High–throughput enantioseparation of Nα–fluorenylmethoxycarbonyl proteinogenic amino acids through fast chiral chromatography on zwitterionic-teicoplanin stationary phases

In this study, 31 racemates of N α-FMOC (fluorenylmethoxycarbonyl) amino acids (AAs) with different chemico-physical characteristics (neutral nonpolar, neutral polar, acidic and basic) have been success- fully resolved in fast enantioselective chromatography on recently-developed zwitterionic-teicoplanin chi- ral stationary phases (CSPs). The CSPs were prepared by covalently bonding the teicoplanin selector on fully-porous particles of narrow dispersion particle-size distribution (particle diameter 1.9 μm) and superficially-porous particles (2.0 μm). Both the zwitterionic-teicoplanin CSPs have proved to be ideal media for the separation of this important class of compounds. In particular, the zwitterionic CSP pre- pared on superficially-porous particles exhibited superior enantioselectivity and resolution, compared to that made of fully porous particles, in virtue of more favorable thermodynamics. The zwitterionic na- ture of these CSPs allowed avoiding the annoying effect of Donnan’s exclusion of enantiomers from the stationary phase. This effect, on the opposite, was frequently observed on a commercial teicoplanin CSP (Teicoshell) employed for comparative purposes. Noticeably, on the zwitterionic-teicoplanin CSPs, by us- ing either acetonitrile- or methanol-rich mobile phases (MPs), it was possible to favor speed over enan- tioresolution and vice versa. This work gives further replies to the request for rapid determination of enantiomeric excess of N α-FMOC proteinogenic (and non–proteinogenic) AAs, typically used as preferred chiral synthons in the solid-phase synthesis of therapeutic peptides.