Recognition of Cis-Trans and Chiral Proline and Its Derivatives by Ion Mobility Measurement of Their Complexes with Natamycin and Metal Ion

Discrimination of isomers is an important and valuable feature in many analytical applications, and the identification of chiral isomers and cis-trans isomers is the current research focus. In this work, a simple method for direct, simultaneous recognition of d-/l-proline (P), d-/l-/ -/ -4-hydroxyproline (4-HP), and d-/l-/ -/ -N- -butoxycarbony (N-Boc-4-HP) was investigated by means of trapped ion mobility spectrometry-mass spectrometry (TIMS-MS). The isomers with cis-/trans-/d-/l-configuration can be directly recognized based on their mobility upon reaction with natamycin (Nat) and metal ions through noncovalent interactions. The results indicate that the recognition of the enantiomers has certain specificity, and the structural difference of the enantiomers was increased in a complex with Nat and metal ions. Herein, d-/l-P can be recognized through the ternary complexes [P + Nat + Mg - H] , [P + 2Nat + Ca - H] , [P + 2Nat + Mn - H] , and [P + Nat + Cu - H] . Similarly, c-4-HP , c-4-HP , t-4-HP , and t-4-HP can be recognized by [4-HP + Nat + Ca - H] , [4-HP + 2Nat + Ca - H] , and [4-HP + Nat + Cu - H] , while N-Boc-c-4-HP , N-Boc-c-4-HP , N-Boc-t-4-HP , and N-Boc-t-4-HP were recognized through the enantiomer complexes [N-Boc-4-HP + Nat + Li] , [N-Boc-4-HP + Nat + 2Na - H] , [N-Boc-4-HP + Nat + K] , [N-Boc-4-HP + Nat + Mn - H] , and [N-Boc-4-HP + Nat + Ba - H] . Moreover, tandem mass spectrometry (MS/MS) results indicated that different collision energies were obtained for the same fragment ions, which implied that the enantiomer complexes that contributed to their mobility separation shared identical interaction mode but had different gas-phase rigid geometries. Furthermore, the relative quantification for the enantiomers was performed, and the results were supported by a satisfactory coefficient ( > 0.99). The developed method can provide a promising and powerful strategy for the separation of chiral proline and its d-/l-/cis-/trans derivatives, bearing the advantages of higher speed, better accuracy, high selectivity, and no need for chemical derivatization and chromatographic separation.