S-Trityl-(R)-Cysteine, a Multipurpose Chiral Selector for Ligand-Exchange Liquid Chromatography Applications

Chiral secondary amino acids, their importance, and methods of analysis

Naturally occurring secondary amino acids, with proline as the main representative, contain an alpha-imino group in a cycle that is typically four-, five-, and six-membered. The unique ring structure exhibits exceptional properties-conformational rigidity, chemical stability, and specific roles in protein structure and folding. Many proline analogues have been used as valuable compounds for the study of metabolism of both prokaryotic and eukaryotic cells and for the synthesis of compounds with desired biological, pharmaceutical, or industrial properties. The D-forms of secondary amino acids play different roles in living organisms than the L-forms. They have different metabolic pathways, biological, physiological, and pharmacological effects, they can be indicators of changes and also serve as biomarkers of diseases. In the scientific literature, the number of articles examining D-amino acids in biological samples is increasing. The review summarises information on the occurrence and importance of D- and L-secondary amino acids-azetidic acid, proline, hydroxyprolines, pipecolic, nipecotic, hydroxypipecolic acids and related peptides containing these D-AAs, as well as the main analytical methods (mostly chromatographic) used for their enantiomeric determination in different matrices (biological samples, plants, food, water, and soil).

Preparation of silica microspheres with a broad pore size distribution and their use as the support for a coated cellulose derivative chiral stationary phase

A templating strategy using crosslinked and functionalized polymeric beads to synthesize silica microspheres with a broad pore size distribution has been developed. The polymer/silica hybrid microspheres were prepared by utilizing the combination of a templating weak cation exchange resin, a structure-directing agent N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride, and a silica precursor tetraethyl orthosilicate. The silica microspheres were then obtained after calcinating the hybrid microspheres. The as-prepared materials were characterized by scanning electron microscopy, mercury intrusion porosimeter, and thermal gravimetric analysis. The results showed that the starting templating beads were about 5 μm in diameter and the formed silica microspheres were less than 3 μm with a pore size range of 10-150 nm, some pores were even extended to beyond 250 nm. It was demonstrated that cellulose tris(3,5-dimethylphenylcarbamate) was readily coated onto the surface of the as-synthesized silica microspheres without any additional surface pretreatment. The coated silica microspheres were uniformly dispersed even with high loading of the chiral stationary phase, which exhibited high resolution chiral separations in high-performance liquid chromatography.

Chiral recognition in separation science - an update

Stereospecific recognition of chiral molecules is an important issue in various aspects of life sciences and chemistry including analytical separation sciences. The basis of analytical enantioseparations is the formation of transient diastereomeric complexes driven by hydrogen bonds or ionic, ion-dipole, dipole-dipole, van der Waals as well as π-π interactions. Recently, halogen bonding was also described to contribute to selector-selectand complexation. Besides structure-separation relationships, spectroscopic techniques, especially NMR spectroscopy, as well as X-ray crystallography have contributed to the understanding of the structure of the diastereomeric complexes. Molecular modeling has provided the tool for the visualization of the structures. The present review highlights recent contributions to the understanding of the binding mechanism between chiral selectors and selectands in analytical enantioseparations dating between 2012 and early 2016 including polysaccharide derivatives, cyclodextrins, cyclofructans, macrocyclic glycopeptides, proteins, brush-type selectors, ion-exchangers, polymers, crown ethers, ligand-exchangers, molecular micelles, ionic liquids, metal-organic frameworks and nucleotide-derived selectors. A systematic compilation of all published literature on the various chiral selectors has not been attempted.