Serine sublimes with spontaneous chiral amplification

The origin of biological homochirality

The single-handedness of biological molecules has fascinated scientists and laymen alike since Pasteur's first painstaking separation of the enantiomorphic crystals of a tartrate salt more than 150 yr ago. More recently, a number of theoretical and experimental investigations have helped to delineate models for how one enantiomer might have come to dominate over the other from what presumably was a racemic prebiotic world. This article highlights mechanisms for enantioenrichment that include either chemical or physical processes, or a combination of both. The scientific driving force for this work arises from an interest in understanding the origin of life, because the homochirality of biological molecules is a signature of life.

A simple explanation of the enhancement or depletion of the enantiomeric excess in the partial sublimation of enantiomerically enriched amino acids

In the partial sublimation of three enantiomerically enriched amino acids, the ee of the sublimates can be explained by the ratio between the saturated vapor pressures of the racemate and the pure enantiomers.

Standard-free quantitation of mixtures using clusters formed by electrospray mass spectrometry

Ion abundances in electrospray ionization mass spectra depend on many factors, including molecular hydrophobicity, basicity, solution composition, and instrumental parameters. A recently introduced method that uses nonspecific cluster ion abundances to obtain solution-phase molar fractions of analytes directly from ESI mass spectra without using standards was evaluated using solutions containing 0.03-24% L-threonine, D-threonine, L-leucine, L-lysine, L-glutamic acid, or diglycine with L-serine as a major component. Because of the propensity of serine clusters to exhibit "magic" numbers, which can be chirally selective, these experiments provide a rigorous test of this standard-free cluster quantitation method, which requires that clusters form statistically from analytes in solution. For each of these solutions, the compositions of clusters containing > or = 32 molecules reflect the solution molar fractions of each component. From the abundances of these larger clusters, the solution molar fraction can be determined to better than 10% accuracy over nearly 3 orders of magnitude in concentration. In contrast, the ionization/detection efficiency of the individual amino acids differs by as much as a factor of 460 in these experiments. The protonated octamer incorporates some molecules statistically but efficiently excludes other molecules that have significantly different properties or chirality. This standard-free quantitation method may be most advantageous for rapidly characterizing mixtures, such as products of chemical synthesis, which contain unknown products or molecules for which suitable standards are not readily available.

Chirality recognition between neutral molecules in the gas phase

Noncovalent interactions are particularly intriguing when they involve chiral molecules, because the interactions change in a subtle way upon replacing one of the partners by its mirror image. The resulting phenomena involving chirality recognition are relevant in the biosphere, in organic synthesis, and in polymer design. They may be classified according to the permanent or transient chirality of the interacting partners, leading to chirality discrimination, chirality induction, and chirality synchronization processes. For small molecules, high-level quantum chemical calculations for such processes are feasible. To provide reliable connections between theory and experiment, such phenomena are best studied in vacuum isolation at low temperature, using rotational, vibrational, electronic, and photoionization spectroscopy. We review these techniques and the results which have become available in recent years, with special emphasis on dimers of permanently chiral molecules and on the influence of conformational flexibility. Analogies between the microscopic mechanisms and macroscopic phenomena and between intra- and intermolecular cases are drawn.

Spoilt for choice: assessing phase behavior models for the evolution of homochirality

The observation of single chirality in biological systems has intrigued scientists for more than one hundred years. Here we discuss several recent experimental studies showing amplification of enantiomeric excess based on amino acid phase behavior. Comparing observations of solution-solid and gas-solid phase transitions highlights the underlying fundamental physical chemistry principles that rationalize the observed enantioenrichment in both cases.

An astrophysically-relevant mechanism for amino acid enantiomer enrichment

The sublimation of low ee amino acids was examined while exploring simple mechanisms by which high ee amino acids can be generated under conditions that exist in space; significant enantioenrichment of a variety of amino acids by sublimation was achieved.