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

Curcumin, the bioactive compound present in spice plant turmeric, has been shown to exhibit selective phototoxic activities toward mammalian cancer cells, and it is being used extensively as a photosensitizer (PS) in photodynamic therapies (PDT). However, so far, the fate of curcumin toward photochemical transformations is not well understood. Here we report our findings of a number of novel photochemical reaction channels of curcumin in water-methanol mixture, like photoisomerization, photodimerization, and photooxidation (H2-loss). The reaction was performed by irradiating the curcumin solution with ultraviolet (UV) light of wavelength 350 nm, which is abundant in the earth's troposphere. Product identification and structure elucidation are done by employing an integrated method of drift tube ion mobility mass spectrometry (DTIMS) in combination with high-performance liquid chromatography (HPLC) and collision-induced dissociation (CID) of the mass-selected molecular ions. Two photoisomers of curcumin produced as a result of trans-cis configurational changes about C═C double bonds in the excited state have been identified, and it has been shown that they could serve as the precursors for formation of isomeric dimers via [2 + 2] cycloaddition and H2-loss products. Comparisons of the experimentally measured collision cross-section (CCS) values of the reactant and product ions obtained by the DTIMS method with those predicted by the electronic structure theory are found to be very effective for the discrimination of the produced photoisomers. The observed photochemical reaction channels are potentially significant toward uses of curcumin as a photosensitizer in photodynamic therapy.

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

The structures of tautomers and rotameric forms of curcumin, the bioactive compound present in spice plant turmeric, have been investigated using ion mobility mass spectrometry (IMMS) in conjunction with high-performance liquid chromatography (HPLC) and UV-visible spectroscopy. Two tautomeric forms of this ß-diketone compound, keto-enol and diketo, have been chromatographically separated, and the electronic absorption spectra for these two tautomeric forms in methanol solution have been recorded separately for the first time. The molecular identity of the HPLC-separated solution fractions is established unambiguously by recording the mass and fragmentation spectra simultaneously. The ion mobility spectrum for the deprotonated curcumin anion, [Cur-H]-, corresponding to the diketo tautomer, displays only one peak (P), and the collision cross-section (CCS) value is measured to be 185.9 Å2. However, the ion mobility spectrum corresponding to the HPLC-separated keto-enol tautomer shows three distinctly separated peaks, P, Q, and R, with CCS values of 185.9, 194.8, and 203.4 Å2, respectively, whereby peak R appears to be the most intense one, followed by peaks P and Q. The theoretically calculated CCS values of different isomers of [Cur-H]-, optimized by electronic structure theory methods, display satisfactory correlation with the experimentally observed values, corroborating our assignments. The spectral attributes also indicate the occurrence of structural rearrangements in the electrospray ionization process. With the aid of the electronic structure calculation, low-energy pathways for the occurrence of the structural isomerization to surpass the energy barrier are suggested, which are consistent with the assignments of the peaks observed in the IM spectra.

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

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)-2-methyl-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 Å2, 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.