Amino Acid Oligomer Brushes as Chiral Stationary Phases: Effects of Side-Chain Architecture and Mobile-Phase Modifiers on Enantioseparation Behavior.

In this study, a series of oligomer-brush chiral stationary phases was developed to systematically investigate the influence of amino acid side chains on enantioseparation. Through a structure-preserving "grafting-to" strategy, three well-defined chiral oligomers derived from l-proline, l-phenylalanine, and l-leucine were covalently immobilized onto silica microspheres. The enantioseparation capabilities of these chiral stationary phases were comprehensively evaluated using 27 representative racemates under normal-phase conditions. The results revealed that the side-chain architecture plays a decisive role in the chiral recognition microenvironment. The stationary phase with a rigid pyrrolidine ring (CSP-P(Pro-An)) demonstrated the broadest applicability for bulky, conformationally restricted analytes and was the only stationary phase capable of resolving the tested axially chiral metal complexes. The stationary phase with aromatic side chains (CSP-P(Phe-An)) showed enhanced selectivity for specific π-rich analytes, while the stationary phase with a flexible isobutyl side chain (CSP-P(Leu-An)) provided the highest separation factors for several 3,5-dinitrobenzoyl-derivatized amino acids. Furthermore, an investigation into the influence of mobile-phase modifiers found that, compared to other aprotic modifiers such as ethyl acetate, tetrahydrofuran, and dichloromethane, methyl tert-butyl ether provided a more optimal balance between retention and enantioselectivity. This is attributed to the steric shielding effect produced by the tert-butyl group of methyl tert-butyl ether, which minimizes its competitive interactions for essential hydrogen-bonding sites. These results indicate that both the side-chain structure and the choice of mobile-phase modifier are key parameters for tailoring the performance of amino acid-based oligomer-brush chiral stationary phases.