Chiral HPLC Separation of Protected Amino Acids

Abstract

This work reports the direct separation of twenty one Fmoc, Boc, Trt, and Pmc, single and double protected amino acids (PAA) using protein-based and macrocyclic antibiotic-based chiral columns. This group of PAAs represents a cross-section of commonly employed reagents in peptide synthesis with diverse structures and protecting groups. The Ultron Ovomucoid (OVM) chiral column was found to be extremely versatile in the resolution of these molecules without the need of any chemical modification. Compounds not separated in the Ovomucoid column, were resolved using Human serum albumin (HSA) and Chirobiotic T (Teicoplanin) columns. Several interesting structure-related factors were found to affect the enantiomeric separations in the OVM column. The relative position and nature of the protecting groups seems to have great influence on the separations, also it was observed that compounds containing more protecting groups were easier to resolve. In spite of the relatively low efficiency of OVM columns, it was possible to determine amounts as low as 0.15% of the unwanted isomer (usually D isomer) in commercial samples. The separations developed have been applied to the selection of commercial suppliers of protected amino acids (PAA), and to the analysis of research samples obtained from peptide-synthesis reactions. Five PAAs previously reported to require some chemical modification to achieve enantioresolution Fmoc-Lys-(Boc)-OH, Fmoc-Asn-(Trt)-OH, Fmoc-Ser-(tBu), Fmoc-Cys-(tBu)-OH, and Fmoc-Thr-(tBu)-OH), were separated without any derivatization step.