Chiral differentiation of amino acids by the kinetic method by Fourier transform ion cyclotron resonance mass spectrometry via a different dissociation pathway

Application of Infrared Multiple Photon Dissociation (IRMPD) Spectroscopy in Chiral Analysis

In recent years, methods based on photodissociation in the gas phase have become powerful means in the field of chiral analysis. Among them, infrared multiple photon dissociation (IRMPD) spectroscopy is a very attractive one, since it can provide valuable spectral and structural information of chiral complexes in addition to chiral discrimination. Experimentally, the method can be fulfilled by the isolation of target diastereomeric ions in an ion trap followed by the irradiation of a tunable IR laser. Chiral analysis is performed by comparing the difference existing in the spectra of enantiomers. Combined with theoretical calculations, their structures can be further understood on the molecular scale. By now, lots of chiral molecules, including amino acids and peptides, have been studied with the method combined with theoretical calculations. This review summarizes the relative experimental results obtained, and discusses the limitation and prospects of the method.

Chiral Differentiation of Non-Covalent Diastereomers Based on Multichannel Dissociation Induced by 213-nm Ultraviolet Photodissociation

Here we present the implementation of 213-nm ultraviolet photodissociation (UVPD) in a FT-ICR mass spectrometer for chiral differentiation in the gas phase. The L/D amino acid-substituted serine octamer ions were selected as examples of diastereoisomers for chiral analysis. Several kinds of fragment ions were observed in these experiments, including fragment ions that are similar to the ones observed in corresponding collision-activated dissociation (CAD) experiments, fragment ions generated with different protonation sites by only destroying non-covalent bonds, and unique non-covalent cluster radical ions. The latter two kinds of fragment ions are found to be more sensitive to the chirality of the substituted units. Further experiments suggest that the formation of radical ions is mainly affected by chromophores on side chains of the substituted units and micro surroundings of the characterized non-covalent diastereoisomers. A comparing experiment performed by only changing the wavelength of UV laser to 266 nm shows that the 213-nm UV laser has the priority in the diversity of fragmentation pathways and potential of further application in chiral differentiation experiments.

Chiral Differentiation of Amino Acids by In-Source Collision-Induced Dissociation Mass Spectrometry

Chiral recognition of d- and l-amino acids is achieved by a method combining electrospray ionization (ESI) and in-source collision-induced dissociation (CID) mass spectrometry (MS). Trimeric cluster ions [Cu(II)(A)(ref)2-H](+) are formed by ESI of mixtures of d- or l-analyte amino acid (A), chiral reference (ref) and CuSO4. By increasing the applied voltage in the ESI source region, the trimeric ions become unstable and dissociate progressively. Thus chiral differentiation of the analyte can be achieved by comparing the dependence of their relative intensities to a reference ion on applied voltages. The method does not need MS/MS technique, thus can be readily performed on single-stage MS instruments by turning the voltage of sampling cone.