Experiments in which the temperature of the heated transfer tube was varied indicate dissociation of the neutral octamer to the dimer. Field-free nebulization of racemic serine forms the separate homochiral species with very high chiral selectivity. We suggest that the homochiral preference is a property of (d-Ser)₈ and that the preference observed earlier for (d-Ser)₈H⁺ is simply an expression of this feature of the neutral cluster.