Neutral Octamer Research Articles

Collection and Characterization by Mass Spectrometry of the Neutral Serine Octamer Generated upon Sublimation.

Serine forms neutral octameric clusters during sublimation, as demonstrated by electrostatically deflecting thermally ionized serine species from the sublimate, then gently ionizing the remaining neutrals for examination by mass spectrometry (MS). The MS results demonstrate a strong homochiral preference in the neutral octamer (measured after its gentle ionization), while the smaller serine clusters are achiral. In the initial stages of its sublimation, nonracemic solid serine generates a neutral serine monomer as the principal species in the vapor phase, with a significant enantiomeric enrichment relative to the solid. The serine monomer, when the flux is sufficient, assembles into the octamer, which displays a much higher chiral purity than the monomer. The serine octamer is separated from other neutral clusters in the sublimate by a new method based on the different distances that the clusters travel in an inert gas stream before they condense in a cooled collector. The deposited octamer is subsequently dissolved, and the solution is investigated by MS. The spectrum confirms that the collected serine octamer has undergone chiral enrichment relative to the starting solid used in the sublimation. The chiral enrichment observed in going from the serine monomer to octamer can be accommodated using a chemical model, grounded on the homochiral preference of the neutral serine octamer. Using the enantiomeric excess (ee %) of the vapor-phase monomer as the input, the model output matches the experimental octamer ee % when subliming solid serine with various initial ee % values.

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

AbstractNebulization 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.

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-d3 Ser)8 H+ as a soft protonating agent ensured that neutral (d-Ser)8 was characterized without interference from serine octamer ((d-Ser)8 H+ ) 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)8 H+ is simply an expression of this feature of the neutral cluster.

Negatively-Charged Halide Adducts of Homochiral Serine Octamers

Negatively charged halide adducts of serine octamers, (Ser(8)+2Cl)(2-) and (Ser(8)+2Br)(2-), appear as magic number clusters in the negative ion electrospray mass spectra of solutions containing serine and the halide. Like the well-known protonated serine octamer, these negatively charged adducts are formed with homochiral preference and also undergo chiroselective substitution reactions with other amino acids. Tandem mass spectra of negatively charged halide adducts of serine octamers show that these ions also have a characteristic fragmentation signature. The fact that octamers of both polarities display analogous chemical properties suggests that these may be characteristics of the so-far-unknown neutral octamer. If serine played a key role in the origin of homochirality on the primitive earth, it was likely through both the neutral octamer and the ionic adducts. Unlike the octamers, the formation of halide-containing serine cluster ions of particular sizes is unfavorable under the conditions of the experiment. Signals corresponding to the ions (Ser(9)+2Br)(2-) and (Ser(15)+2Br)(2-) are particularly low in intensity, giving rise to gaps in the distribution of serine/bromide clusters in the negative ion electrospray mass spectra. These cluster sizes are likely to correspond to unstable "anti-magic number" clusters recently reported by Clemmer.

Pople's Gaussian‐3 model chemistry applied to an investigation of (H2O)8 water clusters

AbstractThe Gaussian‐3 method developed by Pople and coworkers has been used to calculate the free energy of neutral octamer clusters of water, (H2O)8. The most energetically stable structures are in excellent agreement with those determined from experiment and those predicted from previous high‐level calculations. Cubic structures are favored over noncubic structures over all temperature ranges studied. The D2d cubic structure is the lowest free energy structure and dominates the potential energy and free energy hypersurfaces from 0 K to 298 K. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005