CE-MS characterization of negatively charged Delta a-, Delta b- and Delta g-CD derivatives and their application to the separation of dipeptide and tripeptide enantiomers by CE

Sulfated, sulfopropyl and carboxymethyl -, - and -CDs were characterized by CE-ESI-MS using an acidic BGE with anodic MS detection and a basic BGE with cathodic MS detection. Isomers of the sulfated CDs comigrated in both systems. The acidic BGE with anodic MS detection resulted in slightly better separation of the isomers of the sulfopropyl CDs, which were separated according to the number of substituents. In the case of carboxymethyl CDs, isomers with an identical number of substituents but with a different substitution pattern with regard to substitution of the primary and secondary hydroxyl groups of the CDs could be separated using the basic BGE. The separation of the LL and DD enantiomers of dipeptides and tripeptides using the CDs was studied with regard to the amino acid sequence and the nature of the CDs. Standardized conditions with regard to buffer pH, CD concentration and voltage were applied. The peptides were analyzed at pH 2.5 as positively charged compounds and at pH 5.3 as neutral zwitterions. The -CD derivatives were more effective chiral selectors for the investigated peptides followed by the -CD derivatives. The -CDs were the least effective selectors. The enantiomer migration order depended on both the CD and the amino acid sequence of the peptides. For several combinations, pH-dependent reversal of the enantiomer migration order was also observed.