Isomers and Rotamers of DCM in Methanol and in Gas Phase Probed by Ion Mobility Mass Spectrometry in Combination with High Performance Liquid Chromatography

Abstract

An integrated method of ion mobility mass spectrometry and high-performance liquid chromatography (HPLC) has been used to investigate the isomeric distribution of a popular fluorescent dye DCM (4-(dicyanomethylene)-2methyl-6-(4-dimethylaminostyryl)-4H-pyran) in methanol solution. Chromatographic separation of DCM isomers in methanol has been performed by probing the molecular mass (DCMH + ), and two distinctly separated peaks are observed at retention times 3.73 (peak-I) and 3.87 (peak-II) min, where the latter one appears nearly twice as intense as the former. However, peak-I appears much weaker compared to peak-II if the chromatogram is recorded by optical probing at the absorption maximum of this dye (467 nm). The ion mobility (IM) spectra of DCMH + ions corresponding to each of the LC-separated factions show three common peaks A, B, and C, with collision cross-section (CCS) values of 174, 185, and 197 Å 2 , respectively, but their relative intensities in the two IM spectra appear in opposite sequences. The three IM peaks have been assigned by considering the theoretically calculated CCS values of 13 possible isomers of DCMH + ions. The IM spectral features also reveal that isomeric interconversions occur during the ESI process. Electronic structure calculations have been used to optimize the geometries of the four isomers of solvated DCM and the corresponding protomeric structures of DCMH + . The isomerization pathways and associated energy barriers have also been calculated. The gas-phase protomers are found to follow a completely different sequence of stability as compared to the neutral isomers. The analysis reveals that peak-I corresponds to one of the cis isomers, whereas peak-II arises due to cumulative contributions of the other three isomers. The absorption spectrum of DCM in methanol is simulated from the computed spectral profiles of the isomers which indicates a distribution of trans1, trans2, cis1, and cis2 isomers as 33.5, 61.5, 2.0, and 3.0%, respectively. The fragmentation behavior of DCMH + ions in a collision-induced dissociation experiment has been found to be isomer dependent.

Tautomers and Rotamers of Curcumin: A Combined UV Spectroscopy, High-Performance Liquid Chromatography, Ion Mobility Mass Spectrometry, and Electronic Structure Theory Study

“…The mobility measurements were conducted using the Agilent 6560 ion mobility quadrupole time-of-flight (IM-QTOF) mass spectrometer which is connected to an HPLC system (Agilent 1290 UHPLC). Details of the instrument were described elsewhere . Briefly, the ions were passed through an 80-cm-long drift tube filled with nitrogen gas (99.99%) at a pressure of ∼4.2 Torr under the influence of a constant electric field of 18.1 V/cm.…”

“…Briefly, the ions were passed through an 80-cm-long drift tube filled with nitrogen gas (99.99%) at a pressure of ∼4.2 Torr under the influence of a constant electric field of 18.1 V/cm. The measured drift times of the ions were converted to CCS (Ω m ) values following the procedure described in ref . Negatively charged ions were produced in the gas phase by the deprotonation of curcumin molecules in the electrospray ionizer operating in negative ion mode.…”

“…Details of the instrument were described elsewhere. 59 Briefly, the ions were passed through an 80-cm-long drift tube filled with nitrogen gas (99.99%) at a pressure of ∼4.2 Torr under the influence of a constant electric field of 18.1 V/cm. The measured drift times of the ions were converted to CCS (Ω m ) values following the procedure described in ref 59.…”

UV-A-Induced Photoisomerization and Photodimerization of Curcumin: An Ion Mobility Mass Spectrometry Study

“…For analysis of the reaction outcome that involves identification and structure characterization of the photoproducts, an integrated method of drift tube ion mobility mass spectrometry (DTIMS) in combination with high-performance liquid chromatography (HPLC), collision-induced dissociation (CID) of mass-selected molecular ions, and electronic structure theory methods has been employed. With the advancements of recent technology, ion mobility spectrometry (IMS) has emerged as a powerful analytical technique for separation and elucidation of the molecular shapes and structures of a diverse categories of compounds ranging from small molecules to biological macromolecules. − In this method, the measurable parameter pertaining to the molecular structure is the collision cross section (CCS) of the analyte with respect to a buffer gas. The molecules of different sizes and shapes are separated according to their CCS values, as their positively or negatively charged ions are allowed to drift through the selected neutral buffer gas under the influence of an external electric field.…”