Chiral analysis of linear protonated dipeptides by complexing with Cyclodextrins using Ion-Mobility Mass-Spectrometry and DFT structural calculations

The functions and applications of dipeptides have been reported to possess unique characteristics compared to that of proteins or amino acids. Naturally occurring amino acids, except for glycine, are chiral molecules; thus, for each type of dipeptide there are four possible isomers. Isomeric structural analysis of dipeptides is important for measuring the quality of dipeptide products, as well as understanding their chemical and biological functions in humans. In the present study, a rapid and simple method for isomeric recognition of protonated dipeptides H-D/L-Phe-D/L-X-OH, where X = hydrophobic amino acids, including alanine (Ala), isoleucine (Ile), leucine (Leu), proline (Pro), tryptophan (Trp), valine (Val), phenylalanine (Phe), and methionine (Met), was developed and investigated. Mixed solutions of dipeptides and cyclodextrins (CDs) were prepared for the production of gas phase, non-covalent complex ions by electrospray ionization (ESI). Trapped ion-mobility mass spectrometry (TIMS-MS) was subsequently used to investigate the corresponding ion mobilities (IM). The TIMS and density functional theory (DFT) results showed that the chiral structure of different dipeptides can be distinguished by beta-CD, which indicates that different isomeric complexes have different collision cross sections (CCSs) and spatial conformations. The relative content of isomeric dipeptides mixtures was also obtained in the ratio range from 10:1 to 1:10 to measure the detection limits. Our work provides a direct analytical method for determining isomeric dipeptides, which could potentially have wide applications in the field of chiral recognition.

Automated summary

In this study, a rapid and simple method for the isomeric recognition of dipeptides was developed using electrospray ionization (ESI) and trapped ion-mobility mass spectrometry (TIMS-MS). The chiral structure of different dipeptides could be distinguished by beta-CD, and the relative content of isomeric dipeptides mixtures was measured. This method provides a direct analytical tool for determining isomeric dipeptides and has potential applications in chiral recognition.